Military .30-'06 ammunition and SAAMI-standard civilian loadings will almost certainly produce similar chamber pressures which are at or below the safe "proof" pressure for any given rifle in good condition. Fine so far, but in the M1 "Garand," chamber pressure is not the problem; it's pressure at the gas port -- a couple of inches back from the muzzle -- that governs.
Let's assume two loadings, one of "slow" powder, and one of "fast" powder, neither of which exceeds the SAAMI chamber pressure standard when fired in our rifle. ("Slow" and "fast" in this context refer to the burning rates of the respective propellants.)
If the propellant (powder) has a too-slow burn rate, even if the chamber pressure is below SAAMI maximum, the pressure at the gas port will likely be too high, and may very well slam the operating rod to the rear hard enough to damage it: bend the rod or crack the piston, perhaps. It is also likely that the bolt will strike the inside of the receiver heel, which it is not supposed to do. Given enough time with what might be termed "overenthusiastic" ammunition, especially if combined with a weak operating rod spring, and the bolt can end up cracking the heel of the receiver. Yes, M1 receivers have been cracked -- a piece actually pounded loose -- under these conditions; the rifle doesn't "blow up" as that term is generally understood. (A damaged operating rod can be repaired or replaced, but a cracked receiver means that one has now become the proud owner of an unwieldy .30-caliber paperweight or a real nifty wall hanger. And no, a receiver so cracked cannot be safely welded back together; don't even think about it.)
Too fast a powder, given at-or-below-maximum allowable chamber pressure, probably won't do any damage. It's likely, however, that it won't generate enough pressure to reliably cycle the rifle's action. The action may appear to cycle, but faulty extraction and ejection or failure to load subsequent round(s) will probably occur. (It looks for all the world like a classic case of "short stroking," which is precisely what it is.) Granted, with most over-the-counter .30-'06 ammunition, low gas port pressure isn't a concern vis-a-vis physical damage, but it sure plays hob with the rifle's functioning.
Please note that neither of these phenomena are absolutely guaranteed: they may, in fact, be very rare. Can they happen? Surely. And people get run over by busses, too. Rare or not, in either case you have a nonfunctional rifle; I think you'll agree that neither condition qualifies as A Good Thing
John......I've heard others say the same thing. I guess it's one of those things where the potential is there, especially with loadings utilizing heavy bullets. A buddy of mine has a low numbered 1903 (Rock Island) that's "technically" dangerous to shoot. But, his grandfather, father, and he himself have used it with no problems. Whoever eyeballed the heat treatment on that one must've got it right because it's held together all these years.OK, I stand corrected then. I shot it all through mine, and had no issues.