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Bolt carrier slammed hard, I can't pull back the charging handle.
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Do you mean that when you chamber a round, you have difficulty pulling the charging handle back?
When the carrier springs forward to chamber a round, it's OK if there is a slight amount of additional resistance as the bolt unlocks, but it shouldn't be difficult to pull the charging handle. If that is the case, you either have headspace problems (if you checked with known good gauges, it could still be out-of-spec/too-long ammo), or the bolt is binding in the carrier for some reason.
Did you install the spring that goes on the tail of the bolt? Unlocking the bolt will be harder if it is missing.
Are the gas rings still installed on the bolt? If so, remove them. They are not needed in a piston configuration.
Also, verify that the top of the charging handle is not rubbing/binding on the inside of the upper receiver.