12ga adapters are very common for 40mm. The seller of the adapter in the gunbroker link above (Destiny) is a great gal! Buy with confidence in any of her auctions. She knows her stuff. These 12ga adapters CAN shoot normal 12ga rounds. Oh, the grenade launcher will still be a DD, not an SBS.
12ga adapters are commercially available for 37mm flare launchers as well, however, MOST of these will not accept a normal 12ga round and will only accept flare rounds. Even you have one machined out to accept a normal 12ga round, you would run into 2 problems. First, you just committed a felony if your 37mm launcher is not a registered DD which might not matter at the time because you would be too concerned with stopping the bleeding from the launcher not being able to handle the pressure. THat being said, I shoot 20ga (buck, slugs, shot, etc..) out of my REGISTERED 37mm launcher all the time. It does eat at my barrel latch a little, but the ole mig welder takes care of it.
Beehives are great. There are a couple of different models available (neither shoots 50 rounds as stated earlier).
If you want to get good accurate info from folks who know 37 & 40mm stuff, sign up at
Grog's site.