HPA m27 inconsistent groupings.

[Genitalthunderstorm]

I have been having consistency issues with my m27 and am looking for a tech to work on it.
The specs:
VFC m27
Polarstar f2~2000 rounds through it.
Redline SFR regulator
Prommy 455mm 6.03 barrel, lapped and R hopped with a prommy 70 deg bucking. All work done by Sherlockl from reddit
Maxx ME pro hop up
BBs: .32g

The problem: the m27 has very inconsistent groupings. The fps measures fairly stable at 344(.32)+- 1.5 measured with my acetech chrono. The bbs however will underhop and overhop pretty badly. The left and right deviation is great. I can send BBs to the moon and then flat and then slightly under-hopped in as many shots. This is most obvious when firing 2 round bursts

This is my most expensive and beloved rifle in my collection. I have sunk hours of work into it when I upgraded it as a AEG. It is also my worst performing rifle. The problem is either beyond my skills or is something simple that I am too frustrated to think through.

Let me know if you can help. I am happy to ship it anywhere in Canada if need be.
Thanks

Some Troubleshooting I have done:

FCU settings: I have followed guides and am pretty sure I have the f2 volumed for my barrel length. Closed or open bolt mode make no difference. I feed every round in every mag I own.

Air leaks: Little bit of permatex gasket sealent under the bucking. Air lines from Amped. F2 clean and lubed.

Barrel: Cleaned with rubbing alcohol

F2: shimmed with a bit of tape so it doesn't move within the gearbox. Before I shimmed it would move fwd and back about a mm or so.

I can post a shooting video if needed.

[BioRage]

Tbh stock vfc unit and lonex unit are the best.

Maybe your rhop patch and or bb quality, there's only 1 rhop guru I'm familiar with and it's pestobanana @ blackblitz

[R.I.T.Z]

Make sure your nozzle alignment is correct.
Dwell setting are a big factor.
I'm noticing an over/under hop issie with the maxx hopunits..

Gotta make sure the rhop is done properly and clean.

Have had sucess fixing most issues with a madbull red shark bucking.

Closed bolt is your friend.

What brand of bbs?

[pestobanana]

Normally I'd question the quality of the RHop installation, but HPA setups are notorious for being troublesome with RHop. Nozzle alignment could be the culprit, but another issue is that RHop requires a gentle nozzle loading action, as the RHop provides friction to stop the BB instead of a wall/mound as a normal hop up would. For this reason, more violent loading actions can cause inconsistency in the BB chambering location, sometimes moving past the patch and just rolling out the barrel completely.

This is notorious with poor engine designs such as Redline that use a very violent chambering action. Redline also uses nozzles that are over 1mm too long, which doesn't help its case. I'm not familiar with the P* platforms, I am only certified with Wolverine platforms. I would take the upper off and use a finger to subjectively feel how violent the loading action is and see if that could be causing problems. I would also inspect with the hop up unit and see if the nozzle protrudes too much, if it does then cutting the nozzle slightly shorter will help. A hop up rubber with thicker lips can also help slow the loading action, as can drilling a double feed prevention well at the top of the feed tube (edit: MAXX already has this).

[Genitalthunderstorm]

Biorage: I replaced the stock VFC unit because it had a prominent mold line right through the middle of the hop arm. I suppose i could sand it down and try it. I have 2 lonex units in my drawer. The problem was just as bad with them as with the Maxx. One problem with the lonex units was the tiniest hair of an adjustment would cause huge changes in amount of hop applied.

R.I.T.Z.: I have looked at nozzle alignment quite a lot. The left and right alignment is perfect while the nozzle is a hair high. I have checked nozzle alignment with and without a mag full of BBs pushing up against the nozzle. Closed and open bolt make no difference. BB brands that I have tried: Elite force, Valken (I know these are shit), Gryphons ( They dont make bio .32 ). I want to spend lots of money on super high end heavy weight BBs but i cant justify the purchase until I tame this Hop problem. My other rifles get consistently better performance with the same BBs. Rhop was done by Sherlockl from the airsoft Subreddit. I dont know how to install R hops but he comes highly recommended.

Pestobanana: I researched this phenomenon before buying the HPA engine and one of the selling points of the polarstar F2 is it uses a spring to push the nozzle forward and load a BB. I just tried firing the engine into my thumb and it feels identical to my AEGs. The nozzle problem isint one I have explored though. Through the maxx hop window (without lighting module) it doesent seem like the nozzle pushes far into the bucking. However as I detailed in my above post the F2 did move a tiny bit front to back. I would put a strip of tape at the 12 o clock position "cylinder head" part of the f2 engine to 1: move the nozzle down slightly and 2: stop the engine moving back and forth. I would always shim the engine as far forward for maximum nozzle to bucking airseal. I guess I can shim in the rearward position and see if there is any improvement but there really is so little movement that I am not optimistic. I never have had a BB roll out on me when hop is applied.

Thanks for your responses guys but I have spent a LOT of time on this problem and googling around. Ill have another look at it but I am feeling like I need a fresh set of eyes on this problem.

All the best!

[ThunderCactus]

Honestly, if it's an Rhop/nozzle issue, just get a stock barrel from someone with a G&G green rubber or something and try that out.
If your groupings improve, you know what the issue is for certain.