I thought I saw the rules somewhere, but now I'm unable to find them. Where are they?

Compact Weapon is an upgrade in the core book.

Dark Heresy calls it a "Hack Shotgun", and it's in the Inquisitor's Handbook, page 115. My Metallican Gunslinger currently has a pair, which he's used to great effect so far in the close quarters of the mutant infested hive city he's stuck in.

Cheers!

Cifer said:

...but you better leave the "price-add-on" out. Unless, getting a barrel and the "buttom" sawed is quiet costly. The reduction of "ammo" isn´t making much sense in this case, either.

Besides that, it is a good option.