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If the primer strikes on the rounds that did not fire are the same as those on the ones that did (location, depth, etc) then I would blame the ammo, not the gun.
Do get some brass ammo as a control group, at least. Powering up the firing pin strike for the combloc ammo might be needed, but you don't want to take it to the point it pierces primers on other stuff.
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All else being equal (and it almost never is) bigger bullets tend to work better.
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