The problem you run into with explosives is:
- A lot of them were over-nerfed by the Obsidian developers
- Some have tiny splash radius settings
- Line of sight issues (something as small as a box can be enough)
- There's no such thing as armor penetration for explosives by default.
http://www.newvegasnexus.com/downloads/readmes/Deadlier%20Explosives%20New%20Vegas_readme.txt
40mm is 40mm is 40mm... doesn't matter whether you use the Grenade Rifle or the Grenade Launcher, the damage is calculated solely by the explosion data in GECK. The only diff (I think, without looking at GECK) between the rifle and the launcher is the # of shells that it holds before you have to reload. That, and the rifle has a weapon mod (+100% projectile velocity) and the launcher doesn't. So you can get a bit more range with the modded rifle (I think). The only minor difference between the launcher and the rifle is that the launcher's projectile gets a base damage of 30 vs 2, so if you manage to hit someone with the projectile it will do 30 dmg.
And we'll completely ignore the issue that a Grenade Launcher can be repaired with a Minigun...
(The over-nerfing of explosives is why I mod the values so that they're more effective due to DT.)