On the subject of artifact recovery, my brother uses bulk raw molasses to de-rust bits of antique machinery. Process time can be several months, but there is little chance of mechanical damage. In the case of small arms, there should also be no damage to the wooden components, though they might taste funny afterwards.
He also suggest immersing less crusty complete items in diesel fuel (distillate) for similar periods for easing dis-assembly. The distillate may not be too kind to the woodwork, but, in the case of a possibly loaded firearm, may neutralise the primers.
