Whats to stop you from just hacking the barrel off for SBR?

well the title says it all. After receiving a form 1 of course.

Nothing. You void your warranty.

Once you have a form 1 in hand you can do whatever you like, but "hacking" the barrel would leave you without threads for muzzle attachments such as suppressors etc. The barrel can be threaded without being removed from the trunnion.

See:
http://www.krisstalk.com/t205-rethreading-the-sdp-barrel-with-pics

Other than that, nothing keeping you from doing it. The gun is recoil operated so there's no gas system timing to worry about. Chop away my friend.

might sbr this thing after all