so the 15/90 can i use a black oring for that or does it have to be clearJust the bolt you will need a 16/70 for the sail (rear) the bumper on those are a 10/90 or 11/90 either or works (the need to be intalled by streching the ring out (oo err) with a pair of circlip pliers and sliding it into position) and the he o-ring is a 15/90 (same as a tank o-ring)
Unfortunately I am low myself so can't pop you any in the post but now you know the sizes
I thought you wanted the bolt o-rings, my bad.
The 17/90 is the sft o-ring (front of breach)
I only listed the o-rings on the bolt, the 17/70's (most likey green) in the front and back of the firing can and the inner of the bolt sleeve.
The only advantage f using white (urethane) over black (Butadine Nitrile/Buna/Nitrile) is that the tend to outlive them, but Buna tend to be a lot cheaper in comparison (about 10x cheaper in fact)
I use what ever I find first, but I only buy Buna as for the amount I use it's a lot more cost efficient