I recently purchased an M17s, and did a lot of checking beforehand. The prebans had a few different parts, most notably the flash suppressor and the split take down pins. Some people like their pre bans as they came. Some have trouble with them. Some return them to Bushmaster, who for a fee of about $100 or $150 IIRC will replace the lower with a new one, destroy the old one, and report it to ATF so even with a "new gun" you can keep your pre-ban status, and thus keep the flash suppressor. I personally figured I did not want to spend extra for a flash suppresssor and then extra to have the upgrades, but I can see how someone may want one just the way they want it enough to pay. I know I just sent a big check off to Kurt's to have mine done just the way I wanted it. However, with the pre-bans there MAY actaully be some reliabilty/safety issues.