im not sure if they are or not im not really familiar with barrels yet cuse only have had a 98. looking to get a used etha and would like a barrel back for it cuse of the bore size being so large for it and found a cheap barrel back on ansgear that's why wanted to know if the planet eclipse barrel would fit on to a sly barrel back
first guess is probably not. most companies all use different threading between the two barrel pieces. even if the threads are the same, the length of the threads may not be the same. take what you have to a shop and see if they have any pe barrel fronts to see if they fit.
just pick up a sly front in the bst forum if you really want to use the back and pe fronts dont fit.
they will not fit also pe backs are about 6 inches long while sly's are like 3 maybe even two you can find a sly front on ans on sale too i believe. but with the sly barrel you have to be careful how you clean it because it is carbon fiber and can ruin the inside. i would just look for a boost kit from pe for the etha
You can always get your stock Etha Shaft 4 back bored for freak inserts. There is a guy on here that does it, link below will direct you to his ad here on nation. He has done a few backs for me and he does great work with a very quick turn around http://www.pbnation.com/showthread.php?t=3912255
if your short on cash, you could just buy a .682 or .685 barrel of your choice and be done with it. that tends to be the most common paint size as of the last couple years. i have a complete freak kit and a complete 8 piece kaner kit and those are the only 2 sizes i ever end up using anymore.