I mostly target shoot (and fish and boat) but I now find the need to use my shotgun in 6 weeks for deer in this area. I have the above with a 28" barrel and an 18 1/2" barrel. Receiver is tapped. Choice would be between buying a 24" integrated scope base barrel, and a scope and rings, or trying to just mount a scope on the long barrel. What eye relief is best? Are those red dot sights a better choice, or will my circa-1951 pair of eyes need a scope as they do with my rifles now? Will use some manufacturer's slugs (smoothbore) and the shot will definitely be under 100 yards, probably half that. Don't want to chance just using the double bead and only wounding the deer.

I would go with the fully rifled barrel with the scope mount. Good for old eyes and good for 125 yards. MIKE.

Mike,
Thanks for the quick reply.
What eye relief do you recommend?
Would the red dot devices make up for my old eyes or do I really need to go scope?
If a rifled barrel, what type and brand slug would you recommend?

The canterlever rail uses Wever style mounts. The rail has several positions so you can move it to your sweet spot. Mossberg and Hastings both make barrels for your gun. The Hastings is a better barrel but costs more. The red dot is good close in, 50 yards or less, but if your eyes are gone like mine I would go to a scope. I can still use iron sights but only in perferct light. Remington, Federal, Winchester, Lightfields, Hornady all kill deer. I use the Remington copper solids. Looks like a crime scene when you hit them. MIKE.

Edited to add: If you need a shotgun for deer in New York in six weeks you must be hunting the Long Island January shotgun season. If that is so a 50 dollar smooth bore deer barrel with rifle sights and cheap foster slugs will do the trick. The shooting is fast and close. MIKE

Fifty dollar smoothbore barrel!  Where?!  I was looking at $129 smackers for the Mossberg's 24" barrel and felt I could just put a scope on my 28" barrel instead.  Seriously, where could I get hold of a 50 buck barrel?
Yep, its those LI deer- always thought they were all scrawny until last week while pheasant hunting--looked like some of them feed real well!

If you got the 28" VR barrel route...make sure you screw in the IMPROVED CYL choke.

As a side note, if you get the rifled barrel with the cantilevered scope mount, you never have to take the scope off when you want to use your regular barrel. The scope goes with the barrel.

Its also been suggested to me that I could use the 18 1/2 inch "security" barrel as a slug gun- my shots would be in scrub brush and mostly stunted forest, and probably under 75 yards.  Anyone have experience with shooting slugs from such a short barrel?  With two kids sucking me dry (college tuition), going cheap for a couple of years is what I have to do- so if I have to make the cheap choice, 18 1/2 improved cylinder or 28" with the improved choke?

Found two: www.cabelas.com, they have a 18.5" cylinder bore barrel, no sights, bead only $69.95, www.corsonsbarrels.com, they have a used 24" slug barrel, $85.00.

Shooting slugs from a shotgun barrel without sights is a crap shoot. Some shoot fine with the bead only, some can't hit a 4X8 sheet of plywood. If you do not want to spend any money just buy several boxes of cheap slugs from Wallmart and try your modifed and improved clyinder choke tube and see what happens. Cabelas has several sights made to mount on the vent rib to shoot slugs. I would guess about 30 dollars. where do you plan to hunt Rocky Point or Otis Pike? MIKE.

2 of my experienced bow-hunting friends took me to Rocky Point after pheasant last "windy" Friday - at one point I think I played the part of the bird dog as I evidently spooked a trophy buck while trying to scare up a darn ring-neck! Known the one guy for almost 30 years. Basically, they just said, "shut up and just enter the lottery!"  (I had said that if I ever did deer hunt, I'd use my Marlin in .35, never expected to find myself after Bambi's dad this close to NYC.)  We did some scouting around further east around Manorville as well.  
I have a 3x9 40mm on the Marlin that was advertised as suitable for shotguns and I had thought about just getting a mount for the Mossberg's tapped receiver- definitely a lot of options to consider, and not all seem to be overly expensive.
If I can just use this scope I already have -- just have to decide which of the 2 barrels I have would be best, the 28" or the 18 1/2"

The Long Island deer season is a lot of fun. Bring extra ammo! The beat method is drives as you allready found out. The lottery must be mailed in by the end of the month, good luck, MIKE.

There are much better scopes out there than your 3 - 9 for shotgun.  Here are a couple:

www.bushnell.com/productinfo/riflescopes/trophy.html

www.simmonsoptics.com/page12/SimmonsPage12.html

On the Bushnell page, the 1.75-4 X 32MM is a good choice.  For fast and furious shooting, crank it down to 1.75 power.  The 32MM objective gives you excellent light transmission, and the "circle-X" reticle is good for this kind of shooting.  At the other end, you have plenty of magnification if you need to take a longer shoot.

The Simmons Pro-Diamond is basically the same thing.  They also offer an illuminated reticle model.

Personally, I don't like 1X red dot scopes for anything over 25 yards.  For deer hunting, precise shot placement is important, and 1X red dots take a back seat to magnified scopes beyond 50 yards.

I have a Mossberg 24" barrel with the integral mounts.  My barrel likes 3" Federal Hydrashok Sabot slugs (1 to 1 1/2" groups at 50 yards).  Didn't like the Remington Copper solids at all.  Before I got the rifled barrel, I used to shoot standard Winchester Foster slugs through my 20" barrel, with IC choke tube in place.  It grouped about 3" at 50 yards, with the scope mounted directly to receiver.  Do yourself a favor and buy the rifled barrel, with scope mount.  It doesn't cost very much, and you'll use it again and again.