I guess I'm not being clear. The part that fits in to an atty is too big without an o-ring. The rounded portion (the very end - about 1 cm at the very tip) goes in but it will go no further. An O-ring would make it even bigger. It's too big to fit a 510 tube.
ive never had that problem. my donut doesnt like to fit well with my 357 i use it with so i just dont take it off much. i blow it out with the tip on it,lol.


j/k 