Have it examined by a competent gunsmith to be sure
If the gun is marked 2-3/4 on the barrel, that is the maximum length shell that the gun will shoot.
The 1100 mag shoots 3in. shells, the 1100 standard shoots 2 3/4in. shells.
You can only shoot what is printed on the barrel. Mag and short mag are not the same.
No, the 22 Mag will not fit.
Yes
Which caliber?
No.
The difference is that the Remington 1100 3 inch mag will take three inch mangnum loads and shorter where one that doesn't say 3 inch on the barrel will only take two and three quarter inch shotshells or smaller.Trying to put a three inch inch a gun that is not designed to will cause extreme stress on the barrel and will get jamed in a pump or automatic shotgun.
No, the 300 ultra mag cartidge is not interchangeable with the 300 win mag.
i am pretty sure they are 3" You can shoot 2 and 3/4 in.shells in the 1100 3"mag but, the shot weight has to be at least an ounce and 1/4 for it to cycle {fire,eject and chamber the next shell} properly.
yes
About 150 yards.