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pcarpenter
01-21-2009, 03:09 PM
It will be an even better idea (the creation of this new forum) if someone makes use of it so I will ask an initial question.

I bought my first AR (an M4gery by Rock River Arms) and have a quad-rail free float tube that is to go on it. I have familiarized myself with most of the disassembly procedures, and know that I am going to need a wrench that fits the current barrel nut and one that will fit the free-float nut. I will also need a vise fixture to hold the uppper receiver under torque while removing the existing barrel nut. Some of these are clamshell types and go around the outside of the receiver. They are sort of famous for rubbing shiny spots in the bead-blasted anodized finish.

The other type is made by DPMS and is called the reciever claw fixture. It is a milled block of delrin that is cut to fit a lot of the interior cuts up inside the upper reciever, including a notch for the gas tube. It looks like the sort of thing a guy could make given some time. I have seen prices on these at around $35 and up and it seems like it might be worth making rather than buying, although I looked up a piece of Delrin from Enco and its more than that for the material so I may make do with some other material. The wrench is going to be expensive enough. To top that, everyone seems to be sold out with the latest round of "get-them-while-you-still-can-mania"

Does anyone have one of these they could roughly dimension for me? The fixture includes a pair of precisely located cross holes that are used to pin the upper to the fixture using the upper reciever hinge and retaining pin locations. I suppose I could spend some time doing some careful dimensioning just using inside dimensions on my receiver, but I would love to not miss an important feature. Its also a bit hard to come up with easy references for some of it. I figure tight fits up inside the receiver are what it depends on to prevent the pins from ripping out of their locations.

Here's a link to what I am talking about:

http://www.dpmsinc.com/store/products/?prod=1675&cat=1450

thanks in advance if anyone can help.

Paul

TECHSHOP
01-21-2009, 05:06 PM
Don't have the measurement handy, but be advised that depending on the "who, what, and when" of the manufacture, the pins are slightly different sizes (diameter) and in slightly different locations. This was done to "work around" the various bans and still have a "legal" firearm (with the built in "bonus" that you "needed" to buy a "matched" upper and lower from the same manufacturer for an easy build. "Milspec" are standardized, "sporters" are a whole another game to "gunsmith".

pcarpenter
01-21-2009, 05:16 PM
I will sure take the advise, but all the makers I am familiar with (currently) make their guns to print. I can interchange parts with (at least current) stuff from other makers. There are occasionally "tightness" issues with some and I bought a Rock River because they build theirs snug. I understand that some of the places that make "billet" (argh...don't get me started) lowers also occasionally have a pin fit issue because of the fact that radiuses are not made to exactly match the print like forgings can be. Still, I know of folks using this particular fixture with Rock River uppers (and others) with no problem so I should be OK if I could copy one.

I do also recall reading something that suggested that the older A1 vs. A2 revisions of the M16 did use a different size hinge pin, but nearly everything today is made to the A2 prints.

How would pin locations help with the post-ban (1994-2004) requirements during that time frame? There were real specific requirements like no threaded barrels (or at least no removable flash hiders), no bayonet lugs, 10 round magazine capacity etc. I just can't figure how anything about how changing pin locations or dimensions would deal with that.

Its starting to look though, like the cost of materials (and my time) may make it look more reasonable to just buy one.



Paul

JCHannum
01-21-2009, 07:24 PM
I do not know from the AR's, but as far as building jigs and fixtures, that fixture might be made from materials at hand for less than the cost of purchasing a slab of delrin and machining it. I have quite a bit of aluminum pieces for instance, and picture bolting together a T frame. The contact points could be padded with sheet plastic to protect machuned surfaces.

For casual shop use, it is not always necessary to duplicate all of the features of a fixture that will see daily service.

meho
01-21-2009, 08:23 PM
I don't like those types of fixtures. It looks like a good way to spring the reciever open.

The clam shell type fixture I have has an insert that goes inside the race to prevent possible crushing or warpage. I have never scuffed a reciever.


The way I assembled my upper like yours is that the barrel nut is tightened with the standard tool. I tightened the hand guard with a Ridged strap wrench.

I cut a 20 service rifle barrel down to 16" and made my own comp for this one.

http://www.mindspring.com/~abodom/m4play.JPG


http://www.mindspring.com/~abodom/m4comp.JPG

andy_b
01-21-2009, 08:26 PM
first, the only AR-style rifles with different pin sizes (at least on purpose) are the Colt "large pin" lowers built starting around 1994. any other upper and lower should be interchangeable. of course, i have an Olympic Arms lower that is off by a few thou and it is a pain to get the pins in. the only reason i haven't sold it is i'd feel bad for the poor sucker who bought it.

as for that receiver holder, what kind of reviews have they been getting? from the looks of it, the pin bosses are the main points taking the torque when you try to tighten or loosen the barrel nut. those bosses weren't really designed to do more than make sure the upper doesn't fly off the lower when firing. i don't think they were designed to hold the upper for assembly. those funky receiver clamps also have a piece that goes inside the receiver, like a fake bolt carrier, and you clamp the entire thing in a vise and it is almost like one solid chunk of aluminum/plastic. any idea how the manufacturers do it?

andy b.

Bguns
01-22-2009, 04:30 AM
I have NEVER had a Scuff Problem...

Lets See, Proper Anodize Finish 50 RC+, Plastic Soft Clamshell not even Rockwell C anything...

Have done a Few :) in the Military (Was My Only Job),
I see more BS from Armchair Gunsmiths, than I can Believe...

The Old Olys (and Most Off Brands) before say 2000, were interesting to work with, due to Pin Hole Locations wandering around...

Pretty much any New Lower will be fine nowadays (hmmm CNC)

Colt's Old Bogus Large Pin Design, and other factors, actually put them WAY down on my recommend to Customer List..

I see lots of Gunsmithing Tools sold, that are Dubious as to Value, or even working properly.. Right up there with Plastic Miracle Buffers for most HK Rifles (Or almost any other Gun) that already have a properly designed, Large Buffer Unit Built in...

Spring Receiver??? With a Preformed Insert? I guess a Caveman could do it....

Not that much Torque ~35 ft Lbs up to align Gas Tube hole.

GI Gunsmiths don't even get that Luxury, just clamp Barrel in Aluminum Blocks (talk about possible scuffing..) and Torque Nut...

The MacDonalds of AR's, M2 MG's, M60's, 1911's, M240's, M203 GL, 81mm Mortar, M67 90mm, 4.2 Mortar, 105mm Tank Guns, 155 Howitzers and 8 inch M110's :)

meho
01-22-2009, 07:34 AM
"Spring Receiver??? With a Preformed Insert? I guess a Caveman could do it...."

I am a knuckle dragging service rifle shooter. Anything is possible:)

pcarpenter
01-22-2009, 10:28 AM
I got the info I had on what to use and what not to use from a google search of the AR-15 site. I read the "scuffing" thing about the clamshell fixtures more than once...but they were really talking about rubbing a shiny spot as I saw. The trouble is that while anodized aluminum is an aluminum oxide process and really hard, its done on a fairly heavily bead-blasted surface which can make for an easy opportunity to knock down the "points" on the finish. On the other hand, I have found, both with the bead blasted anodizing as well as "bead" blasted steel barrels, you do well to take some fine steel wool and give a gentle rubbing all over. This does nothing to harm the finish, but does knock off the "burrs" --something that will eventually happen anyway with handling. By doing it this way, the finish smooths evenly rather than happening in high-rub locations first. It remains matte in appearance and yet feels much better. In most cases you can then use an oiled rag without leaving the gun looking like a flannel board as is usually the case:D My hunch is that the only reason this is needed is because in a production environment, they probably go overboard on the pressure and use "fractured" (old) glass beads. But I digress...

As for the design and the pins bearing the load-- Someone brought this up on ar15.com and someone else did an analysis of the lever created, and the relative strength of the forged Al receiver and the thin web created in the delrin. The receiver will definitely not fail first. More importantly, the insert parts really should bear the load if the fit is good, and the pins only serve to keep the receiver from camming up off of the fixture.

I won't fear the clamshell fixtures, but the general concensus was that this was easier to use as well as better from a marring standpoint. It turns out that a couple of the manufacturers use these in the factory, probably because you don't have to fiddle with fitting on a clamshell and then clamping the whole works in the vise. The fixture can be vise mounted first and the upper set on top and the two pins inserted. On the other hand, for the most part, this is a very occasional use item for most of us.

Jim-- you make a good point. Aluminum will "track" badly (leave silver rub marks) on bead blasted black-anodizing, but a little tape could solve that issue....and in the ideal, most contact is up inside.

Meho-- how do you like that half-quad tube? I went the full rail direction with snap-on covers for the grip area...but that makes for a real handful of foregrip.

thanks to all for the advice.
Paul

Bguns
01-22-2009, 03:30 PM
Never Had a Scuffing Problem. I do Clean Clamshell and Upper before assembling... Clamshell is Not a High Speed Factory Setup.

If YOU have/worried about a Scuffing problem. put tape in Clamshell.

If you do not have a Mfg License, get a Clamshell. All you will need.
Looking at the backlog of AR Parts right now ....

High Speed way is Clamp Barrel in Jig, Torque down Nut. Nothing to do with Upper except use its Threads. Any Minor Scuffing would be Hidden under Handguards anyway... This is what Military Does...

AR15.COMEDY has some good info, but don't take it ALL as Gospel...

Time to get something better, poor old AR Horse has been Ridden, Modified, Pimped, to Death... Where is the Folding Stock Capability, the Gas not going Directly into Action, The Quick Change Barrels/Calibers, Forward Charging Handle, Sights/Grip Closer to Bore, reliability in Cold/Sand??????? Some of these can be addressed with Addons, but Still leave important ones out...

Strange how Internet works. I can Post some AK building info, and next post will send person to a site that shows a guy building an AK with a 16 ounce Claw Hammer. Clamping Receiver directly in Checkered Vise Jaws leaving deep gouges etc, But he has a Web Page up, and Must Know what he is doing.....

Don't try this at home Folks, I do this for a Living :)

meho
01-22-2009, 07:52 PM
Hi Paul,

The half quad is fine. I got it at a deal at Camp Perry last year. They were also selling blemished lowers and uppers as well. The lowers were $80 and the flat top uppers were $65. Splotchy finish that disappeared when oiled and minor scratches.