40mm Sub-caliber Insert What looks like a normal 40mm grenade shell is actually a sub-caliber insert for a 20 gauge shotgun shell. Simply insert the shotgun shell into the 40mm look-alike and you can essentially turn an M79 or M203 into a shotgun. Why? It’s a cheap alternative to use your 40mm launcher when high explosive rounds are extremely expensive and difficult to get. Would be kind of funny to use 6 of these in a Milkor revolving grenade launcher.