I'm totally unfamiliar with this model, but someone pointed out that the new V3 Tac-13 might be a good start to an SBS.

Looking at photos of the Tac-13 and the field guns, it seems like there's a plate at the end of the receiver on the Tac-13 that isn't there on the field guns. On those, the stock attaches directly to the receiver.

Anyone know if you can just unscrew the birds head grip on the Tac-13 and attach a standard V3 stock?

There is a plate on my field gun (26” black synthetic) at the rear of the receiver where the buttstock attaches....where one would place a shim to adjust fit and LOP. It seems like a metal plate though I’ve never removed mine to find out.

Because the recoil system is located entirely at the receiver there is nothing in the buttstock except for buttstock....so one could imagine the possibilities.