My advice for reloading for M1a’s is to
1. Full length resize in a small base die
2. Trim cases
3. Clean primer pockets, ream to depth
4. Prime all cases by hand, verify that all primers are below the case head, and use the least sensitive primers you can find.
5. Use IMR4895/AA2495/H4895 powders.
6. Seat the bullets to magazine depth, no longer than 2.8 for the 308, shorter is fine.
The M1a has a free floating firing pin, like this M1 carbine. The M1 Carbine and the M1a have a firing pin retraction cam. This cam pulls the firing pin back during extraction. It also has limited utility as a safety device, but is easily defeated by tight or long rounds. If you sized your round smaller than the chamber the odds would will be in your favor to have an in battery slamfire if you happen to have an overly sensitive primer on that case.
Only at final cam down is the firing pin retracted. Up to then the firing pin is totally free floating and tapping the heck out of the primer.
This is a M1 Carbine firing pin retraction cam.
This is the M1 Garand firing pin retraction cam they are functionally identical, just the carbine is easier to visually understand.
This is a M1 Garand receiver and the firing pin is fully forward and just touching the firing pin retraction cam. As you can see there is only thousand's of an inch of forward movement left in bolt cam down and yet the firing pin is out about 0.064" of the bolt face.
This is the location where out of battery slamfires occur.
If the bolt has to stop here to crunch fit a long case or a fat case that firing pin is rebounding off the back of the primer at its highest velocity and the lugs are not engaged. There are many of out of battery slamfire reports in these mechanisms.
That is why it is important to small base size cases used in these rifles and to set up the dies with a case gage and size to gage minimum. (Assuming you don’t know the headspace of your chamber, if you do, always be 0.002 to 0.003” less) You want the bolt to close without resistance. This will reduce the risk of an out of battery slamfire.
I recently bought a set of reamer cut gages by Sheridan Engineering. I love gadgets like this.
These case gauges are dimensionally correct for a SAAMI chamber. You can drop a sized round or a loaded round in the thing and see if there is an interference fit somewhere. The Wilson gauges only measure base to shoulder distance, they are cut large in the middle. The Wilson gage won't tell you if the round is fat. The Sheridan gauges will.
There are some who say small base dies are not needed in these rifles, the Gunwriter Mike Venturino has been one. For years he has been saying in print that only standard sizing dies are need. But in the July 2012 issue of Guns Magazine, he is testing an M1a and a AR10 and his reloads are too tight. I find it humorous to read of him beating the bolts open with scrap lumber. Ha, Ha.
If you attempt to small base size with a spray on lube you will stick the case in the die. I recommend RCBS water soluble or Imperial Sizing wax. These are excellent lubes.
For these rifles it is safety critical to ensure that all primers are below the case head. Reaming primer pockets to depth is a good idea. Seat the primers by hand, and verify that all of the primers are below the case head. There is a chance that a cocked primer, with the anvil firmly seated on something, will cause a primer initiated slamfire. One poster swaged his primer pockets, which shaved brass donuts into the pocket. He left the donuts in the pocket, which resulted in high primers, and his AR10 slamfired in battery. Clean those pockets! A high primer can cause a slamfire but only if the anvil is firmly seated. High primers are one of the most common cause of misfires because the primer won't fire unless the anvil is seated and is pushed up into the primer cake.
http://www.shootingtimes.com/2011/01/04/ammunition_st_mamotaip_200909/ However, given a shallow pocket it is theoretically possible that high primers could slamfire, given debris in the pocket, you can get a slamfire.
When I bought my SuperMatch M1a’s, Springfield Armory provided a copy of Wayne Fattz’s article “The Mysterious Slamfire” which was printed in the American Rifleman in Oct 1983.
http://www.scribd.com/doc/2649554/Th...ious-Slamfire-
Mr. Faatz had an out of battery slamfire, from the clip, with Federal primers. He wrote an article trying to understand why it happened to him.
First on his list of slamfire causes is a sensitive primer.
1. Sensitive Primer (included under this category is a high primer)
2. Minimum headspace chamber and inadequate case sizing (leading to the firing pin hitting the primer with excessive force)
3. Hammer following the bolt
4. Fouled bolt face
Any service rifle that has a free floating firing pin can slamfire at any time if improper ammunition is used. The use of commercial factory ammunition or reloads that use soft or more sensitive primers often create slamfires.
Because SKS’s slamfire so often, there are lots of slamfire reports with SKS’s, Murray’s has a firing pin modification to reduce the chance of slamfires. Also, the SKS boards has this excellent “A primer on primers”
http://www.sksboards.com/smf/index.php?topic=56422.0
Springfield Armory M1A Manual, page 4
http://www.springfield-armory.com/do...=M1AManual.pdf.
Ammunition
The M1A is designed and built to specifications to shoot standard factory military 7.62 NATO ammunition. The specifications for standard military ammunition include harder primers to withstand the slight indentation from the firing pin when the bolt chambers a cartridge. This slight indentation is normal. The use of civilian ammunition with more sensitive primers or hand loads with commercial primers and/or improperly seated primers increase the risk of primer detonation when the bolt slams forward. This unexpected "slam fire" can occur even if the trigger is not being pulled and if the safety is on. Use of military specification ammunition will help avoid this.
Every shooter should use extreme caution when loading this or any other firearm. See page 17 for instructions on proper loading to help avoid a "slam fire". Also see enclosed article on “Slam Fire” written by Wayne Faatz
Federal primers are the most sensitive primer on the market and the most "slamfiring" primer in Garands/M1a's. I have lots of web accounts of slamfires with Federal primers. Don’t use them. I recommend CCI #34's and Tula7.62 primers as they considered "Mil Spec" primers. Which means they are less sensitive than commercial primers, federal being the most sensitive commercial primer on the market
When firing single shot, load from the magazine. Do not put a round in the chamber and drop the bolt. Lots of inbattery slamfires, and a few out of battery, have happened because of this. You want to slow the bolt down. When rounds feed from the magazine the friction between cartridges slows the bolt a bit.
Use powders that are close to IMR 4895 in burning rate or just use IMR 4895. High port pressures are the concern. High pressures will create excessive operating rod acceleration. The NRA used to recommend powders that have burning rates between IMR3031 and IMR4320. This includes powders such as IMR 4064, and Winchester 748. I believe the first choice of powders are IMR 4985, H4895, AA2495. In the 308 I have used a load of 168 grain Sierra Match, 41.0 to 41.5 grains IMR4895, LC cases, and CCI #34 primers OAL 2.800 for years.
Check cartridges for case head separation. Gas guns are hard on brass: the bolt unlocks while there is still significant chamber pressure. Because of this the case gets stretched on extraction. Carefully inspect cases for stretch ring marks at five reloads. They occur about .4” of an inch ahead of the base. You can verify if the cases are internally necking by inserting a bent paperclip in the case, and feeling for an edge. A number of shooters I have asked claim various case lives in the M1 or M1A. Some have case head separations about the fifth reload, others have taken their cases up to ten reloads. The useful lifetime of a case is determined by case head separations, case neck splitting, or primer pocket enlargement. When any one of these failure mechanisms happens to a case, it has exceeded its operational lifetime. In my experience, US military brass holds up better than commercial cases. But this is a broad generalization. You want to use heavy cases over light cases.
