View Full Version : Gun Smithing Question

02-06-2008, 08:07 AM
For all who work with guns and their repair, where can I find a reference that lists the dimensions of the chamber of the barrel? Cartridge dimensions can be found in a reloading manual but I would like to get the dimensions used to cut the chambers for 7.62x51mm (.308 Win.) and 7.92x57mm (8mm Mauser). Appreciate any help.


02-06-2008, 08:32 AM
Due to the extreme critical nature of "Head-space". Cambering is normally done with a chamber reamer and GO/NOGO gauges. If you want to get these try Brownells:
(PS the tech folks there are great, call them and they are always ready to help....)

02-06-2008, 08:32 AM

Contact some of the companies that make reamers and they can provide that info. Clymer or Pacific Tool and Gauge Co. are a couple.


02-06-2008, 09:17 AM
The tool companies can provide you with the tolerances on the reamers and gauges that they make, but SAAMI (Sporting Arms & Ammunition Manufacturer's Institute) is "the" word on official dimensions. http://www.saami.org/ They publish both min & max chamber and ammunition dimensions, but remember that these are voluntary standards. For a great discussion on headspace/headspace gauges go to the Brownells site at http://tinyurl.com/2g7unt for Part 1 and http://tinyurl.com/2fjyzk for Part 2.

Montezuma, IA

02-06-2008, 10:07 AM
Although it is slightly dated at this point, the Roy F. Dunlap book "Gunsmithing" has chamber dimensions for many popular cartridges including the .308 and 8mm Mauser.

It is also an excellent book that gives much valuable information. I recommend it for any potential gunsmith's library. It is still in print, available from Brownell's and many other sources.

Al Messer
02-06-2008, 02:33 PM
The NRA's Reloading Manual has all the info you need about chamber dimensions and ammo dimensions. Also, Roy Dunlap's "Gunsmithing" has chamber dimensions in it

Sorry JC, I started typing before I read all the posts!!

02-06-2008, 06:22 PM
The NRA book is out of print, and Dunlap's book is ca. 1960 or so. Lots of new cartridges, and many old ones that were not in his book. JGS reamers publishes prints of their tools, only source I know of that will give you both chamber and cartridge specs for all current cartridges is SAAMI. If memory serves, 7.62x51 Nato and .308 Winchester are slightly different. 8mm Mauser drawings will be of the current spec., and may not match a specific old Mauser made either before SAAMI or CIPS (its European equivalent) dimensions were standardized.

Montezuma, IA

02-06-2008, 09:45 PM
Hi Bill,
The Datum is .400" and the minimum headspace is 1.630"
The reference book is "Gunsmith Kinks" by Bob Brownnell.
Page 178. The techniques used are in it also.
The 8mm is Datum .375" and minimum headspace is 1.896"

02-06-2008, 09:54 PM
Heres a start. I didnt read much of it. I had it bookmarked so when I had time, I could read it. No pictures though. Looks like some dimensions in there at the end. Double check these against something you know, be careful, dont blame me, boom....was that my hand? - etc etc etc.........



02-06-2008, 10:13 PM
Roy F. Dunlap, Gunsmithing, 2nd ed. Stackpole 1963.

Page 564, 8mm Mauser. (http://mysite.verizon.net/vzeehv3p/8mm.gif)

Page 733, .308 Winchester. (http://mysite.verizon.net/vzeehv3p/308.gif)

Very common book. Here are the libraries which are known to have it.

I haven't found SAAMI too useful, myself, perhaps because it's weak on my own category of interest, European military calibers.

02-07-2008, 12:15 AM
Dave Kiff at Pacific Tool & Gauge has a book of reamer prints with over 400 chambers listed, each of which has all the critical dimensions listed. It's priced at around $50.

There are usually several variations on each cartridge (eg. - '95 Palma Team reamer for the 308W), so if you're thinking of ordering a reamer, or even renting one, it's essential that you tell whomever is supplying the reamer what you're going to use it for (hunting, target shooting & what bullets you intend to use).

02-07-2008, 06:12 AM
My advice is don't try doing it without a chamber reamer,Buy the rougher and finish if you plan on doing several, With a one time chamber job a finish is ok.

02-07-2008, 07:19 AM
This site is an excellent source for information. I'm not cutting a rifle barrel chamber but going to make a subcaliber (.22LR) device and I wanted to make the "extension" on the .22 caliber barrel match the chamber and put a nut on the muzzle end to hold the subcaliber barrel device in place. Thanks for the help.


02-07-2008, 10:11 AM
This site is an excellent source for information. I'm not cutting a rifle barrel chamber but going to make a subcaliber (.22LR) device and I wanted to make the "extension" on the .22 caliber barrel match the chamber and put a nut on the muzzle end to hold the subcaliber barrel device in place. Thanks for the help.


Get some CerroSafe from Brownell's and read up how to use it. A reamer print may give you certain dimensions, but there is no guarantee that your chamber was cut to those dimensions. The only way to truly find out what you have is to make a chamber casting. Cerrosafe is easy to use and if measurements are taken within the proper time frame, they will be very accurate.

In re-reading this, I would suggest that all you need to know is the shoulder angle of the chamber. All other dimensions can be a couple thou undersize because when you draw up the muzzle nut, the shoulder taper is going to center the device. You might want to try to get the case head area closer to chamber diameter to provide support, but you run the risk of jamming it in there if it is too big and the shoulder can't bottom out.


Rusty Marlin
02-07-2008, 10:32 AM
cool idea.
How do you plan to handle the offset firing pin? With a threaded base that threads in over the rim? as the nut is going to hold the chamber and barrel sleave from the front, I assume this is going in a break open gun as reaching into the action of a bolt gun would be a mother to load and unload as a single shot and get an offset breech in and out too.

The usual way of using a sub caliber is to use a c'fire pistol cart with the same bullet diameter (or close enough) as the rifle. In 8mm any of the .32's would work. This way you can load and unload the chamber reducer just like a standard cart, then use a stick to push out the spent case.

02-07-2008, 10:55 AM
Should have explained what you were doing up front...would have saved a lot of typing.

.22LR chambers are straight walled (completely) as I recall. Get out your small-hole gauges and measure...nothing like having real world numbers....especially since even the SAAMI specs supposedly have a lot of "range".


02-07-2008, 11:28 AM
I hope he is putting it in a T/C, That would take care of the F/P problem.

02-07-2008, 11:21 PM
I know that the project I have taken on will require A LOT of work. I am going to try to make a .22LR conversion kit for my MG42. I'm still in the data gathering and thinking mode so have a long way to go before I have anything to show. Since the feed mechanism uses full size cases, I'm thinking that I will have to use "full size sub-caliber cartridges." These are like the .30-'06 cases used in the .22LR convsersion of the M1917 Browning. These are full size case into which a .22LR cartridge is inserted and they act like a mini-chamber for the round when it is fired in the sub-caliber barrel. This way the feed mechanism in the MG42 can be used as is. Still working on the how "full size sub-caliber cartridge" will interface with the sub-caliber barrel. The striker in the end of the "full size sub-caliber cartridge" will take care of the center fire, rim fire offset problem. I may have to go to .22LR Magnums if there isn't sufficient energy to recoil the bolt and operate the cartridge feed mechanism. But that is down the road a bit. My thinking at this point is to use the double recoil guide rods as in the M3A1 Grease Gun as the means to guide the bolt so it doesn't have to ride on the sides of the receiver, losing energy to friction along the way. But at this point is it just a thought.


03-07-2008, 01:11 AM
I saw your MG42 at Cabin Fever. Interesting idea, but I think you will need to "totally" redesign to create a "functional" conversion. IIRC the barrels of the MG42 has a short extension into which the bolt engages for locking, and the barrel returns to its forward position after recoil, independently of the bolt. I don't think the WWII (or at least many of them) have the "bolt catch" of post war guns. At the instant of firing the "bolt catch" acts, by inertia, against the firing-pin holder to prevent premature unlocking. I think that during WWII there were four possible "assembly" of bolt parts, but only three were "authorized", but even then the resulting "assembly" might not function properly in "any gun". Add in all the post-war production and developments, most of which are "compatible" in the "assembly" but not "functional"...

The belt feed mechanism in the hinged receiver cover, both pawls alternately move the belt, "half" on recoil and "half" on the return to "battery". Require a "good bit" of energy to operate.

Don't let me stop you, but most belt feed machine guns are fairly "tight" designs with regards to ammuntion and functionality.