I recall a few recent bayonet charges (in the news), but if I recall correctly in each case it was the Brits.
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Here is the story
http://www.freerepublic.com/focus/f-news/1138046/posts
From what i have seen the M7 bayonets will fit a SBR
but if you have a 16" or 20" then you're going to want an M9
It kinda has to do with where that "hole" (sorry brain not working and can't remember the name) that goes over your A2 flash hider sits.
The M7 will go to far back and not make contact with your A2 flash hider
thus causing it to be to loose.
I tried one on my S&W M&P and it fit fine. but fit my mossberg 590a1
great also.
**Warning, if you plan on stabbing a head of lettuce with a bayonet attached
to a mossy 590a1 12g shotgun then shoot it.
Make sure you are wearing not only protective eye wear
but also rain coat and pretty much dressed as if you were
going to be sitting front row at a Gallagar show**