Gunsmithing BRASS SHOWING OVERPRESSURE

[cmike41]

I have built an FAL using two different new take off remington 243 barrels. With either barrel the brass is showing over pressure. The barrels were checked with headspace gauges one would close on .631 and the other .632 which is in spec. I thought something was wrong with the first barrel so I ordered another, but have the same problem.The primers show flattened and the brass is stretched. It is built on an entreprise receiver.(for what that is worth). The gun functioned fine but the brass shows over pressure. Has anyone here had this problem or have a solution? Pictures at:
https://sites.google.com/site/cmike1...mike1941-brass
More info- I chambered new rounds to see if they touched the shoulder or the rifling The ammo is Winchester 100 gr and 80 gr. When I chambered the rounds two of them chambered with no resistance and one with slight. Two of the rounds have rings where they touch the shoulder. Also I made a .004 steel shim for the bolt (with a firing pin hole) and fired the gun. That would make it .003 under min headspace.It took slight pressure to close the bolt. After firing the case still grew about .006 near the base, but the gain on case length was less. The primer looked less flat, but the center was of course shaped like the shim. I am testing as a single shot since this test barrel is not drilled for the gas port yet.

 

[E. Bryant]

Re: BRASS SHOWING OVERPRESSURE

Your link did not copy over properly, so I cannot view your pictures.

Without any additional information, troubleshooting this is a bit of a crapshoot. However, I can say that "flattened primers" are not at all unusual with rifle rounds. Keep in mind that we are talking about pretty substantial peak pressure (60,000 PSI). If you are seeing cratering (flow of the primer into the firing pin hole) or signs of brass flow into the ejector, then we can start making assumptions about excessive pressure.

Also understand that the FAL is hell on brass! Since there is nothing to stop the bolt and bolt carrier from moving forward after chambering a round, the brass will get shorter by several thousandths because it has to absorb a substantial amount of energy. When the round is fired, the flexible nature of the rear-lockup system means that the headspace can grow by a few thou beyond its nominal measurement. This combination, obviously, is pretty abusive - but consider that the FAL is a battle rifle, intended to run on military ammunition that is loaded in thick-walled brass and fired a single time.

 

[cmike41]

Re: BRASS SHOWING OVERPRESSURE

Try this for pics:
https://sites.google.com/site/cmike1941/https-sites-google-com-site-cmike1941-brass

 

[E. Bryant]

Re: BRASS SHOWING OVERPRESSURE

What you are seeing is completely normal in my experience with factory ammunition and factory chambers. There is a substantial amount of brass working due to the fact that factory ammo is smaller than the SAAMI maximum cartridge dimensions, and factory chambers are often a bit larger than the minimum chamber dimensions. That adds several thou of clearance - necessary to guarantee function under all conditions, but certainly not advantageous to brass life.

Here are the maximum cartridge and minimum chamber dimensions:

http://www.saami.org/pubresources/cc_drawings/Rifle/243%20Winchester.pdf

You'll note that the drawings call for a minimum of ~0.001" difference in diameter, but in reality, the gap is often much larger.

Do you have another other factory-barreled rifles that would allow you to perform the same measurements?

 

[Greg Langelius *]

Re: BRASS SHOWING OVERPRESSURE

Just make certain the case mouth is not jamming beyond the actual chamber neck area. This will spike pressures, usually dangerously, on what seems to be an otherwise benign load.