Bushmaster ACR Pistol
The tailhook adapter was a pain in the @ss. You have to disassemble the ACR stock, remove the spring-loaded buttons and install them into the adapter. Luckily Dan Haga has a video that shows exactly how to do it, but it still cost about 1.5hrs and some sore fingers.
Ive always wanted one of these, and finally went for it. It also seems that Bushmaster will finally start releasing some new barrel calibers. Regardless, the controls are awesome and its a solid shooting firearm.