RE: 870 slug gun question
Pinning the barrel would certainly help with the accuracy as it ensures that there is always a constant alignment between the barrel and receiver. This becomes supremely important if your scope is mounted on the receiver.
Another option (and probably a less expensive one) would be to switch to a cantilever barrel and mount your scope there. With the cantilever, the scope is mounted directly on the barrel, not the receiver. Any sort of misalignment from the receiver to the barrel becomes far less significant.
One distinct advantage I can attest to is that removal and subsequent reinstallation of the barrel has little to no effect on the zero of the scope. The Hastings cantilever I have for my BPS gets switched on and off a couple times per year and I have yet to have it not keep zero.