Originally Posted by
JoshNC
The commercial import 550s have circumferential hump for the grenade tensioning ring and iirc the flash hider turned to smaller outer diameter. The 16” OAL barrel 551s also have a smaller outer diameter on the flash hider. The StG90, PE90, and 551 LB barrels are all slightly larger diameter iirc than the 551 short barrels. But I don’t have any more 551 LB barrels around to measure.
I believe Gemtech sold two different adapters to accommodate different O.D. and length of various flash hiders.
Edited to add: a friend just mic’d his PE90 barrel. The flash hider is 21.74 mm O.D. I mic’d one of my 551 barrels and the flash hider is 21.49 mm.