Added as a second opinion
Opinion from another source: I am an avid fan of the Rem 1100 in general, but find the 20 ga in used condition to be a very difficult find. If you are looking for one that also has a slug barrel it would seem easier to find a unicorn. They only seem to come up for sale during at estate auctions, or during divorce cases. The low end you will find one used (and pretty beat up) is around $300, I bought one that was very clean last night for $525. Note that there were different models of the 20 ga made. The first was on the large 12 Ga frame. Then there was a LT model and a standard model, their barrels do not interchange. One has a short tang on the barrel the other a longer one.
$200
No such model in the Remington line.
3
300
D081337n
Made by Remington in 1985 &1986; 28" or 30" vented rib barrel with various chokes; basicly same gun as the 870 Express model; most if not all 870 parts are interchangable. Should probably hold value comparable to the Remington 870 Express model which retails close to $400.00 after taxes.
The average price of a Remington 870 Express Magnum pump 12 gauge shotgun "brand new" is about 300-450 dollars. I got mine used for $250
NOWADAYS $100 , NO SCREW IN CHOKES= NO VALUE
Model 870 Magnum 1982 ducks unlimited edition
In good condition $1200.
Only if chambered for 3"
Check the Remington Society of America, date barrel codes and location. http://remingtonsociety.com/rsa/questions/barrelcodes