I'll build on this a bit.

Powders for the M1A and M14

The M1A/M14 gas system isn't as sensitive to gas port pressure as an M1 Garand, where slower burning powders can bend the operating rod, but there is still a fairly narrow range that will produce reliably cycling without beating up the tappet or operating rod.

M59 Ball, M80 ball, M62 Tracer and M276 Tracer (dim) all used WC 846 powder with their 146-150 gr projectiles.

M60 HPT and M61 AP used IMR 4475.

M852 Match ammo used IMR 4895 with the 168 gr SMK.

M118 Match used IMR 4895 and the later M118 Special Ball used WC 846 under a 173 gr FMJBT.

BLC(2) is essentially the canister grade equivalent of WC 846, while H335 is essentially the canister grade equivalent of WC844. Bear in mind however that WC 844 is just one end of what used to be a broader specification for WC 846, so the original M59 loadings encompassed the whole spectrum with the powder charge adjusted accordingly to develop the desired pressure and velocity. That's why you need to take any nominal loadings for a military cartridge with a large grain of salt.

Even with canister powders 42 grains of BLC(2) from one lot is not necessarily going to develop the same pressure and velocity as BLC(2) from another lot.

If look at powder burn rate chart the range between BLC(2) and Reloader 15 is where you probably want to stay with an M1A.

My preference was always IMR-4064 with my M1A match rifle and that was reflected by most of the M1A match shooters I knew back in the day.

Brass issues

The 7.62x51 chamber is .013" longer than the .308 Win chamber. You do not need this much excess headspace if your rifle is clean and well maintained, and I order my service match barrels with a .308 chamber, in the USMC fashion. I then use a small base sizing die to bring the brass back to new dimensions.

If you have a 7.62mm NATO chamber you will get significant stretch on firing new brass or ammunition in the chamber, and if you set the shoulder all the way back to the new condition, you'll get that stretch each time you fire the cartridge. However, if you don't set the shoulder back far enough it won't feed reliably. If you don't also reduce the base diameter enough, it also won't feed, so full length resizing or even small base resizing to new dimensions are the only valid options - and that means short case life.

That initial stretch is one of the reasons military brass is thicker (the other is to survive the more energetic extraction forces on some full auto weapons - the M60 and M14 in particular. But it's not thick enough to endure repeated stretching in the .013" longer chamber, so case life is seriously limited in an M1A or M14 - about 3 reloads and 4 firings in total.

The HK 91 and similar clones use a delayed recoil roller locking system that has rollers that are designed for very narrow recoil parameters. If you shoot a load with excessive recoil, the bolt will open while the chamber pressure is still too high, and the brass will both take an incredible beating, and be ejected about 25 yards. In that condition, the striations on the brass will be so deep and the brass so deformed that reloading is very difficult if not impossible. If your ejection is that strong, your recoil impulse is way too strong for the locking system.

This means that while the HK 91 is not powder sensitive, it is very load sensitive and the total recoil impulse has to match the engineering of the rollers. It also means that brass like in the HK 91 is also very short.

Primer pockets

You do need the primer to fully seat so that it is not standing proud above the base of the cartridge, but again I've never seen any need to clean primer pockets to accomplish this. In fact, tumbling media stuck in the flash hole is far more likely to create a primer or flash hole issue.

You will however need to remove the primer pocket crimp from any non-match brass as this will interfere with seating the primer. I prefer to swage the crimp rather than a cutter to remove metal from the cartridge head and I use a primer pocket swage from RCBS.