Seems you have the wrong barrel, but…
First some background:
Remington 870-type guns come in 2-3/4", 3" and 3-1/2" length actions, and generally the barrels are interchangeable.
Rule #1 Don't use a cartridge longer than the is marked on the barrel!
Rule #2 The length of the action will determine the maximum shell length cyclable.
•Barrel's chamber shorter than the action
A 2-3/4" barrel on a 3" frame works fine with 2-3/4" shells; but, if you cycle 3" into the shorter chambers, you're courting disaster. Although a gun in this configuration works fine as long as you use the shorter shells, there is a potential danger if you stuff longer shells into the short barrel.
•Barrel's chamber longer than the action
A 3" barrel fits fine in a 2-3/4" with not problems; but, you can only cycle 2-3/4" shells. This configuration doesn't pose a safety problem. Remington used to sell 2-3/4" and 3" barrels separately, but now sells 3" chamber barrels for both 2-3/4" and 3" chambered 12-ga 870 guns. Some folks mistakenly think this makes there old 2-3/4" long action into a 3" -- not so.
Remington's extra barrel data page contains:
NOTE: Model 870 Extra Barrels with 3" chambers will fit Super Magnum, Magnum, and 2-3/4" receivers. These barrels will handle both 2-3/4" and 3" shells when used on Magnum receivers. Use only 2-3/4" shells when these barrels are used on 2-3/4" receivers.
It seems your brother-in-law's gift is a strange set up; but, it isn't necessarily the wrong barrel. Perhaps Remington is marketing a standard barrel on a Super-mag action; but, I'd think their legal department would object. Before contacting Remington, check the gun's model number (found on the box) on the Remington web site -- I suspect you'll find it's the wrong barrel, or it's an off-catalog item.
Brent is correct,
Do no use 3-1/2" shells in a barrel marked for 2-1/2" & 3" shells.