Nozzle-to-Bucking Seal and Midcaps

[volteco]

I have searched for this but the issue seems to be non-existent. At best people talk about mag compatability.
I run mid caps on both of my guns (VFC SR15 and G&P SR16) and they both experiencing this issue to some extend. If I load almost a full mag (80-100 Bbs) when I chrono the guns they have variarions in FPS +-5FPS until the amount of BBs left is about 30 or less. Then the issue dissapears and FPS stays the same, at most +-1FPS.
My guess is that the spring in the mag pushes BBs out and BBs in turn push on the nozzle so hard that it causes the nozzle to misalign slightly in the hopup chamber causing a very small airleak when it meets the bucking. When the amount of BBs in a mag is low, the spring in a mag is not compressed as much and doesn't make the misalignment. This would be only relevant for midcaps, not hicaps or lowcaps.
I tried different cylinder heads (War bear, VFC, G&P, Lonex, Guarder) with different nozzles (VFC, Prometheus, systema, lonex, G&P), and when a nozzle is in forward position it can wiggle up/down relative to the direction if airflow quite a bit depending on a setup.
Has anyone experienced this or am I making things up and I need to look in another direction?
Is there the best combination of cylinder head - air nozzle - hopup bucking?

Just to mention, hopup sits tightly against the gearbox, done tappet plate mod, floss mod and teflon tape mod. A few o-rings on the inner barrel to insure tight fit. Tried on G&P and prometheus hopup, PDI barrel and bucking. Mid caps are Magpul and Beta, pushing them up and forward doesn't do much. Airseal test cylinder to barrel shows a good seal.
Thank you!

[volteco]

Double post. Please delete.

[lurkingknight]

how loose is the nozzle on the cylinder head?

Tappet return speed can cause these issues, say if the tappet is binding on the forward stroke or if it's hitting the trigger spring...

You can also try cutting a couple loops off the tappet spring and bending out a couple loops to replace it. Putting the tappet spring under constant tension can return it forward faster.

Other things like the pistol grip screws being too tight can also pull the back of the gearbox down which raises the front of the gearbox.

[volteco]

With the best combination the amplitude is ~0.5 mm

[volteco]

I haven't tried fiddling with the return spring. Thank you! I will give it a try.

[pestobanana]

Find a nozzle that has a lot of taper at the end and a small inner diametre to reduce wobble. Find a cylinder head with the widest nozzle. Try using a soft hop up rubber.

There was one batch of SHS cylinder heads that had very wide nozzle bores, very little nozzle wobble when paired with stock VFC, Lonex, and Modify nozzles.

[volteco]

It is not easy to find one because in a store they are all in a packaging and you can't measure it with caliper. Unless you buy evething that is available and measure it.
Based on my testing the nozzles with a lot of taper create more leak than the ones with almost no taper even if they are longer. That only applies to PDI W-hold rubber though, because the lips are smaller compared to lips on the other hopup rubbers that I have.
Thanks for the advice with SHS!

[pestobanana]

Only reason I know about the SHS batch was because I sorted through about a hundred of them prior to installations, and realised some of them had larger bores. Unfortunately the ones with larger bores sometimes have an O ring so it is harder to install a sorbo pad, and they are all off spec in that the side holes must be enlarged or they do not fit into mechboxes.

You're using a PDI W-Hold, you're already losing 30 FPS from it. I've never had FPS loss from tapered nozzles. SHS, Lonex, Modify and G&P and Retro Arms nozzles are all tapered, I've managed to get them to seal well, although some of those nozzles are a pain in the ass to seal to cylinder heads.

[jordan7831]

Originally Posted by pestobanana

Only reason I know about the SHS batch was because I sorted through about a hundred of them prior to installations, and realised some of them had larger bores. Unfortunately the ones with larger bores sometimes have an O ring so it is harder to install a sorbo pad, and they are all off spec in that the side holes must be enlarged or they do not fit into mechboxes.

You're using a PDI W-Hold, you're already losing 30 FPS from it. I've never had FPS loss from tapered nozzles. SHS, Lonex, Modify and G&P and Retro Arms nozzles are all tapered, I've managed to get them to seal well, although some of those nozzles are a pain in the ass to seal to cylinder heads.

Out of curiosity, what fps variance would you consider acceptable? Say I have a gun with +/- 2 fps variance from the average. Would you consider that to be a fairly consistent set up?

[volteco]

Replaced PDI W-Hold rubber bucking with Prometheus purple bucking. When chrono-ed, got results anywhere from 405 to 425 FPS with 150% PDI spring. I am surprised I could get such a big increase in FPS from 150% spring...
Replaced VFC nozzle with much more tapered Lonex nozzle and cut one loop off from the tappet return spring. When I pressed the hopup against the open gearbox I could see tappet plate goes 2mm back inside the gearbox when the nozzle seals with the bucking lips, so it was supposed to be a good seal. When I assembled everything together and chronoed I got 394 - 399 FPS consistently... which is still inconsistent. I guess I need a slightly longer nozzle. Not sure what else to do at this point. VFC nozzle has larger inner diameter then Lonex one and it is not round (like on the pictures below) but the VFC one is 0.3 mm shorter. There are no leaks when I do cylinder to barrel airleak test.

VFC nozzle:


Lonex nozzle:

[lurkingknight]

5fps variance is very good but a bit concerning if that spring topped out at 425, I'd be a bit worried if it sealed on certain shots you'd shoot over the limit. I'm not familiar with what pdi springs are supposed to be in terms of output.

Did you floss or teflon tape the rubber onto the barrel to get a good seal inside the hop up? a good amount of fps can be lost in blowby if the barrel/rubber assembly is not tight int the hopup.

[volteco]

Originally Posted by lurkingknight

how loose is the nozzle on the cylinder head?

Tappet return speed can cause these issues, say if the tappet is binding on the forward stroke or if it's hitting the trigger spring...

You can also try cutting a couple loops off the tappet spring and bending out a couple loops to replace it. Putting the tappet spring under constant tension can return it forward faster.

Other things like the pistol grip screws being too tight can also pull the back of the gearbox down which raises the front of the gearbox.

Not sure how tappet plate can hit the trigger spring? The tappet plate moves freely and very smoothly.

[Hectic]

yeah, ±5 fps isnt really all that bad, my bolt action has a ±2 fps and its a bolt action so it will be consistant when tuned like mine is. But yes the issue (a non issue imo) is caused by the fullish mag putting more upward pressure on the nozzle. Again tho i dont really think ±5fps on an aeg is an issue, thats actually really consistant if u ask me. ±10-20 fps is an issue, more than 20 is a prety wonky setup

[volteco]

Originally Posted by lurkingknight

5fps variance is very good but a bit concerning if that spring topped out at 425, I'd be a bit worried if it sealed on certain shots you'd shoot over the limit. I'm not familiar with what pdi springs are supposed to be in terms of output.

Did you floss or teflon tape the rubber onto the barrel to get a good seal inside the hop up? a good amount of fps can be lost in blowby if the barrel/rubber assembly is not tight int the hopup.

According to the spring chart here:Spring Comparison Chart PDI 150% spring supposed to be giving out 395.45 FPS. The field I play at doesn't allow anything above 400FPS so I have to be in this limit...
I only did teflon tape over barrel and rubber and then teflon tape over the hopup and barrel. Will try again with the floss...
Actually one thing I changed while changing nozzles is I replaced piston head from SPEED to Lonex, because occasionally I was getting shots less than 100FPS and suspected that to be the piston head issue...
Thanks!

[lurkingknight]

inspect the piston head you pulled out for a piece missing off the back of it. I had one crack and it no longer sealed.

A heavier piston assembly can increase FPS in certain setups, say if you were building a gun to joule creep.

The tappet can hit the trigger spring when the spring isn't sitting straight, in some gearboxes or with some springs, it will move side to side on the post that anchors it to the gearbox shell.

Are you still seeing issues with your new parts?