if the barrel threads are not timed as well, it doesn't make a difference. Somewhere on the web are the instructions i posted on changing SIG 550/551 barrels - their threads are timed as well as the threads in the trunnion. Net effect is when installing a barrel (new or the old one) you would screw the barrel in till it's hand tight, then with a straight edge on the base of the front sight / gas block mount (new barrels come with the barrel already drilled for the gas block port and the mounting pin), and a similar straight edge on the dovetail at the front of the receiver, you turn the barrel in until the rear straight blocks all view of the front straight edge. I had a swiss army armourer demo that for me, and when he was thru i asked "are you going to use a headspace gauge ? His response, "NO! SIG wants 1200 francs for that gauge - they are crazy". And he was right, i'd already explored buying that gauge, and all it was was a single blade dog leg feeler gauge. I really think it was a misprint in parts manual. maybe 12 francs should have been the price
PS - just found the picture tutorial on the SIG 550/551 barrel changeout - it is pretty interesting, but as this is the HK Talk section, i just found the "non-HK" section and will be posting it there. Give me about 30 minutes as there a lot of photos and i'm slow as molasses
posted
hhttps://www.hkproshop.com/forums/threads/sig-550-551-barrel-changeout-procedure.2069/