try a heavier bullet, different powder, and try shoot'n from a strapped lead sled...
i have never had any luck with varget----but thats my onion--
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others here will prolly flip-out on that one !!!!, but all barrels and there shooters are not the same.
the 60 grain nosler partition or the horny v-max 60gr should do you well.
i have 1/12 and a 1/9 both shoot the same boolit great!!!!!
i also have many bolt guns, but i can't stop tinker'n until my handi's will perform just as good or better.
i'll send you a pm with a load that will do it right with both twist barrels, according to my testing..
if your getting 2 shots off great, then the 3rd one goes tits up!!!----HEAT !!!, even tho you did say its an ultra heavy barrel right?,,,,,, should take more heat than a standard or ultra-lite barrel before walking.
jmho.......