37 mm flare or "1.5 inch" caliber is the specification for a common launching system for non-lethal and less-lethal ammunition. Such launchers are also often known as "gas guns" due to their original use by police for launching tear gas projectiles. 37 mm systems are typically smoothbore as rifling is unnecessary or even detrimental to the performance of the usual projectiles.
37 mm rounds can be fired from a variety of devices, including standalone launchers, and M203-style launchers mounted on rifles using clamping systems or rail mounting systems.