Two options, buy a replacement barrel chambered for 2 3/4" shells, it will have two gas ports instead of the single port in your magnum barrel, option two is to open up the single port in your existing barrel. Note that if you choose option two you should no longer shoot 3" ammo as increased gas volume will damage the gun or accelerate wear.
I chose option two for a gun that is no longer used for hunting, original gas port was .073", increased to .093" and will now cycle target ammo.

 

What he said^^^^.

 

^^^^^ yep