I have an old H&R 20 ga topper. It says '20 GA 3" MOD" on top. Does the “3 inch” marking mean it’s safe to use magnums?
20 gauge is obvious; 3" is the length of the shell; MOD is the choke.
I’m not sure how to answer the question about “magnum”, as that usually isn’t a size issue, but a grain issue.
It can take 3" magnum shells as the orignal standard 12ga shell is 2-3/4" IIRC but the longer ones are pretty normal. Most magnum shotguns today can take a 3-1/2" shell which were made so a 12 gauge gun could hold as much lead shot as 10 guage.
http://www.hr1871.com/ is the manufactuer’s website, get the straight dope from them.