Change a 1100 2 3/4 into a 3
Jim, yes, that's the way to do it.
Except possibly the very earliest receivers, which are said to not be convertible to 3" but I have yet to find the reason why, a 2-3/4" gun can be converted to 3".
This is going to require a heavier action sleeve. The action sleeve is the hollow cylinder that moves forward and back on the mag tube.
The increased mass of the 3" magnum action sleeve slows down the bolt velocity moving rearward. The combination of the lighter action sleeve and magnum loads results in the bolt slamming hard into the back of the receiver, which can and will result in damaging parts and eventually cracking the receiver.
Remington made a 3" steel shot barrel that they approved for use on both 2-3/4" actions and 3" actions. But there was an explicit warning for 2-3/4" guns due to the lighter action sleeve. Only 3" steel shot could be used, not 3" lead. The latter would result in high bolt velocity, as I outlined above.
The question is, where can he get a magnum action sleeve? I don't know if Remington has them anymore. And the 1187 action sleeve won't work. It's the same as the 1100 2-3/4". I'd suggest checking with Remington but you might also consider putting a WTB ad here.