Bugger. It's already had a stock on it? That's beyond my ken. Check carefully before going the pistol route then. If it's 1st assembled into a rifle, ie, a buttstock & a bbled upper, it's a no-go for pistol.
If you start from a lower sold as an "other", w/o a stock, & build as a pistol, you are GTG. If it's already got a stock on it? That I don't know.
Gig 'em,
backbencher