I noticed that VEPR sells both 16 inch and 20 inch barreled ak rifles.

I know there is a slight velocity differnce between a 16 and a 20 inch barrel, but what is the accuracy difference with open sights (longer sight radius) and with a scope (probably not as dramatic as the open sights)?

Does the slight increase in velocity with the 20 inch barrel make any noticable difference in accuracy due to its slightly flatter trajectory?

Thanks in advance

robarm says there is very little difference, i specifically asked them that when i bought my first one.  

As with any firearm, barrel length has absolutely nothing to do with intrinsic accuracy.  Any enhanced "accuracy" due to a longer sight radius is going to be user dependent.  The longer radiusdoes help.  As far as  increased muzzle velocity due to increased barrel length, you are only going to get about 200 fps more out of the 20" barrel.  Again, aflatter trajectory has nothing to due with intrinsic accuracy, only bullet drop at a given distance.