a friend of mine has a mount on his .410 shotgun for slug shooting..it's very old and have a scope in it thats very small...mounted in rings that we believe are 7/8"...the scope is currently mounted with shims inside the rings.....we were wondering if 7/8" mounts actually existed or if we are measuring wrong....and if they still make scopes in 7/8"...we know for sure it's not 1inch....and 7/8 works out to be about 22mm so that sounds odd also