The major changes with this shotgun over the Saigas are that they run out of the box with everything, they come with AK-type rifle sights, they come in pistolgrip configuration, they feature a very well-designed bolt hold-open that features an external bolt release as well as an external hold open button, and most importantly, the magazines are much, MUCH easier to load on a closed bolt.
The major advantages over other shotguns I've run are the super fast reloading and bolt hold-open. I can operate the controls, shoot, and reload mine as fast as an AR. Mag changes are very slick.
I'm a rifle/pistol shooter who didn't like shooting shotgun. I love shooting with this shotgun.
ETA: It has been confirmed by one of our Russian friends that Vepr shotguns can use Saiga 12 bolts and carriers, though the Vepr 12 oprod would need be be swapped onto the Saiga carrier. The handguards and pistol grip are AK/RPK compatible (though the gas tube is unique) and they accept any stock that'll work with an RPK- regular AK stocks need some fitting to fit in the receiver.
These Vids show off the shotgun pretty well:
Vepr 12
SGM Vepr 12, 12 round mags
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