Nothing wrong with shooting what ya have!
I've been tempted to put down my 870 and join the M2 crowd, but it would be for gun games only and I prefer to shoot my SHTF gear and work on my skills more than chase a higher ranking at a match. I have no illusions of becoming an "Alien Rockstar" of 3gun. (don't ask me what that means... Heard it at last weekends match for the first time, LOL)
Since my 870 is an 14" SBS and 5+1 home defense gun.
For 3gun I change out the barrel to an FO topped 21" railed model and add a +4 Nordic tube for 8+1 capacity.
Since I like the pistol grip (speedfeed IV-s) I weak hand load off California Comp 4 shell caddies (7) on the belt only.
I recommend breakin the edges of the loading area with a file and emery cloth.
I've got a question posted now on how to solve the lift gate over travel issue, but once that's resolved the gun will be finished for me.