Of the 12 gauge guns, I prefer the pump for just about any kind of social encounter.  A double -- side by side or O/U -- only has two shots. 
Forget the self-loaders because they're too maintenance intensive and some are sensitive to ammo.  That leaves the pump.
 
I assume you want one that's relatively compact, so get either a riot type (18-inch barrel) or a trench type (20-inch barrel).
A good, used pump gun would be my choice.  In no particular order: (1) Winchester 1300, (2) Ithaca Model 37, (3) Remington 870,
(4) Mossberg Model 500.  The Winchester 1300 comes with an extended magazine tube (8 shots); the Ithaca Model 37 loads and ejects
through the bottom (perfect for a leftie); the Remington 870 is very popular and well respected; the Mossberg 500 is tough as nails and the
military models are qualified to a Mil-Spec.  Numbers 1 thru 3 have safeties on the trigger guard (in back or in front), but the Mossberg is on
top of the receiver.
 
Riot and trench guns are smooth bores -- no rifled barrels in these models -- and that's good.  Rifling will cause even more shot spread than
a smooth bore, not to mention deforming the shot.  Tactical loads: (1) No. 4 Buckshot (.25 cal) is minimum for social encounters and 00
Buckshot (.33 cal) is most popular for general purpose use.  Usual load for 00 Buckshot is 9 pellets, but some enhanced loads pack in 11. 
Aquilla brand ammo has mini-shot shells (2 inch vice 2-3/4 inch standard).  You can put at least one to two additional rounds in the gun by
using mini-shells and the range is equivalent to the standard round a normal combat ranges.  For slugs, use Foster-type.  These are the lead,
hollow lead slugs, with the hole in the middle and the slant grooves around the sides.  The slant grooves cause the slug to spin in flight for
stability.  Don't bother with sabot rounds.  These are designed for rifled barrels (which you won't have) and they are very, very expensive
in packs of 5.
 
Shotguns are picky about the kind of slugs they like to shoot.  I had an Ithaca Model 37 that would group 6-inches (three rounds) at 100 yards
with Remington slugs.  It was lucky to get all three on the paper with Winchesters.  Brenneke slugs have a heavy lead projectile with the wad
screwed to it.  They're expensive and don't shoot much better (sometimes worse) than the 'plain Jane' loads.
 
Ranges: Buckshot about 15 to 25 yrds (max); slugs to 100 yds with 50 yds about the median.  The nice thing about Buckshot is you can bounce
the shot off hard surfaces to get to targets -- like a perp hiding behind a car -- shoot beneath the car and the shot will take out his legs. 
 
There are aftermarket kits that replace the butt stock of the standard riot gun or trench gun with a pistol grip.  These are strictly for use in
confined spaces like buildings.  The pistol grip will amplify the felt recoil of the standard 2-3/4 inch shot shell and you should never use them
with magnum loads -- never use magnum loads with any shotgun -- unless you want to develop one hell of a flinch!  If you have a pistol grip,
consider the Aquilla 2-inch mini shot shells.
 
That's basic Shotguns 101.  I can go with more detail when you make a decision on what to get.

--------------------------------------------------------------------------------
From: [email protected]
To: [email protected]
Subject: Gonna get a shotgun!
Date: Thu, 17 May 2007 21:25:04 -0700

I've decided to sell my .32 to Paul and get a shotgun. Preferrably 12-gauge, doesn't matter single, double or pump. The very first gun I had
ever shot was a shotgun and I had a sawed-off 10-gauge in Vietnam. I am first off a rifleman, with pistols being my present pursuit. But I
have always had a soft spot for shotguns because of 'Nam and their history. I plan on taking it out to the range and use slug rounds ( I know,
expensive!) at 25, 50 and maybe 100 yards. It'll be too un-weildy for home defense and I don't hunt, but should be a fun gun to shoot.
Master Guns: Recommendations on ammo-type-shot-etc for best results?