I think it's been covered but it's also been drowned out in the noise of other posts, so I'll say it again.
It's unlikely anything is wrong with the rifle, other than inadequate lubrication.
The M1 Garand, M1 Carbine, M1/M1A and Ruger Mini 14, all use a very similar rotating bolt design that relies on an L shaped firing pin and a bridge in the receiver to prevent out of battery firing.
Simply put until the bolt rotates all the way into battery and allows the arm on the end of that firing pin to clear the receiver bridge, the firing pin is physically blocked from going far enough forward to contact the primer. Additionally in the M1A there is a camming surface on the back of the bolt that prevents the hammer from contacting the firing pin until the bolt has rotated fully into battery.
A few things can prevent full bolt rotation into battery from occurring:
1) Inadequate lubrication. The general rules on any of the above mentioned firearms are:
- if it rotates, oil it;
- if it slides grease it; and as a catch all
- if it's shiny, grease it.
It ain't a Glock. Run it wet, and grease *all* the shiny spots: inside the receiver; in the slide track; on the slide; on the bottom of the barrel; on the bolt; on the op rod guide and spring; and on the hammer. Too much is better than not enough.
2) A fouled chamber or out of spec case, that doesn't allow the cartridge to fully enter the chamber and allow bolt to rotate all the the way into battery. This is usually obvious on careful inspection before you try to eject the case, and if it's sticky on ejection that's another good indication of a problem.
3) In rare cases there can be some tolerance issues on commercial M1As that may prevent the firing pin from clearing the bridge.
If and only if you have verified 1 & 2 are not the issue, contact SA customer service and let them know you've excluded the first two issues.
But given it runs fine for a couple magazines before you have issues it's far more likely to be 1 or 2.
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I'm not normally a big fan of chrome lined barrels unless you live in a swamp. But one major advantage of a chrome lined chamber is that the surface is very hard and any crud in the chamber is more likely to be mashed into the brass and ejected rather than left in the chamber. That has a clear advantage in reliability in an M1A being run with a dirtier powder.
Unfortunately replacement barrels are usually shipped with a .010" short chamber that is not chrome plated. It's then easily head spaced with a chamber reamer. If you order a chrome lined chamber, it will come fully reamed and chromed and the barrel that is properly timed and headspaced, but only on a correctly spec'd receiver.
