Comments directed at a COMP Shotgun:
Yeah, 500’s aren’t set up to take a mag tube. You can get a conversion kit from Brownells which give s you a longer mag tube and the differant barrel to make it a 20" 500 model.
On the SPX. I would look at one of the non-SPX 930’s like the HS. They cost less and can be set up just about any way you want as you grow in the sport. If you go that route add a Nordic Comp mag extension, the Noveske sling adaptors, and one of the over sized charging handles. After that shoot it for awhile, before you look at other mods.
If you want your head to explode with 930 information read here:
Brian Enos Forum, Mossberg 930 thread
You can also call AimPro Tactical about having a 930 Comp Gun built. Mike’s good people, but not much on this internet thing. He also works on shotguns for Jerry Miculek.
For an out of the box comp 930 look into the Jerry Miculek Pro series 930 which was released at SHOT. Not sure when they will start shipping though.
Shooting Wire article
The versatile 930 platform features a dual gas-vent operating system which aids in recoil reduction and reduces stress and wear on operating parts. This 12 gauge semi-auto reliably handles 2 ¾ and 3-inch from low-brass target loads to hard-hitting magnum rounds. Standard features include Uni-Line™ stock and receiver design for instinctive eye/rib alignment; stock drop spacer system for customizing fit; drilled and tapped receiver; interchangeable ACCU-CHOKE™ system; durable black synthetic stock and forend; and matte blue metal finishes.
Performance-enhancing features on the JM Pro Series include Mossberg’s extended tactical bolt handle; forend cut to facilitate faster reloads; oversized safety button; orange magazine follower; and protective barrel clamp for the extended mag tube. The 10-Shot Tactical Class has a 24-inch, non-ported, vent rib barrel with standard ACCU-CHOKE Modified tube; red fiber optic front bead sight; extended 9-shot magazine tube; adjustable trigger stop; and anodized receiver with chamfered loading port opening. The 9-Shot Tactical Class version features a 22-inch, non-ported, vent rib barrel with standard ACCU-CHOKE Modified tube; red fiber optic front bead sight; extended 8-shot magazine tube; adjustable trigger stop; and anodized receiver with chamfered loading port opening.
On fighting shotguns. There are better choices of fighting tools availble if you are also shooting 3 Gun.
Simply swaping out the slugger barrel on your 500 for a 18" barrel will give you a servicable “fighting” shotgun, and is basicly what the Army issues, the Marines swap out a couple more parts. You can sex it up as much as you want from there, but a “fighting shotgun” is all about simple. Add a Surefire or one of the new Insight forends to make it more suitable for night time and call it good if you have to go the extra mile.
I have to much time invested in the process to believe other wise, unless you have very specific duty requirements.