I’m looking to get a new barrel and I was noticing that MidwayUSA doesn’t seem to offer a barrel that has a 3" chamber. My receiver is a super magnum, that is chambered for 3 1/2". Do you require a 3 1/2" barrel to shoot 3 1/2" shells? I can’t find a reference online to such a barrel. Thanks… I couldn’t find a definitive answer to this question anywhere online. 
Yes, the chamber is part of the barrel. YOU NEED A 3 1/2 MARKED BARREL
That’s what I figured. I was just kind of bewildered that I can’t find a replacement barrel to fit my gun. Just checked Remington’s site… they don’t make a 3 1/2 870 replacement barrel. Very odd.
You can use a 3" chambered 870 barrel on a 870 Super Mag, but not vice versa.
Check gun broker
2nd edit ***** my bad, you didn’t specify 870 **** getting old, in any event the following applies to the 870 magnum
Interesting didn’t realize the magnum 870 supported a 3.5" barrel.
thought 3" was max for the magnum, at least that is what my 870 magnum supports.
Learn something new everyday.
Edit: curiosity got the better of me, check out this table:
I think you are correct the magnum only supports the 3in , but the super mag is 3 1/2. I had one for goose and turkey hunting wasnt a fan. My regular 870 will kill anything i need with a 2 3/4 shell, and not beat the hell out of me