Need info on shotgun customizations

[Mazryonh]

I need some info on customizations for shotguns for something I'm writing. After playing too much S.W.A.T. 4, I began wondering whether the Benelli Nova Tactical in that game, if fitted with the 3-round tube magazine extension, could still mount a bayonet. To my knowledge bayonets are usually mounted underneath the barrel, but the 3-round magazine extension would block this on the Benelli Nova Tactical. Would it be unsafe to mount a bayonet on the tube magazine extension instead (much as it would block a weaponlight in the handguard)? Or would one have to use a side-mounted bayonet on the barrel instead? How much would it cost to fit some bayonet lugs for a side-mounted bayonet on such a firearm?

I'd also like to know how difficult it would be to convert a semi-automatic shotgun with a tube magazine, such as the SPAS-12, to accept a detachable box magazine. On a strange note, Hitman: Blood Money lets you apply both a side-mounted shell holder and the "feed from a box magazine" conversion to the ingame SPAS-12, though in real life you'd only use the first to help load a tube magazine more quickly and wouldn't need one if you switched to detachable box magazines.

Finally, the civilian model of the M4 Super 90 offered by Benelli USA has a 4 + 1 shell capacity on their catalogue, but the fact that the tube magazine still reaches the end of the 18.5-inch barrel puzzles me. Is the tube solid past a certain point? If it is solid, does that mean you can't add a tube magazine extension without illegally modifying the shotgun? And why didn't Benelli just shorten the magazine tube so there is less "dead weight" on it?