Because shotgun shells are large. This is a Saiga AK shotgun. Check out how long the 10 round magazine is. It's almost to the point of impracticality. This is a mossberg 500 with an upgraded tube. read more
Yes, this new shotgun is built on the decades-old 870 workhorse pump-action platform, but Remington has removed the tubular magazine under the barrel. The DM stands for “detachable magazine,” which, in this case, is a six-round box magazine that fits in a magwell under the receiver where you'd normally feed shells into a tubular magazine. read more
However, I have a Mossberg 590A1 pump-action shotgun that, although I once kept loaded with a round in the chamber, I now leave the chamber empty with the magazine loaded because I am not willing to take the chance if Mossberg is not willing to claim their shotguns are drop safe. read more