It sounds to me that your barrel is "walking" as it heats up. This is VERY common among factory barrels and especially on lighter barrels. You COULD cure this by sending it off to be stress relieved by cyrogenically freezing it. I've "cured" several barrelsby doingthis. Just don't fall for all the claims of vastly increased accuracy,being easier to clean,less fouling,etc. The ONLY thing I've been able to determine for SURE is it stops barrels from walking as they heat up(to reasonable degree). For a barrel to be easier to clean and produce less fouling,you'll need to have it lapped. If it was me,and since it only does that after several shots,I'd be content to leave it as is unless you have a particular reason to want to shoot it a lot fast,like when prarie dog town.