Ok, I will divulge.
as you all know, the barrel recoils inside the receiver. It is held in place against the receiver via ribs on the barrel itself. One of 2 ribs are fore and 1 rib aft.
The ribs are a smaller diameter than the aluminum cast receiver and thus a prominent barrel slop occurs. If you wiggle the barrel you will notice at leas a 2 MOA travel. If you push down on the barrel, it will move rearward.
The rearward travel is stopped by the barrel shroud - not a big problem since the energy dissipated to the receiver is not that great.
But what stops the forward travel when the breech block slams against the barrel?
I have taken some exact measurements to make sure, but it is the barrel slamming against the receiver. What is worse, there is only a small amount of aluminum in a form of a ring that is preventing the barrel from
going any further forward. When the gun is assembled, the push down disassembly pin aids in the damage, but because it is not part of the receiver, it only helps from total disintegration of the ring.
So what gives? The barrel has a bigger diameter at the chamber (for about 2 inches) then it has a shoulder and tapers down to a thin barrel that you can see is inside the long spring that retards the cocking handle.
This shoulder slams on the receiver's ring of aluminum that is only about 1/4 inch thick. I have fired about 500 rounds through mine and I can already see some peening.
hence - this whole process of deforming the receiver is exacerbated if you do not have a round in the mag. As the breech block travels forward, it has to stripe another round (looses velocity and momentum) then the friction of the round going into the barrel scrubs some of this momentum also.
Without the round - you will hear this LOUD and very metallic CLANK!!
That is bad, very very bad for your weapon. I have done it before not realizing what the internals were doing.
Of course, pictures are forthcoming on the web site, so please be patient!
Between babysitting my 3 year old princess and the yard, I will try to manage to post them tonight.
There ya have it fellas.