I emailed Spikes Tactical about the missing o-ring, and they are sending me one to go on it as well as a few spares for no charge. Good folks.

This conversion is by far the best thing Ive bought for my AR so far. I can shoot it as much as I want. I think the most I shot it in one day was like 15o rounds before this. Now Im shooting 500+ a when I go. lol