Madbull Shark bucking. PDI W-hold bucking. Must share my success

[ThunderCactus]

What part of the PDI did you need to teflon tape?
Every time I've seen one the ribs on it jammed it so freaking hard into the hop chamber it was impossible to get it to come out with the barrel

[airsoftjunky]

Originally Posted by ThunderCactus

What part of the PDI did you need to teflon tape?
Every time I've seen one the ribs on it jammed it so freaking hard into the hop chamber it was impossible to get it to come out with the barrel

I personaly had to give the outside of the bucking about 2 wraps. Perhaps my King Arms hop up has looser tolerances than some..

[Trev140_0]

Got a discussion going right now on the Hill437 Private forum on the PDI W Hold.

[Amoki]

Epic necro, but... this is still literally one of the best thread on the PDI W-hold bucking on google after 5 years with almost all the iimportant discussion sitting one after another. (A testament to a quality ASC thread?)

I am currently in the same conundrum: put in a PDI delta barrel + PDI W-hole bucking in an E&L AK that was doing 355fps on .25g (Maple Leaf aeg bucking + stock barrel+ firefly namazu: shocking accuracy, could only hit an A3 target 7 out of 15 tumes at 25 meters...), getting only 335 fps on 0.25 g after the install, go eh...

After some tinkering, here is my conclusion:
1) the reason why a loss in FPS happens: The PDI bucking has a shortened sleeve mount. Long enough to have good sealing with the nozzle, but way too short to "hold" bb. You can verify this by droppin a bb through the pdi w-miond bucking vs a Madbull shark (or any other bucking for thst matter)- the PDI sleeve has zero contact with the bb at all. With the hop off, the bb will roll out from the barrel in a gun. Not only is there a shortened stay in the barrel, but you also lack the resistance from the bucking dleeve(which builds up additional pressure/energy behind the bb).

If you can rule out air leak from your bucking-hop unit-nozzle interface, then a fps loss is actually "normal". The way you test for this is by resetting your mech such thst the air nozzle is fully forward in an assembled mecbox, fit your hop unit and barrel onto the mech as it would be in thr assemlled aeg, put a finger to block off your hop unit, and blow from the front. If you are not losing pressure, then everything is as designed.

2) but because the w mound is doing the work of holding the bb, you have significant consistency now. In most conventional hop rubber, you have 3 degree-of-freedom in the bb vector : the bucking sleeve, the drag imparted by hop up mound plus nub, and the barrel. The pdi bucking cuts that down to 2 dof and get the bucking mound to do the work of determining the spin vector and the travel vector. In most other bucking, this work is shared by the bucking, the sleeve, and even the barrel.

3) at the same time, the mound design and the rubber material is good enough to affect the drag on the bb such that the bb behaviour at the tail end of the bb flight path remains consistent.

In my quasi-control gun (CA SA 58) shooting slightly higher fps (340-345 fps at .25gs) and using the Madbull shark bucking + stock brass 6.05mm barrel + a firefly namazu nub, the accuracy beyond 60m was terrible. In my E&L Ak105 + m-nub + PDI w-hold bucking + PDI delta barrel, the gun can ping target out to 70m easy, measured using Google Earth, even with though it has 10-15 fps less.

In summary: don't be alarmed if you see a fps drop. If you can verify that you are not losing air from your bucking-hop unit-nozzle interface, be sure to make a decision about whether to ditch the bucking on the basis on the actual performance and not just a fps loss.

[ThunderCactus]

Or maybe it has something to do with there being a gap in the middle of the hop rubber, so more air escapes past the BB while it's moving through the hop rubber, which is also when the chamber pressure is highest on a ported cylinder.

Doesn't take much to change your fps when you're messing with stuff at highest pressure.
I managed to bump my bolt action up .3j by chamfering the air brake back just .02"

Air pressure doesn't hit the BB until the air nozzle has pushed the BB past the seal of the rubber. So unless it's so tight that it's causing jams, it won't be affecting fps.