Barrel thickness makes a big difference as well in heating. My savage has a thicker barrel than my Ruger, and the ruger will be effected by heating pretty quickly. I usually can get 3 to 4 shots, but then I have to let it sit 10 to 15 mins to cool completely or it will become pretty erratic. My savage can handle less cooling time. I wouldn't rule out barrel heating. Try waiting until the barrel is completely cool between groups. If it still does it, you can rule out barrel heating and move on to the next possibility.
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