When adding a Launcher to a gun, the model glitches out and moves everything that isn't the gun (magazine, smoke launcher, even the charging handle) forward, with some moved to the end of the barrel. When you remove the smoke launcher, everything is normal. Other underbarrel attachments (including the Masterkey) do not glitch out.
Note: This only happens when viewing your whole character, not when just looking at the gun.
As far as I have seen, this only affects the guns in the customization menu, not in any matches.