That was my thought too. Everyone with black rifles make trades with others. Confuse ownership so much no one knows who has what. Make bills of sale on each trade to make it look like a sale.
I was not placing any particular genre of firearm in the mix, except ALL of them, from Bond Arms derringers to Barrett fifty cals.
Here in swampy Louisiana, the ONLY record of sale, by state law, is the motherloving federal NICS paper. NO bill of sale required, NO transfer of ownership, NO hand me down documents, either.