I have decided on a Green Mountain SS fluted but unknown as to length. Help me out: could someone (who actually knows, not just think they do) explain the pros/cons between the 16, 18 and 20? thanks in advance....

Velocity will be highest at 16".

The longer the barrel, the quieter the sound will be.

Other than that, it's pretty much a matter of personal preference as to what you like in looks, weight, balance, stuff like that.

the longer the heavier

if shooting bench the longer/heavier bbl will be more stable and less prone to human error/movement/jerking so the accuracy could be better. just something i noticed.

Quoted: Velocity will be highest at 16".

Not necessarily.  Not all 22LR ammo burns its powder by 16".  Some burn slower and can take 20"+ to burn completely.

I went with the 18" figuring it would be a compromise between too short and too long.

Quoted: Quoted: Velocity will be highest at 16".   Not necessarily.  Not all 22LR ammo burns its powder by 16".  Some burn slower and can take 20"+ to burn completely.

I've never seen any data to support that. I've seen several tests where they start with a 24" or longer barrel and cut off two inches at a time and highest velocity is almost always in the 14"-16" area. I've you've got hard facts otherwise, please share. For example, what brands and SKUs of ammo work better in 20"+ barrels?

I have a 20" unfluted .920 simply because I always shoot (50 & 100yard target and plinking) from prone.  I always use a bipod and no rear bag.  I like the extra weight, and it's very quiet when shooting subsonic ammo.  A lightweight bbl. is of no advantage to me, for my uses.  

From the past year or so of browsing rimfirecentral.com , I've seen that a 16" .920 can/will group just as good as the longer ones.  So, to me it's more of a matter of what your needs are for the rifle as a whole.

Quoted: Quoted: Quoted: Velocity will be highest at 16".   Not necessarily.  Not all 22LR ammo burns its powder by 16".  Some burn slower and can take 20"+ to burn completely. I've never seen any data to support that. I've seen several tests where they start with a 24" or longer barrel and cut off two inches at a time and highest velocity is almost always in the 14"-16" area. I've you've got hard facts otherwise, please share. For example, what brands and SKUs of ammo work better in 20"+ barrels?

I think I read the same articale on the 24" barrel and right about 16" was the highest velocity for 22lr.