Impact Puff of air...cause

2x1 oring on the end of the valve stem

https://m.youtube.com/watch?v=tzPTiQUenv4

This should help....

I did mine a couple of weeks ago and its not so bad. You should be able to just drop off the scope rail/ top plate/ breech block and unscrew the valve house to access the oring ( after making things safe of course!)

Check the barrel for the internal o-ring at the lead-in. If that's gone you can get air in the face and you'll see a drop in velocity.