View Full Version : Muzzle brake/flash arrestor - anyone ever make one?

Jim Shaper
11-24-2009, 04:21 AM
I'm building an AR and can't believe the prices people are charging for a 1/2-28 threaded slightly milled chunk of drop. :eek:

I know the crown is the primary feature in directing a bullet down range where you want it to go, but how does the brake interact with the gasses as they pass the bullet between the crown and the fixture hanging off the end of the barrel?

If I was to make something, what kind of ID profile should I be shooting for? Clearances, angles, and such.

11-24-2009, 08:51 AM
There must be a gazillion different AR muzzle brakes designs. Since that is not a controlled part, and there is currently no "assault weapon" ban in place, you could always experiment until you find one that does what you want it to do. Noise and flash would be the major concerns, and it is difficult to design one that limits both. Probably the two simplest designs are the original AR-style that is just a series of cuts made around the brake that look like they could be made with a small diameter double-angle cutter, and the designs that are a series of holes machined around the brake. I have seen the holes drilled perpendicular to the bore as well as angled. Who knows, if you come up with a good design you could always sell them. :)

andy b.

Bob Ford
11-24-2009, 12:00 PM

A flash hider disperses the gas, so it is harder to see a flash. A muzzle brake redirects the gas so some of the force is pushing the firearm forward. The main consideration with ether is that the bullet does not strike the inside of the device. The better brakes make very good hearing protection mandatory.

Thread for barrel and bore the brake in one setting. Baffles or holes are your design all work, some better than others. When finished install and using a bore size rod slide into the brake and muzzle to make sure the bullets will not strike the brake. On a .224 a front hole of .250 would not be too large and a length of about 2.5 inches from muzzle. Use anti seize on the barrel threads as high pressure gas will force crud in them. These are easy to make and you might want to try different designs.


Jim Shaper
11-24-2009, 02:16 PM
I've already got something cooked up in my head, it's just a question of how to implement it without inducing instability to the tail of the round as it exits.

Any prototype is an experiment. I was hoping for some guidelines to help smooth the development bumps.

11-24-2009, 07:49 PM
On most brakes/flashiders the inside is a cone with the wide end facing away from the muzzle of the firearm. That seems to be pretty universal in every muzzle device I have ever looked at. Other than that, there doesn't seem to be anything standard to base a design on. I'd say if you built it from 1-3" long with the interior a cone starting at 0.25" diameter and opening up to ~0.6" diameter (for a standard 5.56mm round), then that is about as general as you can get. Everything else will directly affect the performance and either make it a flashider, muzzle brake, or "noise enhancer". :) You can drill holes, machine slots, cut grooves, combine several of these features, etc. That is the part you'll need to experiment with. In fact, I think it would be difficult to come up with an entirely original design that hasn't already been produced.

It is sort of like asking, "what are the basic design guidelines for me to build a chair". If it keeps your ass off the floor, that is about the most generic description you can make. Anything else is up to you. ;)

andy b.

Jim Shaper
11-24-2009, 07:55 PM
But how the chair keeps you off the floor doesn't affect how you walk when you're 500m away... :p

Is there reason to make the bore of the brake that large?

I understand the JP's (the one that looks like this -> (() ) use a rather large passage, but they also have considerable space between the barrel and the first wall of that brake.

What I have in mind is similar to porting the barrel, but the lack of remaining in the lands of the rifling while those ports become exposed has me nervous to the impact on the trajectory.

11-24-2009, 08:59 PM
Two different designs for two different purposes. I can't seem to lay my hands on it at the moment but there is a book sold by Precision Shooting magazine that treats muzzle brakes. The early designs like Magna port for bigbore pistols and Skeet/Trap shotguns were EDM cut into the existing Barrel. The purpose was to reduce recoil and muzzle rise. They work but increase the muzzle blast. The old Lyman Cutts Compensator was years ahead of the competetion but people standing near the shooter didn't like them. After a lot of experimentation, the ones that look like a flash hider on a tank work better than the fancy multi hole models. I have shot a friends 300 win mag with a good muzzle brake recoil is about like a 243 Win but you need both ear plugs and muffs.

Jim Shaper
11-24-2009, 09:10 PM
What you're attributing as a muzzle brake is actually a recoil reduction device. The JP does both, but the intent of a brake is to prevent the gasses from the charge from continuing to thrust at the round and cause actions after the round has left the muzzle from affecting it's path; such as muzzle rise. So to minimize the effects, the gasses are diverted to the sides rather than being allowed to continue to push the round from behind.

Flash suppressors also divert the gasses, but their intent is to shorten the flash seen from the end of the weapon which helps mask the location of the shooter. Now since I don't particularly care what my location looks like to those around me, the reason I'm interested in suppressing the flash, is because I intend to shoot this gun at dusk and dawn - so retaining my night vision is very desirable. Diverting the flash to the sides prevents it from interfering with my "eyes on target."

We're talking about a .223 here - recoil is a joke.

11-24-2009, 11:15 PM
The long range varmint hunters use the muzzle brakes to hold down the muzzle rise to be able to visually spot their own shots. The heavy bbl varmint rifles like the 22/250 don't recoil as much, but it is enough that it is hard to spot your own shots. There are piloted reamers made to provide uniform predictable clearances. I have bought a few different designs of brakes to look at but have never installed one. There are clearly lots of different nitches and purposes for these type devices. I had never thought about night vision. I used to do a lot of load development shooting at night. There is a wide range of muzzle flash with different burning rate powders. Darrell Hollands brake that looks like a Tank flash supressor might intrest you.

Jim Shaper
11-25-2009, 04:56 AM

This guy fell short of giving the ID bore size, but listed everything else to 3 decimal places. lol

I was thinking of something similar to these, only rather than placing the holes all the way around the top, having them radially to the sides like this > < and angled back slightly.

I'll probably see what kind of stainless I have on the shelf tomorrow and have a go at making one.

11-25-2009, 07:53 AM
In the 1950's, an American engineer had come up with a radical new idea for a muzzle brake. It was presented to the US Army, who laughed the guy out of their offices. The engineer published his design and that was the last it was heard of. Fast forward 25 years. When the Russians introduced the new AK74, the US Army was really intersted in the unique muzzle brake that was being incorporated into each weapon. They spent much time and money studing the Russian muzzle brake, determining how it operated. After they had stuidied it, it was pointed out that it was not a unique Russian design but a slight moderinization of the muzzle brake the US Army could have had 25 years before!

11-25-2009, 04:25 PM
I have tried a LOT of different brakes on the AR15. Several on my own and many on customer's rifles.

I find that the Smith Enterprises brake works the best of most of them. It is relatively inexpensive and I have found that it in some cases improves accuracy. WARNING..it is LOUD! It makes a mild .223 sound like a short barreled .308. BOOM! I use ear plugs AND muffs with it.

Now, that being said..there is zero muzzle rise or recoil on the M4 after install. You can actually lay the forend on the flat of your palm and pump rounds without the disturbance of muzzle rise.

IF I was to copy a brake...I would copy that one. But for $45..I don't bother as time is worth something.

The following information is copied from the site.



The SEI 5.56mm Muzzle Brake is designed for extremely rapid follow-up shots. This time-proven design,
with its 14 gas ports is painstakingly made from heat treated 4140 chrome moly steels and provided in a
manganese phosphate finish. The SEI 5.56mm NATO Muzzle Brake is not compatible with the SEI
M4DC Sound Suppressor, which requires the use of the Vortex DC Flash eliminator, P/N 1001V.
SPECS: M4/M16 Muzzle Brake 8620 chrome moly steel, heat-treated, black parkerized per MIL SPEC.
Diameter 0.860 inches (in), Length 2.000 in., Jam Nut length 0.200 in. Wrench Flats are cut for 3/4 inch
wrench. Weight, with Jam Nut 0.22 of a lb. Threaded 1/2 X 28 TPI (RH).
Part Number Item Description
Part No. 1002 5.56mm Muzzle Brake (M4/M16)
(Information current as of 19 Sep 2005)

11-25-2009, 04:33 PM
JS- I have made many,many custom muzzle brakes from scratch for rifles and handguns. They are not that difficult. I have tried experimenting with different port angles but discovered that 90 degree exit angles work fine unless you are shooting large caliber, FAST, heavy bullets such as .338 Lapua Mag up to .50 BMG. Only in those large, Fast moving bullets is a rearward exit angle beneficial.
Realizing the cartridge you are shooting, the length of the barrel, and the length of barrel required to burn 90% of the gasses the cartridge is loaded with is important when dealing with heavy, large caliber bullets.
For instance a .45-70 Govn't cartridge in 350 to 500 grain bullet is going to burn up much of the powder in typical barrel length for that cartridge (22" +) and have less gas force as the bullet exits the barrel. Applying a muzzle brake for that cartridge would only benefit about 5-12% felt recoil (Almost not worth the effort). I experimented with a muzzle brake on a lever action Marlin XLR .45-70. I felt almost NO reduction in felt recoil until I ported almost all of the gasses straight up.
Fast moving bullets in weights around 140 grain and higher, in typical lightweight to medium weight hunting rifles (not bull barrels) get the most benefit of a Muzzle Brake (MB) (Rifles chambered for example in 270, 7mm Mag up to .338 mags). The powder burn rate creates excessive gasses to get the bullet velocity up so a lighter weight bullet can do damage comparable, or more, than a slower moving heavy bullet. The MB takes advantage of the excessive gasses by expelling them in a controlled direction, and volume, with the result of less felt recoil, and less movement of the muzzle. Also longer barrels have less of a need for a MB than a short one since the bore length allows the gas volume to expand before exiting the muzzle.
I have quite a few AR-15's. Muzzle brakes are not necessary on them since the bullet caliber / weight / mass is so small (55 to 64 gr typically) and the gas operated system with the buffer spring absorbs much of the felt recoil. I have muzzle flashes on most of the AR-15's which really don't offer any measurable reduction in felt recoil.
I also have a few AR-10's (.308 cartridge) where a muzzle brake starts to become beneficial on 155 + gr. bullets with velocities at 2500 + fps.
A good example of where a MB is a dandy is on my AR-10(TU) in .300 Remington Short Action Ultra Mag. It's pushing 190 grain bullets at 2910 fps. The muzzle really likes to jump when shooting from a bench.
You can get more detailed info from the web. Also look at common designs from the Brownells catalog. The mass produced VAIS and KDF brakes are quite efficient at expelling the gas as fast as possible in all directions. They are typically sold in small calibers and must be bored out to your caliber before use.
I have also experimented in creating chambers where the ports are (widening the ID of the MB where a port exits. I actually lost accuracy doing that and found that the bullet is more accurate where there is a clean exit port. The rear of the bullet acts as a brief plug in the MB forcing the gasses to escape through the ports. I found that the accuracy of boattail bullets were not affected in the calibers I have tried.
A few tips on making a MB from scratch.
1). The MB bore must lign up with the center line of the barrel bore. This is VERY important! Often the barrel contour is not concentric with the center of the bore. I have seen the bore off center of the barrel contour by as much as .040". If you thread your muzzle to the circumference of the barrel contour, it does not mean the bore will lign up with the more concentric MB you just made on your lathe. Use a live-center in your lathe to find the center of the barrel bore and then thread the end of the barrel concentric with the bore. You can blend the O.D. of the MB body with the barrel taper/contour after you have test fired the MB and know everything works well. If you contour the MB with the barrel contour before test firing it, it makes it next to impossible to rechuck in your lathe, or indexer to machine further.
2). I recommend making the MB out of stainless steel regardless of the barrel steel. You can agressivly clean the SS MB ports without affecting the bluing finish like you would have on carbon steel. 416 SS works and machines well, but 303 SU SS machines even better and doesn't need passivation.
3). The thread quality & tolerance on the internal threads of the MB must be sharp & tight. The threads on the barrel and the MB will wear-in quickly, expecially when you are screwing the new MB on and off quite a few times to get a proper fit. This is especially important if your MB is 'timed' having certain ports facing directly up due to its design.
I usually make a 'timed' muzzle brake about 20 degrees short of it's final rotation allowing for wear, and light stretching of the threads.
4). Do not use any tools to tighten the MB onto your barrel muzzle. Your thread tolerance and hand tightening should be just fine to keep the MB tight and aligned. Using tools will stretch the threads unpredictably, causing your MB to lign up differently each time you install it.
5). Make the MB removable, but don't take it on and off too much for the same reasons as 3 & 4 above. I make the threads so tight that the brake needs to be a little warm and expanded before it will fit onto the cold barrel threads. After it cools it contracts and makes for a snugger fit.
6). Cut the threads on your barrel a common UNF thread, such as 1/2 -28 tpi. If your new home-made brake does not work, or gets lost, you can easily replace it with a mass produced MB with the same thread pitch.
7). when cutting the threads on the barrel, avoid cutting off the original factory crown. If your accuracy goes to hell after installing the MB, you can rule out the new crown you may have cut wrong.
8). After installing the MB onto the barrel, use a hole pin gage and run it through the MB into the lands of the barrel. The exit hole of the MB should have an equal distance gap around the pin gage larger than the bullet diameter (You should read about the clearance needed. I hate to give advice on that) If you made a mistake, it is easier to find out then rather than have the bullet get shaved exiting the barrel into the MB, and having bad things happen.
9). Before shooting the rifle for the first time after installing the MB, shoot it from a bench vise with a remote trigger. If there is going to be a problem it is better not to have your face too close.
10). Dip the MB in Dykem before shooting. If the bullet, or fragments strike the ID of the MB, or if fragments exit the gas ports you will see it clearly and know there is a problem with your machining. Use a target for testing. If the bullet is keyholing the target, you will also know that the bullet is hitting the MB.

Machinists and gunsmiths that make custom muzzle Brakes usually use their own design, or appearance, so they know it is their own whenever they see it.
I have a design that is my signature and it is proven. There is no other MB like it and I know it is my own work on any gun I see with it on. I also don't mass produce and sell MB's to anyone unless I install it on their firearms, or make it part of the barrel such as in handguns.
I hope this helps and I would like to see what you have come up with when you are done.

Jim Shaper
11-25-2009, 04:47 PM
YM just made my point about stainless before I got a chance to hit post. :)

My barrel comes from a small outfit out of Washington - Black Hole Weaponry. While not hammer forged for rifling, it is 416. I chose stainless for the simple reason of corrosion resistance. This is a functional gun that will see service in the woods, rain, snow, or shine. The end is pre-threaded, and the barrel has been heat treated and surface treated to matte.

Thanks for all the input guys. I can bang one of these out in about half an hour or so. So if I'm not spending 50 bucks, I'm paying myself shop rate in making it. I have lots of material on the shelf, and I've got lots of tools. This is something I want to do for more reasons than just being thrifty.

11-25-2009, 04:53 PM
Excellent write up.. You pretty much summed up what I have discovered in my experiments from the past.

I tend to use purchased brakes as it is time consuming to do one correctly. That is certainly so if you are experimenting with hole size..and postion. A person can make several just in learning the ropes. It is fun if you have the time to play with it and are willing to accept that some ideas won't fly.

I agree on the part about stainless as it does lend to better cleaning. I do suggest that when you land on a design that works you should go back and polish the machine marks out as it makes clean up easier later.


11-27-2009, 10:58 AM
Jim are sure that's 316? That would be a horrible material choice for barrel. It is soft, not hardenable, and tends to gall at the mere thought.
I think you will find that it is 416, if not rethink that purchase.

Jim Shaper
11-27-2009, 05:22 PM
Sorry, that was a typo. They're 416 and heat treated post machining.

Jim Shaper
11-28-2009, 02:14 PM
Barrel showed up today (USPS is fast right now), but it's not 16" as advertised - it's 14! :mad:

The barrel itself is really nice and fits snugly into my upper, so I'm going to keep it. I just hope they give me something to compensate how I have to pin and weld the damn thing now, but it does give me a little more latitude for brake/suppressor design without making the gun longer than I want it. The whole idea behind this build is to make a shorter "brush" gun.

11-28-2009, 03:15 PM
You have to wonder why the heck they would do that. Threaded muzzles are legal, so why force you to pin the suppressor on. I'm kind of thinking I'd send it back and tell them I want an actual 16" barrel.

One thing, where are you measuring the length from? On a rifle the chamber isn't included in the length, just the bore. On a shotgun it measures from the breechface (basically the entire barrel).

andy b.

Jim Shaper
11-28-2009, 03:35 PM
With this gun, the shorter the better. I'm not super bothered by having to pin the brake on, since it's not like I'm going to spin it off and mount a silencer anytime soon (MN is a kook liberal state that bans them for EVERYTHING - you can't even put one on a paintball marker :rolleyes: ).

The outfit that makes these barrels isn't taking any more orders. It could be months before I got a real 16" barrel in stainless, and they typically run far more than what I paid ($200 shipped).

Like I said, the whole intent of this weapon is to be short so it doesn't scrub branches when I raise it to aim. I've missed 2 deer due to hitting trees while trying to shoulder my cetme in the woods (that gun is 39" long with a 16" barrel in .308 - and weighs 12# with glass), not to mention it makes noise scrubbing it while walking through brush.

Smaller and lighter is the goal here. I'm not one of the freaks building an AR for home defense. I even used to be in the ".223 is too small for deer" camp, but I figure I can shoot well enough to place my shot to make sure it's a humane kill. I like semi-autos for the rechambering without breaking cheek weld, not because I need 30 rds to hit what I'm shooting at. ;) My shots will all be well under 300yds on the outside fringe. I could get away with a 9" barrel if it was legal (I'm not registering this thing as a SBR).

I'll run the barrel up to a gunshop I want to look at some stuff at anyway before I crown it. We'll see if it's that much shorter than a regular 16". I've been under the impression that the barrel was always measured from a closed breech for any gun. Do you have a reference to the contrary?

11-28-2009, 06:49 PM
There I go spouting my mouth off before verifying. I was thinking of the rifle 16" barrel vs. shotgun 18" barrel and not the measuring points. Yes, it is from the muzzle to the breech face for both.

"For purposes of this definition, the length of the
barrel having an integral chamber(s) on a shotgun or rifle shall be
determined by measuring the distance between the muzzle and the face of
the bolt, breech, or breech block when closed and when the shotgun or
rifle is cocked."


It still seems weird they would send you a 14" barrel assuming you were going to pin the brake on. It sounds like that will work out better for you anyway.

andy b.

Jim Shaper
11-28-2009, 06:56 PM
I ran up to the store and put it next to a 14.5" and it's pretty darn close. I haven't stuck the bolt in it and measured it properly though.

I'm not thrilled about having to pin it, but it's also not the end of the world. I think they just eyeballed them and thought they were 16's.

Later tonight I'm going to go crown it and press it into my upper. The fit is perfect. I can get it to where the alignment key is just barely below the edge of the receiver, and no more. Super nice tight fit to that point. I've verified the pin will fit the slot, so it's not just binding on that.

Jim Shaper
11-29-2009, 04:34 AM
Sure enough, it's 14.6" when the bolt is in.

Sorry the pic sucks, but here's my first ever crown job:
http://i6.photobucket.com/albums/y242/FishyJim/th_BHWCrowned.jpg (http://s6.photobucket.com/albums/y242/FishyJim/?action=view&current=BHWCrowned.jpg) (click for bigger image)

Since I'm looking at performance of the available designs, I found something of interest.
Don't blame me for spitting beverages on anything while watching this - I'm warning you in advance! :D


11-29-2009, 08:01 PM
Sure enough, it's 14.6" when the bolt is in.

Don't forget to get a shorter gas tube now!
Never mind. I was thinking of going from a 20" down to a 14.5 / 16". It was the scotch talkin'.

Jim Shaper
11-29-2009, 08:10 PM
The fore grip section is stock carbine length. If it wasn't, the grips wouldn't fit right and the barrel would be in the mail already.

I've got the whole upper assembled with exception of the front gas block (since I'm probably making one of those too) and I've verified the gas tube lines up and fits well within the bolt carrier.

I won't be able to mount a bayonet, but then again, I have no desire to either. Since I've passed the bayonet assault course at Ft Knox, I'm well aware how much work it is to beat someone up with a non-firing rifle and am certain I could find better devices should the need arise. ;) I'm also certain that need wouldn't arise while I'm out hunting bambi and coons. :D

Jim Shaper
12-13-2009, 04:20 AM

Here's what I decided to go with, because my original design is likely to get me in "federal pound me in the exit prison" should the ATF not like it. I'll draw up a print and mail it at some point. This one gets me legal for the time being.

The holes are bored straight across rather than through the center so that there's turbulence created as the gases pass around the brake. This does a couple things - it dissipates the blast, but it also should promote full consumption of the powder and thus reduce the flash. Any flash that remains will be an "X" and not cross my line of sight.

The chamber is wide at the barrel's crown, then steps down as it progresses through the brake. This should peel off most of the blast and result in a very true flight. Only the range will tell, so I'm hoping to get to shoot it in the next week.

12-13-2009, 01:07 PM
Cool-looking design. I am interested in hearing how well it functions.


I like the "it looks factory" comment. I'm not sure what "factory" this guy was referring to, but I certainly wouldn't purchase anything from that factory. :)

andy b.

Jim Shaper
12-13-2009, 02:55 PM
Mine doesn't even look factory due to the deburring of the big holes. I had it chucked in the 4 jaw and wasn't happy with how long my emery cloth was taking, so I used my angle grinder with a 2" rol-loc disk and had at it. Well, that left a lot of funky swirls in the 304. I didn't quite get all of them out, but it's not an issue since I'm going to sand blast it to make it matte and possibly paint it black after that.

The barrel is 416, but I might paint the whole works black.

Jim Shaper
12-13-2009, 08:04 PM

I'm pretty sure I'm leaving it like this. :D I really like how it looks.

Now I just need to get down to a buddy's house so I can torque the barrel and buffer tube nuts and get this sucker to the range!

12-13-2009, 08:20 PM
I'd leave it blasted. That looks cool!!!!

andy b.

Jim Shaper
12-13-2009, 08:29 PM
I really like how it looks too, but the fact that it's 416 makes me nervous that it'll rust. I'm pretty sure I blasted off the passivisation that was done, and due to the gas block being "captive" now (thanks to the stupid gov't requirement for pins and welding) - I can't repassivate it.

Jim Shaper
12-15-2009, 10:08 AM

The scope is a cheapy temporary deal, but here's the finished product in it's entirety.

I still need to torque the barrel nut, but I'm hoping to hit up a friends tonight to do that (he's got the vise blocks and such).

I also found out that the chromium oxide layer that makes stainless - stainless, will naturally repair itself if left exposed to oxygen. So I just need to keep water away from it for a while (not a problem).

Jim Shaper
12-23-2009, 10:08 PM
Scope flat out sucks, but the gun will hold MOA at 50yds even with the crappy optics.

Brake works really well as a brake, but it's still a flaming whore on cheaper ammo. :D The design does work as intended. Some of the flame is projected forward while the side flames are kept well below and to the sides of the line of sight.

Since muzzle flash seems to be limited to repeated quickly fired rounds, and not an issue on the first couple - it's gonna work just fine!

12-24-2009, 03:50 PM
i have built dozens of muzzle brakes for airguns, alot depends on the style and bore size what it will do or not do..

for example one that has severl holes will redirect the sound up down left right and so on

forward holes or forward angles holes direct all the sound toward the target while back ward holes sound comes to you,

size of holes also makes a difference on how the sound it altered, in cases it can increse the sound alot, as does how you do the crown on it. an inner what i call the speaker design will increas the sound or amplify it, you can also make other adjustment to for pitch control or make it really bark the sound and i do mean bark it out,,

you can attacch thes things either threaded on or set screws just make sure if you thread it on you r are extreamly percise on your thread cutting on the barrel and the brake same goes for the set screw holding, the boar can not be the same size it must be slightly larger then the bullet or projectile,not alot but a few thou is bestthe corwn on the brake is not all that importante but for looks a nice crown is cool ,

also your POI will change when using a muzzle brake so you will be re siting in your gun with A each style of brake used and also when you take it off your POI will also alter as well.. so again resiting in again ...

when you drill the hole for the boare is must be perfectly round so a reamer would be handy to have to get a perfect hole ID for the bore

some brake stlyles als help to reduce and or elmimate the muzzle flip almost completly to 0 ..

do not use ALUIM for muzle brakes on Regual fire arms

Brass maybe ok but Steel is best as well as SS steel

Jim Shaper
12-24-2009, 06:08 PM
While the report was much louder than I would've expected (it's about the same as my cetme), there's absolutely no concussion felt by the shooter with my brake.

The JP howitzer blasts the shooter with the gasses while pulling the gun forward - I don't need recoil elimination so much as I didn't want the muzzle lifting. Rather than compensating rise with down force, I simply evacuated the gasses laterally to keep the flash from the line of sight.

I wouldn't use brass on a firearm. It's far too soft and malleable. If you dropped it or whacked it into something, you could have a serious problem the next time you shot.

I also wouldn't make the brake's bore that close to that of the barrel - the barrel is smaller than the bullet, so your bore needs to compensate for the major diameter of the lands in the rifling, as well as any deviation from perfect concentricity. Most brake manufacturers suggest no less than .040 over.

If your brake has no internal chamber difference at the crown, the angle you cut on the crown isn't likely to make a lick of difference. All the gasses will still rush right past the bullet as soon as the diameter increases from the gas-tight seal the bullet had in the barrel. You want to make that transition as unobtrusive as possible, and for me that meant making the first bore change as big as I possibly could given the size boring bar I had which would fit in a 3/8" hole.

On my barrel, it's too short to be legal without a "short barreled rifle" tax stamp (200 fee from the ATF). I had no option of removability - it had to be pinned and welded. Even so, I wouldn't use a set screw. I also don't expect the threads to hold concentricity - that's asking for trouble. Keeping things square and straight is the best approach (IMO), and that means the shoulders need to be true and the threads will do as they do without appreciable degradation to the mounting alignment. I put a cone on the back edge of the brake so it seats centered on the end of the barrel, held by the threads, held square by the shoulder. If you really want to get crazy with concentricity, you could do your final reaming pass with the brake mounted to the barrel and the barrel mounted in the 4 jaw.