What I am looking for is a multipurpose shotgun, one that will fill many rolls. I want one for hunting, shooting trap and tactical as well. I know for the most part this can be accomplished by changing out the barrel, different stocks and so forth. Mag tubes are the sticky point as many models either have dimples or no way to extend their mag tubes.

So is there a good base model? Should I go with say a tactical model and then convert from there? Then there is also the question of pump verse auto. I never hear much about auto's jamming, is this an issue to consider?

T/C Encore.  Get as many different barrels for as many different uses as you want.

Remington 870HD

Get the model with the factory full length mag tube. Can be had NIB for around $300-$315. Get a plug, mag tube end cap, and a "sporting" barrel with interchangable choke tubes. Done.

Quoted: T/C Encore.  Get as many different barrels for as many different uses as you want.

yeppers

A used Wingmaster with 2 3/4" chamber is ideal.  The gun will get beat up switching hardware back and forth so why buy new?  The Wingmaster action is by far better than almost any other pump action so it will be smooth.

Basic shotgun you are looking for would be a Remington 870 pump, they can be found for very cheap prices.

But if you have some extra money get a Benelli.

My mulitpurpose shotgun is a Benelli Super Black Eagle 24" and four shot extended tube. Benelli shotguns come with 5 choke tubes which is great for hunting and trap.

I hunt with it, shoot sporting clays and trap with it. And if I want to just 3 gun match it can do that too. I also have a Benelli M1super 90 with a vent rib 21" barrel and four shot extend tube that can do eveything like the Benelli SBE.

Winchester 1200 or 1300 pump.
You can buy a the field version for skeet and hunting and buy a 18" riot barrel to fit the short magazine from ebay OR buy an 18" full magazine barrel and a mag extension also found on ebay all the time.

I like the aforementioned 11-87 auto and the 870 but love the winchester alum receiver and locking lug bolt too.  No question when a shell is chamber when the rotating bolt is slammed forward.