Ordered R1 Carbine, received long barrel R1 instead...

[Motorhead]

That is a PICTURE PERFECT example on where a stock breaks when barrels been let go of   

How does this happen?  ... By pulling the trigger while the barrel is in fully open position?
Almost like throwing the transmission in reverse when you're going 50mph....   I always wondered it you can really do that.  LOL...

Barrel break actions have No Bear trap or Mid cocking stroke safety catches.  Until barrel is taken ALL THE WAY TO FULL BATTERY any loss of tension or worst case HOLD on barrel will Flip the barrel under full spring energy back to the closed position !!
With a Rifle Butt stock having much mass, as barrel violently snaps closed the receiver end of action tries to rip itself free of the stock. Being action is anchored at trigger guard, all this energy basically breaks the stock as shown in your picture.

Seen it and done it too