What's the cause of this? The bullet jamming in the lands?
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Aero
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Bushmaster lower for comparison
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For reference this is the round that required the mortar to clear...
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That can happen with some bullet profiles and chambers that are pretty short. Sometimes mortaring will get you a gunpowdery mess in that case. Happens with some handloads with pistols, too.
Other times, I really don’t know. And in his pics, I don’t see rifling marks. I have a hypothesis that the bulge in the case from the bottom of 147s make some stick in tight chambers, but I really don’t know. It happens, but I’m not sure of the cause.
I don’t know for sure if this is applicable in OP’s gun and ammo.