My dad used to own a mosin as well, he told me it was a russian mouser, which it is, but I prefur calling it the mosin. He had to give it to my uncle to hold onto it when I was born, and my uncle sold it WITHOUT my dads permission. But I found the bullets to the gun in my garage in an ammo box. They were massive, 7.62x54mmR, big bullet. I asked him how fun it was when he had it. He said that it was fun to shoot, but expensive due to the bullet sizing. He used the iron sights, and I assume he was accurate, and it was painful to the shoulder after a coulpe clips of shooting.
Sawing off a rifle requires a tax stamp as well, so unless the rifle came as a pistol (read: no stock and short barrel), then sawing one down is still illegal and will land you up to 10 years in prison.
And if you somehow managed to turn it into an Obrez, the bullet drop would be INSANE, due to the lack of barrel, which leads to a lower muzzle velocity. Ammo is still expensive as hell, personally, turning a mosin into an Obrez is a waste of money, personally, just get a pistol. Something simple like G17, m9, or taurus g2c. Simple 9mm, good for concealed carry cause they are smoll and the ammo is relatively cheap, i think.
Well it's partly meant to be a fun gun and I'd spend money on something like that for the novelty of it more than it's practicality as a CCW. But the low muzzle velocity isn't going to be much of an issue so much as it's inaccuracy do to a lack of a full-length barrel, and remember. Pistols also lack a long barrel, as well as having a smaller cartridge and consequently less propulsion strength than that of a mosin. So even then a sawed off mosin will still have higher muzzle velocity than your average glock.