Today I tested a new bullet- powder- gun combo. 115gr FMJ (9mm) and AA#5 5.6gr out of an 8.3 inch barrel. I loaded up all 25 rounds in the ETS Glock mag and shot them over a chrono without much time between shots. I had a camera rolling on the chrono so I wouldn't have to stop to write anything down. Well, the first 10 or so rounds were very close together in velocity, but then they tended upward, and the last 10 or so ended up close together too, but about 35 FPS higher than the first group. What can explain this? Without having looked anything up, I think it must be either a hotter chamber causing the powder to burn faster, more completely, or the inside bore diameter changing due to expansion. Your thoughts?