There was a reported test (Winchester advertisement) comparing American guns and the M21 reportedly utlasted the NID and others by a considerable shot count. The guns shot heavy loads, exceeding recommended pressures, until they could no longer shoot.
According to Researcher:
"in the famous Winchester Model 21 Proof Test, the NID failed after 56 proof loads, the Fox-Sterlingworth after 80. Of course the point was the overdesigned Model 21 digested 2000 proof loads and was just fine."
The rest of the story (the M21's stock broke after 15 rounds):
http://www.doublegunshop.com/forums/ubbt...e6172d7d6ab57c4