ltcnav, The standard used by ammo manufacturers is a 24" barrel. Ammo will chronograph differently based on brand and bullet weight. If you use a normal 40gr bullet in a high velocity 22LR cartridge, you can expect about 1250 fps from a bolt action 24" barreled rifle.
Ruger 10/22s typically have a 18 1/2" barrel and because they lose a little velocity from the blowback action, you can expect the same ammo as above (40gr bullet in a high velocity load) to chronograph at 1150-1200 fps.
Ruger does make a 10/22 with a 20" barrel but I have never chronographed them. I would assume it would be a little more than an 18.5" barrel and a little less than a 24" barrel.
I chronographed a Charger with a 10" barrel using normal 40gr high velocity ammo and it averaged about 1000 fps.
If you use different brands of ammo or even different bullet weights, these numbers could be quite different. Chargers typically run about 80% of full velocity (24" barrel).
ltcnav, BCs for 22 LR 40gr bullets are pretty grim. Looking in my Ballistic Explorer data base, I found the best to be .157 (Eley Club Extra) and the worst was .120 (CCI Green Tag). Lighter bullets are even worse .... as low as .084 for a 32gr CCI Stinger. Generally, hollow points have a lower BC than solid nose bullets of the same weight.
When I crank in BC for computing 22 LR bullet drop, I use .120 .... the lower end of the spectrum. BC makes very little difference because the velocity of these bullets is very slow (compared to a centerfire 22 cal bullet) and not very uniform. The goal for computing bullet drop is "time to target" where more t-t-t allows gravity to pull the bullet down more. Lower BCs generate more air friction that slows the bullet down faster, thus a longer time to target. If you go with match grade 22 LR ammo, the rated velocity (1080 fps) is quite uniform but these bullets typically have a very poor BC, which is why they are intended for 50 meters (or closer).
I have chronographed many different brands of 22 LR bullets then ran the results through Ballistic Explorer to get the actual BC. In all cases, the actual BC was not nearly as good as the advertised BC, no matter what brand ... no doubt due to the rifling engraving. The advertised BC for all bullets is done with no rifling engraving because the manufacturer doesn't know what type of gun you will use. The same ammo fired in my Marlin 39A (Micro-Groove rifling) developed a considerably higher BC than when fired in any other rifle with conventional rifling .... but still not quite as good as advertised BC.
So to answer your question .... use a BC of .120 and you will be in the ball park. If you have a specific cartridge (brand and load number) I can look it up for you and at least come up with the advertised BC.