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is the pin suppose to protrude out?
Yes.
Second is how can you ensure that the rifle is centered on the recoil lug?
After the receiver is in the stock, give it a bounce on the recoil pad. If the barrel lines up perfect in the barrel channel of the stock, it's seated properly. The barrel will stick way up in the air if it's not seated properly.
Try Talley Light Weight (low), one piece rings.
