I just upgraded from a PDI v.1 to AA and its night and day. I could only get a good seal with the PDI using rtv sealant at the barrel securing hex screws and the end retaining collar. The AA unit is much more user friendly, and you can remove the barrel easier without destroying the bucking like the PDI unit does. Like stated above, use some teflon tape with it to ensure a seal. I chronoed mine with and without teflon and saw no change in FPS, so it does have a good seal. I am using a maple leaf MR hop 80% bucking and prom 6.03 barrel. If your cylinder head nozzle is under 6mm in diameter, you will have a poor seal. I have an old Laylax stainless cylinder head that was 5.88mm dia. Had terrible seal so I tig welded on it with silicon bronze filler metal, then turned it on a lathe to be 6.15mm thinking it would seal better. I chronoed it, and it was same power as my "newer" PDI cylinder head that is exactly 6.00mm (5fps variance with .40g bbs). So i turned the modified head down to 6.05mm so it wouldn't be stretching the entrance o-ring. I have a Modify cylinder head on its way i am going to try next. When changing up parts like cylinders, outer barrel, receiver, hop chambers, and heads, i find its important to know where your cylinder head lands in relation to your hop chamber. I put a tiny ball of clay on the head, and cycle the bolt. Then take the bolt out, and see how flat that ball of clay is. It will tell you how far your nozzle is feeding into the chamber.
