Can someone validate or fact-check the last paragraph of this Wikipedia link?
M203 grenade launcher - Wikipedia
So, a 40mm grenade is a “destructive device,” but a 37mm is not? Is the ATF just fixated on size, that those 3mm make so much difference?
40mm can fire military grade projectiles, no one makes them for the 37mm. 
Does that mean this is false?
No… No military or civilian arms manufacturer makes high explosive rounds in 37mm.
The ATF doe’s what it wants. 
All things 37mm. 37MM Cyclone Projectile | EF Industries LLC
The 37mm “flare gun” is a smooth bore while the M203 has rifling the 3 mm is the difference between the two, I’ve never measured them but the rifling in a 203/M-79 is STOUT. I may be wrong on this part but the chamber for the 37mm is shorter than the one for the 203. In other words you can get a 37mm in an M-203 but not the other way around. I never looked much farther than that into than that. There may also be something with the “cocking feature” in that when you close the breach on a 203 she is ready to go other than the lever safety alongside the trigger guard. Most all of the 37mm’s I’ve seen have a “cocking lever” of some sort as well as a lever safety.
It’s been a long time since I looked into things but I believe it was either Hatcher or Ackley that wrote about the recoiless rifle in their respective books which is where the concept of modern grenade launchers came from. There is something with how the round is launched so that you don’t rip your shoulder apart by using them. Again it was a long time ago that I looked and never gave it much thought after.
Cheers,
Craig6
For more info. 