I reload for my m1a that’s in 6.5creed and I reload for my m1 garand in 30-06. Both are hard on brass. When I need brass I wish usually by once fired hornady brass from members on hear and then fully resize and reload.
Try to find some L.C(Lake City)brass in bulk. Shouldn't be too hard to find. It's thicker and can take at least 2-3 reloading/bolt cycling beatings. A quick 3 second search found this for you ..... .308 Brass
Since you mentioned reloading .......41.5 grains of IMR-4895 pushing Sierra MKs 168's seated at mag length is the "Go To" load for M1A's. If your rifle doesn't shoot it well, you have a sick rifle. Getting an adjustable gas plug like this Schuster is also worth it's weight in gold for the longevity of your rifle.
Google "once fired brass". Diamond K, in post #7 is one of the ones that come up. Everglades is another thats reputable. As others have said, these rifles are rough on brass, so I would be looking at military cases. At one time IMI was pretty good brass.
Since you said you were new to M-14's and 308's I would suggest staying with a powder no slower than 4064. 3031, 4895, 4064 have all worked well for me.
One quick note. If you go the once-fired brass route you will probably have to remove the primer crimp. I would also recommend a small base die and a 308 case gauge to insure that the rounds will chamber. As powders go the 4895 and 4064 are the go-to powders along with BL-C(2).
M-14 / M1A's are hard on brass , and my past experences was that I could get 3 to 4 reloads that were in the FGMM loads range. 7 reloads with my cast bullet loads. One thing that helped my was using an RCBS X die. On your SB die, DO NOT force your press, it's better to take two pass's in the sizing procedure that to be shut down due to a case stuck in your die.
If you don't have your F/L die set properly to move the shoulders of your fired cases properly, neither die will work. 98% of the time, most chambering issues are due to user error.
Do you have a way to measure the datum length of your fired cases?
Only one time, I needed a small base die to take down the web of the case to .468". That was with a tight chambered Steyr SSG 69.
M-1A's have a violent extraction. This can cause the case shoulders to be a little more forward than the chamber actually is. One way to get a better measurement on your fired cases is to turn your gas spindle off and fire 5 shots and extract the cases manually. This will work fine as long you don't have a match rifle where the gas spindle has been locked to open only.
The FL die is set so it bottoms on the shell holder. I do not have a way to measure the datum length. The only measuring devise I have right now is a Sheridan Slotted case gauge that I got yesterday. The case will not go far enough into the gauge for the shoulder to contact the shoulder in the gauge by about 300 thousandths.
Just be aware that one of the downsides of using a drop in case gauge like the Sheridan, Wilson Dillon and others, is a semi auto rifle can bend the rim of the case so it will not fit into the gauge and sit proud, giving you a false reading. Make sure to try a few cases.
For your M-1A, your cases should be sized so the rims of the case sits flush with your Sheridan gauge. I reload for a couple of M1-A's, a 3 digit serial # Devine Tex. gun I bought in 1974 and a Scout Squad.