QUOTE (Stahlseele @ Aug 31 2010, 07:57 AM)
why?
Because it was a joke from the original Street Samurai Catalog, iirc.
In reality, there'd probably be no real consequences other than the barrel not being able to handle the rate of fire and being more prone to melting/warping under the stress of sustained autofire. Also in reality, weapons like the M249 (an LMG) have been fitted with collapsible stocks. The real interesting part is how they make the feed mechanism modular. As written, the weapon would need to be chambered in at least three different calibers with variable barrels for each caliber as well.
If that's the case, then it would likely be almost impossible to mismatch parts because things just wouldn't fit properly and one would hope it was designed that way. If you fitted a barrel that was too small, the round wouldn't feed. If you fitted a barrel too large, the round might fire (I've seen AK-47s fire off 5.56 ammo before (don't ask)), but it would most certainly misfire due to a lack of sufficient gas pressure to move the action, and who knows where the projectile would hit because it wouldn't be properly catching the rifling.
Regardless, the end result would be the complete weapons system would weigh a ton. Figure about two to three times the weapon's actual weight, considering the only common part would be the frame. It would have to have at least two lower receiver parts (one for the low caliber rounds and one for the larger (unless they went for common sized magazines adapted to fit smaller rounds), three upper receiver parts (, five barrels (AR, Rifle, SMG, 2 barrels for the LMG, and add an extra weapon for the HK since it has a shotgun config)). The Steyr, iirc uses a fixed stock for all configs, so it's probably no big deal. It would also need magazines for each configuration (LMG and AR could share mags). Not really something you could carry around, at least if that was the misguided intention of the player.
Of course, if you look at the illustration for the XM30, it's a firearms design nightmare, lol. Bullpup configuration belt feed. Yikes. Believe the AUG was similar. Probably be only rational to eliminate the belt option for these weapons.