Yeh, what Bob said.
Generally this will start appearing in rounds such as the 6mm-06 AI or 257 STW or something like that. The sharper the shoulder the more this will tend to happen. It is not common in the 257 STW but I have seen it in three of the rifles I have built for customers and my personal rifle slightly.
My 257 Allen Mag is terrible for this. You simply can not shoot any loads with any stick powders, every one will bridge and cause severe pressure spikes. If you use ball powders problem is solved totally.
Generally neck diameter needs to be 6.5mm or less. My 270 am does not seem to have a problem as the neck diameter is large enough to allow the stock powders to pass well enough to prevent bridging.
The worst powders are IMR with their very long granulars. The large Hodgdons are next I would say with the RL line being third with their slightly finer granular size.
The new short cut powders help alot and the newer ball powders are a god sent for my AMs, espeically the 257 AM and AX as well as the 6.5mm AM and AX.
There is really nothing you can do to prevent this in a case where it occurs. I have noticed that heavy for caliber bullets tends to help limit this to some degree. Probably because it causes a better in case burn then the lighter bullets.
Like I said, its not common in the 257 STW but it does happen enough to be aware of it.
Good Shooting!!
Kirby Allen(50)