While it could be argued that the average civilian is not going to shoot enough 40MM for it to really make a difference, IIRC, the LMT rail-mounted M203 was more or less designed as an MRP accessory since the LMT MRP does not have a removable bottom rail to barrel mount the M203 - however, since it's a "true" monolithic, it has the strength and rigidity to support the M203.
I would think it would more or less fairly well on a one-piece quad rail, but I don't think I'd be comfortable putting it on a removable bottom rail design and/or "modular" handguard where only a couple of bolts are supporting the launcher and attaching it to the rifle.
Not the the M203 is a super-heavily or intensely recoiling device... but I'm pretty sure I've seen some bent rails from using them with the rail-mounted '203.
If/when I get a '203, it will be the barrel mount version.
The ironic thing, however, is that the M4A1 RIS II was designed with a specific requirement to free-float an M203... but many of the units using the RIS II are now using the M320A1, and thus having to go back to using it with the KAC M4 RAS.
~Augee