.020
no
0.604 inches http://www.wildfowlmag.com/tips_strategies/shotgun_062304/
Check the internal diameter of the barrel (disregarding the choke diameter). A 10 gauge is 0.78" (19.7 mm) A 12 gauge is 0.73" (18.5 mm)
Nominally the inside diameter is about 0.72 inches, but will vary depending on choke.
ABOUT .72 caliber. I say about because the degree of choke on a shotgun changes the muzzle diameter.
yes.
The Winchester 140 12 gauge has a choke barrel at 28". This is the same choke size as the 20 gauge model.
The model 120 ranger shotgun ranges in price from 90-170 dollars depending on the overall condition of the gun and a good bore.
only if your choke is improved cylinder or modified, full chokes are a no,no. the barrel will bulge or burst!
about a 150 bucks
Modified choke with the 28" barrel.
You don't give us the model, so it is hard to answer the question. Assuming the gun does not have an obvious adjustable choke, the choke is probably not easily adjustable. A gunsmith can open the choke (from Full to Modified for example) or can install internal interchangeable choke tubes (costs around $125).