G&P M4 Issue - FPS 180 ~ 300
 

Hello All,
Hope someone can chime in and give me some ideas where could be wrong.
I recently purchased a new G&P M4. I had the a new spring swapped in to lower the FPS, it was a M90 spring.

Yesterday I took the gun out for a game, it chrono'd at 180 to 190. Later I chromed it at about 300... and then again it chrono'd at 180 ish.

What could be causing this inconsistency? Is the spring a poor fit for this gun?

Thanks in advance

It's probably not the spring but the compression parts.

Make sure you don't use this gun until you have solved the issue, because certain types of compression failures can cause piston stripping (especially ones where you shoot a really low fps and then sometimes two BBs come out at the same time, etc).

Who did the downgrade for you? If it was a retailer that sells G&Ps, take your gun back and insist that they get the compression stable and chrono'ing across a whole mag before they give it back to you.

Buy a new air nozzle
The stock air nozzle works great until you take it apart, then you can never get it lined up perfectly ever again lol

Haha, c'mon man, you usually give great advice, you can do better than this ;)

The G&P stock nozzle is actually a great nozzle and has an o-ring for a better seal around the cylinder head. I've seen it give great compression even with lots of play on the tappet plate.

ThunderCactus, you do you really advocate buying a nozzle every time you go into a stock G&P gearbox ? :) Sure G&P internals are psycho-crazy but...

I stand by my original advice. Go to the place you bought this from and have their gun doctor re-assemble the compression parts until he sees consistent results through the chrono. Insist on a demo before you leave the store.

they've got a .02-.03" chamfer on the end, they're practically square ended. Unless they're perfectly aligned to the hop rubber, they don't seal very well.
Normal air nozzles have much larger chamfers on the end so even if they're misaligned, they'll still find the center of the rubber and seal.
The G&P nozzles are good, but it's too finicky.

Oh and if it's brand new, you should make sure the gears are shimmed properly before you game with it, otherwise you might break a shaft.

This should go into a "how to have your beautiful G&P rifle not explode at its first game like all these other ones did" FAQ

:D

For the amount of times it's been said it should be common knowledge by now lol

Based on the number of reports on the net, I always get the impressions that reshimming the gears will just be delaying the inevitable destroyed shaft.

that's right, but sometimes it's the difference between 2 games and 40 games.
I had one go 3 years and 300,000 rounds without breaking a shaft, all stock parts lol

The nozzle is maintained in a forward position by the spring inside the gearbox that either pushes or pulls on the tappet plate. The nozzle can "stick" a little in the hop up chamber due to a variety of reasons (slight misalignment, foreign particles=dirt) thus preventing it from sealing against the bucking as it should. I LITTLE spray of silicone oil "up" the hop up (turn the AEG upside down, so you are spraying down) does wonders in my experience.

well, I took it back to the shop today. when they chrono'd it, at first the problem came up... shooting at around 180 and than 300 back and forth. Then it stabilized at around 330 fps - so they told me it may because the gun is cold, and once it warms up, the o-ring will create a consistent seal. I thought it was okay so I left and went to the game tonight.

I ran through 2000 bbs tonight, and till the last game, it did not shoot consistently.

It would be back and forth between 180 ish to 300 ish, sometimes it blank fired.

I'm taking the gun back to do a full compression test next week.

Amos Airsoft how to: AEG Compression test - YouTube

:)

thanks for the video... I might give it a shot tomorrow and take apart the gun

Unless you have a whole new compression set to throw in there, or have extensive experience in debugging hopups, piston o-rings, etc, you will get more mileage out of this by taking it back to the shop.

To be blunt, the excuse about cold is nonsense. Myself and several other team members just gamed a bunch of guns, including ones with G&P mechboxes, at a game that averaged -12C yesterday and they all had absolutely fine compression.

You should politely insist that the gun is not working properly and ask them to stabilize the compression parts and not just keep shirking away from the supporting their own guns. Don't accept the cold temperature excuse. People game their AEGs in very cold weather. Ask the guys on this forum from Calgary who have gamed in -30C temperatures.

Side note:

I am really thinking that we need to start to maintain a highly-visible list of outright lemon brands. G&P is an outright lemon brand. Every G&P I personally have ever seen simply did not work out of the box properly, and needed to go through hours of work and/or expensive changes before shooting correctly. Compression issues, snapped gear shafts, defective hopup units, stripped bevel gears, all on stock guns.

Hell, maybe the OP should ask for a KA M4 as a replacement.

Well My Guns compress fine in cold as well, but it's primarily due to the fact I use high performance grease.
I know for a fact the poorer quality grease gets very stiff in cold temps and affects compression.

G&P isn't GREAT out of the box for sure, it's half decent, I'd say on par with CA for performance.
I've seen them break first game, but shimmed out of box they consistently last a full season unless the piston strips. And I've seen one last 3 years and 300,000 rounds with zero maintenance and zero breakdowns.
But keep in mind people who buy G&P's don't buy them for the internals, they buy them for the EXTERNALS.
Internals are easier to upgrade than externals, and people who buy G&P's immediately put $350 upgrade kits in them, because it's honestly one of the best platforms to upgrade.

But I do agree the KA is a REALLY excellent alternative to G&P.