Is it safe to fire 37mm Smoke/flare/gas rounds from a 40mm?
Just curious

Yes, if you can get the casings to center, and the barrel to close. A few wraps of electrical tape will help. Some 37MM casings have thick rims that won't work in 40MM launchers. GROG

I'm glad you saw that, and thanks for the information. Reason being is most of the 37mm's on the market (that i have found so far)
will not chamber an 8", eg. the spikes are for 5 3/4". Only a break-top launcher would work, and the only one i have seen
is either a B&D, or an older fedlab/s&w gas-gun on gunbroker.

any advice as to my best bet? just get an older gas gun? only reason i'm hesitant there is the issue with barrel erosion.

I recently registered a B&D EX-D launcher as  a DD, and I wouldn't have done that if it was a cheap launcher. They, as well as the old S&W type, are good products made to last, and handle any 37MM round on the market, as well as the new ones from Reloadableshells that kick harder. As long as you clean it well after firing black powder casings, there should be no bad erosion in the barrel. I know B&D launchers are warranted for life, and they have fantastic customer service. Each launcher is hand made in their shop, and it is a small business, so they put time into each launcher they produce. I've heard of only a few customer issues with launchers, and those were resolved very quickly and at no cost to the buyer. Hope this helps, GROG

Can you check the chamber on your B&D factory 37mm and let me know the length? The issue i'm having is all the commercial launchers i have found so far have a 5 3/4" chamber, and i need one with an 8" chamber. Thanks again grog.

The B&D launchers do not have a "milled" chamber in them. The ID of the barrel goes all the way through the launcher barrel, so no step to prohibit longer rounds. I recommended this when I spoke with George Bates, when he was designing the launcher, in order to allow customers to use any length casing in his launchers. The S&W does have a chamber, so you would be limited with those barrels.

Excellent. I was worried i would have to chuck up the barrel in a lathe and bore out the chamber longer to fit the 8" rounds. Guess the B&D will be the solution, unless i find a cheaper S&W or FL gas gun in the next week or so, as these already have an 8" chamber. thanks again grog!

Not a problem Bro. We like videos!! GROG