As to using GI mags with JHPs, yes I've tried them.  Sometimes they work fine.  When they don't the problem can be the shortness of some loads not being able to properly bounce off the hood and into the chamber.  This usually happens with target SWCs, though.  Today's JHPs tend to be round-nosed and mimic ball ammo in shape, however, unlike earlier JHPs which tended to be SWC in shape.  Remember the old 1911 saying, the closer your load resembles ball ammo the more reliable it will be?  I believe JHP ammo makers took that to heart.  That being the case, a late release isn't a problem like it can be with short and stubby SWC target loads.

It isn't so much the width of throating, in my opinion, it's the irregularity of the bullet shape bouncing off the underside of the barrel hood and then the into the throat.  If they get a late release (via GI mag) they are less likely to smoothly transition from bounce to chamber.  Russ Carniak was smart to bevel that edge of the hood, this is custom touch too often ignored.  Then, throating about halfway between I and what was shown in the photo at the start of this thread is probably enough to insure reliability with most loads.