What kind or specific slug ammo can I use for a 20 gauge, smooth barrel, 2 3/4" with an old Winchoke choke tube? Thanks in advance.

Since you have a smooth bore sabots will not work.
You still have many selections available in what most people refer to foster type. I have done a considerable amount of shooting/testing and to save you some time I would stick to Brenke slugs. My belief on chokes is, if you have removable choke tubes I would stick to a cylinder bore and choose the tube with equally spaced tightening recesses, better off with a knurled extended, crown of rifle barrels are extremely critical for accuracy so it would be reasonable to say that would hold for slugs if you want to shoot extended ranges. No substitute for a scope

Thanks for the reply. What setting would I put the choke on? IC, M, F? I'm concerned that I may damage something. I appreciate the help.

Also what is the effective range for deer going to be? I'm thinking 75 yards?

Any factory foster type slug will shoot through any choke since they are soft lead. Common sense dictates that if you deform the lead by choke restriction it will affect its flight capabilities, I would use the IC. with a scope, practice and proper shooting technique the max range is whatever the distance that you can group in a pie plate. For the average shooter with iron sites and smooth bore probably 50 to 75 yds. For an experience shooter who has put in the time and has a scope generally twice that
A fully rifled barrel scoped in a capable shooter 200 yds maybe more with certain sabots. I gave up on the smooth bore and went strictly rifled and scoped and I sight my shotguns both 12 and 20 at 3" high at 100 and have shot them to 200 but the longest kill so far was 115yards

Here is a site that deals strictly to shotgun slug shooting, I have a write up on a recent deer I took this year "hastings at 60 yards"

http://www.shotgunworld.com/bbs/viewforum.php?f=118

Thanks for the replies. This is an uncommon probably one time situation. Where my son will be hunting is a shotgun only area. Not the norm for Colorado. Did not want to invest in a slug gun setup. Thanks again.