Two different AR's, a 10.5” & 10.3”, are getting stuck on live rounds, 1/4-1/2” out of battery with carbine, H1 or H3 buffers.  But an H2 buffer seems to fix both of them.  Why?  

What is causing the bolt to get completely stuck so I have to remove the upper from the lower and tap the bolt carrier into battery with a small hammer then reset the trigger manually so it will fire?  Mortaring, yanking charging handle, and the forward assist don’t fix it.

Both uppers where purchased assembled (DSA & Stag), properly lubed with CLP, no debris in the action, shooting factory ammo and reloads (that function fine in a carbine/rifle length AR).  Gas tube looks fine and aligned on both. I have been troubleshooting the 10.5” longer, tried swapping the lower, bolt, charging handle, FCG, etc.  Nothing worked except the H2 buffer.

I want to understand what the issue is/was and why an H2 fixed it when an H1 and H3 buffer didn’t and are so similar in weight.  Has anyone encountered this before? Are AR pistols just finicky in general? Thank you!

Is it happening with different types of ammo? I am assuming they are both chambered in .556?
Chamber measured?

Yeah, don't do that.

Happens with various ammo, both are 5.56.

And I realize using a small hammer to get the bolt into battery is a bad idea, but as I noted the standard fixes did not work, and I would rather have the round fire and shoot the bolt out the back of the upper than try and knock a live round out from the front with a cleaning rod.  I did manual reset the hammer a few times first, and that was not giving the carrier enough of a smack to get it fully into battery.  In hindsight trying to pry the bolt back probably would have been a better idea.

There's nothing inherently special about a buffer that stops a BGC from going into battery or makes it go into battery better. Given that you had to use a hammer (worst idea ever) to get your bolts into battery, I'm going to say that somewhere along the lines you consistently did something wrong with reassembly, as it happened with 2 different guns from 2 different manufacturers, and you stopped doing whatever it was when you popped in the H2.

If I had to guess I would point at firing pin retainer not fully seated. It's really impossible to say without personally inspecting the weapon and reproducing the failure but, the logic says the operator is the only common denominator.