You have 3 choices.
1) pin the stock
2) purchase a pre-ban lower for about $600. In which case you can put any combination of stocks, bayonet mounts, and/or threaded bbls/flash suppressors on your gun that your heart desires
3) ***best choice if you don't want to spend the $$ on a preban***Get yourself a nice Vltor rifle stock. Look, why would you want a non-collapsing collapsable stock?? Its stupid. Just get a fixed stock.
I know the answer - because you want it to look cool. I understand. The Vltor is a great stock, and it looks cool. Its also designed to be fixed and works great.
I hate collapsable stocks, so I have rifle stocks even on my pre-ban gun. Here is a photo.
As time passes we learn what we like. I like rifle stocks. I like short light guns. I like shooting open sights.
So for me, its a rifle stocked gun with a short barrel, light weight hand guards (no rails)
I also like shooting suppressed with both .223 rifle ammo and .22LR ammo. So a threaded bbl is what I have. This gun with a .22 kit and a YHM Mite on the bbl is a great way to introduce people to shooting. Light weight, very quiet so instructing is easy, and effectively no recoil.