About a week ago I took my Contender in .17HMR to the range. I bought this set up brand new and put a UltraDot Pan-A-V Reflex Red Dot Sight on it. Sighted it in at 100 yards and for ever after was able to put most shots in a 2" square.
So all of a sudden my shots are all over the southwest corner of the target. Now this could certainly be shooter error.
The real problem, which has me baffled, is, every second or third (sometimes more) shot the case cracks from the mouth about 1/4 to 1/3 of the way down. I also noticed the shoulder of the case is expanded forward a miniscule but noticeable amount, almost as if the chamber is suddenly too long.
Anyone ever seen this before. Any thoughts as to a cause.
I'm gonna' have my local GS look at it for sure.

Jeff