So I'm taking apart the gas system on my M1A the other day, finally got the shims to make sure the gas lock properly indexs on the gas tube. And as I'm taking it apart I realized the O-ring on my plug is missing. No idea what happened to it. I just want to know some thoughts on if I should...

I recently purchased my first M1A and I have been swapping out the parts I can with USGI. I recently came across a marked HRA gas system and was wondering if it's any good or if I should avoid HRA as I have heard mixed messages on them