m4tactical,
Brass is annealed from the start. This allows the necks to be formed with collapsing the case wall. Over the life of the brass, the side walls will become thinner from the multitudes of fire forming stretch, and at some given point, the case will split (normally at either the mouth, or at the web from being set back too many times). Now when referring to brass, the more that it is shot, cleaned, and resized, it actual becomes more brittle (solvents play a big role).
The fact that we are talking about a case that has only been sized and not fired yet, in order for the brass metal to even begin to flow/stretch from contact heat, you would have a barrel/ heat on the verge of cooking off the round.
As I posted above, most likely his problem is that the brass being either sized or crimped wrongly causing the problem. Now granted that he may have fouled the chamber to the point that the cases may have been sticking (lack of lube), but my guess since he was shooting for groups, he never allowed the either the chamber, or the barrel to foul to this point.
Also, when considering brass, unless the cases have been fired threw a M-249 with a way worn out chamber that causes the brass to only last a few reloading, the primer pocket is the first to go. Since the question did not come up regarding either bright rings at the base/web, splitting necks, nor blown primers, this leads to me believe that his brass is still in it's useful life, and should still be good for several more reloadings before it needs to be trashed.
Bottom line is that some brass is better than others, and the deciding factor is the softness/ integrity of the brass from the start. The factor regarding this is determined not by the case sidewalls, but the rim it's self. If the brass is not up to par, then on the first firing, the extractor is going to tear the hell out of the rim. Granted that the this may be due to the case over expanding (loosing it's elasticity during normal working pressures), resulting in it plasma forming to the side chamber walls, leading to the rim torn extraction problem, but this problem shows it self in the first firing of the brass. To sum it up, his brass could have been defective/worn, which would cause an after ignition problem, but not when the round is only setting in the chamber waiting to be fired.
Note: When firing for groups, you don’t even begin to get the barrel to any considerable amount of heat the can either cause excessive throat erosion, nor to the extreme of ammo cook off concerns. Long before these problems, the barrel is going to start to over whip, and destroy the relationship of the barrel harmonics and the load tuned to correspond to it.
P.S. Feel free to respond. Often the best advice given here is when we as a group take point/counter point stands, and go well beyond the initial discussion, to expand the subject to the multitudes of aspects of not only the rifle, but the ammo that can lead to the original problem.