I like the Defender, great shotgun. But no good for 3 gun. Barrel length is not the problem, it is lack of choke. I found that steel targets, particularly small or heavy targets, just do not reliably go down with cylinder bore, even with 36 gram loads. Now I run a 22 inch Remington 1100. With improved cylinder, even 28 gram loads will reliably take down most steels.
Sure, use the Defender to start with, but expect to fire a lot of extra shots, and to suffer the bruised shoulder that goes with it.