Of the 3 tight chamber barrels I have had (currently have 2) utilizing minimal freebore, all have generated excessive pressures. For example, my current 308 has a tight chamber (tight meaning a factory case will fit but there is no room for much if any real expansion) and has a throat short enough to seat AMAX's to the lands at near SAAMI OACL depths. Shooting factory Federal GMM 168 SMK's, they run at just under 2800 FPS. Published is 2600 FPS. The previous 308 I had utilized a slightly larger chamber and lots of freebore. The same loads ran just over 2600 FPS. The 300 RUM's I was using showed the exact same thing EXCEPT with 100 grains of powder, the pressures were much more dramatically high. In short I have NEVER been able to get close to factory published charge weights in any of my custom barrels and maintain normal velocity levels. I have reached normal velocities and even higher than normal velocities. Just never with published charge weights. For example, published charge weights for the 300 RUM are about 100 grains of RETUMBO using a 180 class bullet in the 300 RUM. Published velocity is around 3300 FPS. For me to get 3300 FPS in both my RUMs took 90 grains of powder. 100 grains would have posed a serious problem. I made the mistake of starting with 95 grains and destroyed my extractor on the very first shot. I am shocked that in all the custom barrels you say you have had that you have never experienced this.
My opinion is that when a case has no room to expand and the bullet engages the lands sooner than later, high pressures WILL result. Offer the case a bit of room to expand and the bullet to get a running start and the pressures fall. Hence my comments about using factory ammo in a custom stick with tight tolerences.
Take it or leave it, these are my experiences with 'match' chambers and short throats. Short meaning getting the bullets to engage the lands and have them near standard OACL.
If I were the only guy I knew of that had experienced this (or if all the rifles came from the same smith) I wouldnt neccesarily take it at face value. I know of plenty of other caces that this has happened. I know one case where he was using a snug chamber and minimal freebore. When he shot one round down the pipe using factory 300 WSM loads, the smith had to turn the barrel off of the action to get the case releived so it could be removed. I guess what I am saying is that if you want to shoot factory ammo in a custom stick, pay attention to chammber and throat demensions. Just because factory ammo will fit and fit smoothly, doesnt meen that it will be safe.