Originally Posted by WildRose
20" just isn't enough length to stabilize many bullets properly and the longer barrel also allows for more complete powder burn.
How long are the barrels on the pistols that folks are shooting very successfully at 1k?
I could be wrong, but I don't beleive barrel length has much to do with stabilizing bullets. ...unless you need the extra length to attain sufficient velocity for the required RPM.
Several of the cartridges mentioned will completely and efficiently burn powder in a 20 inch barrel.
You will be able to get additional velocity from a longer barrel which may be welcome, if not essential, out past 500 yards.
That said, I don't own anything with a 20 inch barrel.