As part of Navy boot camp we had to qual on the Mosberg 500. Years later I qualified on the Mossberg 500 as part of our pre-deployment to Iraq. We went through a bunch of close quarters combat situations and then had to show proficiency in a high speed target range. Needless today EVERY military Mossberg 500 I have seen has a full butt stock. The ONLY pistol grip ones I have ever seen were for use in vehicles as a last ditch defensive gun.
With that being said I have shot the 18 inch barrel pistol grip 12 gauges with full 00 buck and slug loads. It was quite an unpleasant experience that I really don't care to duplicate. You can't hit a damn thing with them at any range over 25 meters. The recoil makes you feel like someone smashed a baseball bat into your hand. It hurts, bad.
To convert from a pistol grip to a full stock is not expensive at all. I bought a used SWAT team stock for my Mossberg 500 for all of $20. If you get a good deal on the gun then go for it. Just make sure to change the stock. It takes only a few minutes and it is a huge upgrade.