Accurate 2520 and Leverevolution

[Bassfish1952]

Both show up on Hornady reloading as the only 2 that will go 2850 at Max. I read about 2520 on their site , supposed to be a little slower and better for longer barrels. 
LR seems to be real popular for 6arc and shorter barrels.

My question 

What is considered a longer barrel ?

Anyone used the 2520 and how did it work for you ? Anyone use both and can give a comparison ?

Thanks