Quote Originally Posted by AR_ART View Post
I'm not intimately familiar with piston driven rifles, but I took it down and watched how the parts worked.

I removed the piston block, and stuck a dewey rod in the piston chamber. I pushed down and watched the BCG in the ejection port. I noted that the when the guide-rod spring was fully compressed, the BCG was only half-way open.

I then removed the guide rod spring and repeated my test. This time I could move the BCG all the way open via the piston rod.

So, I'm wondering if the guide rod is supposed to go all the way back OR the rod only opens it part way and the momentum opens it completely?

Checked the things you suggested and it looks good....
Given this info I would be questioning if there is an error in the assembly.
I am only shooting in the dark here Ray, I have only fiddled with an ACR once or twice and that was in a retail environment.
Anxious to take a look though.....