Yes, as fiasconva asks, it was all the same ammo (Remington Thunderbolt). I'll clean it, lube it, and take it back to the range and try the three other ammo brands I have. (I actually thought that might be the problem, but I wanted to get some input--maybe this was a known problem, with a known solution. If I still have problems I'll try your suggestion Viking Dude. My thanks to both of you.