Believe it or not, I agree with your post. The .300 Win Mag can be a frustrating rifle to shoot. Loaded with a round it likes, it can outshoot almost any other rifle, IMHO. Out of the box, and with factory ammo, it can cause gray hairs.
My M1A, though--now, that's a different story. I tried a recent range experiment: First loads were as follows:
Federal Gold medal factory new cases,
Federal LR Match primers
175 grain MatchKing
40.0 of IMR 4895
Cases deburred, pockets uniformed, neck turned, etc, etc, etc.
Each charge handweighed
Each bullet weighed.
Second lot:
150 grain mil surp pulldown bullets, complete with leftover tar and sealant (eeewwww)
42.0 IMR 4895, measure thrown
Federal Match primers
Different cases from the brass bin
Uniformly crimped to h-e-double toothpicks with the Lee Factory Crimp die.
This was done to see just how much of a difference all the accuracy steps made.
Rifle: SA Nat'l Match M1A (old model) with Kahles 3-12x56 scope mounted
What happened?
The MatchKings shot like doo-doo! Patterns at 100 yards never fell below 2 inches.
The mil-surp, throw together, bottom of the brass barrel plinkers?
Groups comparable in size to the .300--five groups fired, ALL under one-half inch. Two screamers--.250 and .298, respectively. Rounds literally stacked on top of each other.
So, again I agree--the M1A is a much more forgiving rifle to shoot with standard ammo loaded to factory length.
(Doesn't kick the bejeesus out of you on the bench, either......
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"Be not afraid of any man, no matter what his size;
When trouble rises, call on me and I will equalize."