I had a frustrating experience with my Dan Wesson PM9 jamming. Finally did a drop check of 300 rounds using the barrel and found that ~50 were slightly oversized. Figuring that I should be checking them the right way, I bought a Wilson chamber gauge, then for fun checked all the bad rounds again. Every single one of them fit the gauge just fine Apparently the chamber on the PM9 slightly intrudes into the acceptable size range for rounds (meaning that the chamber is slightly undersized, assuming of course that the Wilson chamber gauge is accurate.) An undersized chamber might be great for accuracy, but it's a pain because a given brand of 9 mm ammo - even if it's within spec - won't necessarily fit...
So my point is, using anything other than the barrel for a gauge is no guaranty that the rounds will work flawlessly. Even if the chamber gauge is exactly accurate, it won't help you if the pistol camber is undersized.