37 mm flare or "1.5 inch" caliber is the specification for a common launching system for non-lethal and less-lethal ammunition.
37 mm systems are typically smoothbore as rifling is unnecessary or even detrimental to the performance of the usual projectiles.
In the United States, 37 mm launchers are exempted from the National Firearms Act of 1934 so long as no anti-personnel rounds are in the user's possession.
The specific wording of the BATFE rule is: ATF Ruling 95-3: 37/38 mm gas/flare guns possessed with cartridges containing wood pellets, rubber pellets or balls, or bean bags are classified as destructive devices for purposes of the Gun Control Act, 18 U.S.C.
Likewise, you cannot possess or use 37mm anti-personnel ammunition or you will have to register the launcher platform as a destructive device per BATFE ruling.