I bought a Binelli M2 field for trap shooting.  I took it out a few days ago with my wife and we did not like it.  Both of us shot much worst (not that we are any good to start with ).  I'm also not so thrilled that the tube only holds 3 shells .  Pretty weak in my opinion.  So, instead of putting it in the corner of the safe where it'll never be used again, I'd like to upgrade it to make it "tactical".  I'd start by replacing the tube and stock.  Any place thats good for Benelli accessories?  Any suggestions?

I would keep the Field Stock.  If you just have to have more shells get a +2 Extension.

I would keep the stock its gonna help take some of the recoil. I have a 24" m2 thats my turkey/yote gun and setting it up for 3 gun also. Its a great 12ga and remember dont give up on trap/skeet with it when you get a new gun it may take a while to get used to and use the right choke!! lol

Keep the stock, add a Nordic tube and clamp and a 3-gun gear side saddle.
If you want to step up a notch, send the barrel to Briley and have it cut to 21" and re-threaded for the thin wall tubes and add a 10/22 rear sight on the vent rib for shooting slugs.
Now you have the #1 shotgun used in 3-gun.