My M3 performed flawlessly for the first couple months after I purchased it. I had it tuned to 1/2” groups at 50 yards with the 2nd reg at 150bar and velocity at 990fps +/- 3fps. Every time I took it out it was dialed in. But I did noticed that whenever I took it out after it had been sitting for a day or two that the first 5-6 shots were slow. They would be something like this: 930, 950, 960, 970, 980, 990. I figured I could deal with this. Before hunting I would dry fire the gun around 6 times.
I took a break from shooting during the past 2 months. I did not make any changes to the gun during this time. It just sat in its case. I took it out to shoot. I checked to make sure that the reg pressure, valve and hammer were all set to where they were they had been when the gun was dialed in at 990fps. The first shot was 850fps! Over the course of 20 SHOTS the velocity eventually climbed to 990. I thought maybe things just needed to get warmed up after being unused for so long.
But then I tested a shorter time interval between shots. I took 10 a minute break, then the took a shot. 960! After just 10 minutes a drop of 30fps! Then over the course of 5 shots it climbed back to 990. This is not how it behaved before. If I had taken a 10 minute break 2 months ago, there would be no change in velocity.
The next day I took the gun out again and the first shot was 930fps. 15 more shots and it’s back up to 990.
Any ideas guys?
UPDATE: PROBLEM SOLVED SOLUTION(S):
I did several things, all addressing the issue of friction between the valve rod & plenum nose o-ring. I made all of these changes at the same time because I didn’t want to disassemble the rifle, make a change, reassemble, let the gun sit overnight, test, then repeat the process for each change. As a result I’m not sure which change had the most significant affect on resolving the issue, but all of these changes could only help resolve this problem in the present and prevent it from happening again as quickly in the future.
For reference there are 2 images of the valve rod and valve tube assembly schematic below.
Here’s a link to the entire FX Impact M3 schematic:
https://fxairguns.com/wp-content/uploads/2021/04/F10370-Impact-M3.pdf I disassembled the gun down to the valve rod and on the 2 inch section that slides in and out of the Valvetube End (part 10338) there was a grippy grey residue. This was from the o-ring (part 19768, O-ring 2.84x2.62 PUR90) in the valvetube wearing away onto the valve rod, perhaps due to just normal use, not enough lube being applied at the factory, o-ring shrinking overtime - likely multiple reasons.
I did several things to reduce valve rod friction:
- DEBURRED the ends of my HAMMER SPRING with a rotary tool. Some hammer springs have burrs on the ends that can cause inconsistent velocities. Not all springs have these burrs, but if you notice that yours does, then sand them down with a rotary tool, fine file, or sand paper.
- CLEANED o-ring residue on the valve rod with rubbing alcohol.
- POLISHED the valve rod: Used a polishing compound and microfiber cloth to polish the valve rod until it had a shinny mirror-like finish. Took about 10 minutes. I used Flitz Multi-Purpose Polish and Cleaner, available on Amazon: https://www.amazon.com/gp/product/B000MUSOW4
- UPGRADED THE O-RING THAT THE VALVE ROD SLIDES THROUGH/AGAINST: I purchased an improved valve rod o-ring, which is made from a harder and smoother material than the stock o-ring. Here is a link to where you can buy this o-ring: https://www.ebay.com/itm/267168330592 For reference it is O-ring 103 (part 19768) in the schematic below.
- LUBED THE UPGRADED O-RING with SuperLube Synthetic Grease rather than silicone oil. The maker of the ungraded o-ring said that SuperLube keeps this o-ring lubed better and longer than silicone-based lubes. Here's a link to the specific SuperLube he recommends: https://www.amazon.com/dp/B000XBH9HI?ref He said that he uses this SuperLube on ALL of the o-rings in his M3 because, again, he has found it to keep the o-rings better lubed and lubed for longer.
- (NOTE: The last step I took below is likely unnecessary, and the above steps will probably solve the problem for most people)
- UPGRADED THE VALVETUBE: I purchased a custom valvetube from AGN member mubhaur. This valvetube has a PEEK SPACER/BUSHING inside the valvetube next to the o-ring (part 19768). The valvetube that comes stock on the M3 doesn’t have a threaded end that allows it to be removed so that the two o-rings inside the valvetube can be easily accessed. So it was difficult to replace the 2 o-rings inside of FX's valvetube. The stock valvetube also doesn’t have a high strength peek spacer/bushing to support the sealing function of o-ring 19768. The peek bushing also ensures that there isn’t any metal-on-metal contact between the valve rod and the valvetube. He explains the rational for his design in the video below at the end of this post.
After these changes, when the gun sits overnight, not even first shot is slow. The fps is exactly what it was when I last shot. Velocity is also more consistent. Fps spread used to be +/- 6fps. Now it’s +/- 2.
As described in the original post, before these changes the first 20 shots would be slow: starting at 100 fps slow and gradually increasing with each shot. And if the gun sat for just 10 minutes the fps would drop by 30 fps.
***MORE INFORMATION***
The seller of these o-rings
https://www.ebay.com/usr/mudpro1 is very knowledgeable about this issue with the M3/M4. He answered all my questions regarding how this o-ring could make a significant difference. He owns an M3 Impact and was experiencing this same problem. FX didn't offer him any help after months of contacting them, so he decided to make his own fix. He is a machinist who makes precision parts for a living, so he has access to many manufacturing resources. He had these o-rings made specifically for this issue with the M3/4. These o-rings are harder and smoother than the stock o-rings.
The seller of this upgraded o-ring also makes an upgraded valvetube that utilizes a peek bushing held in place by a thumb screw, which results in less o-ring compression and ensures that there is no metal-on-metal contact between the valve rod and valvetube. I didn't purchase his valvetube because the upgraded valvetuve that I purchased from AGN member
mubhaur has been working very well now that I have the upgraded o-ring. Both of these improved plenums have a similar design, incorporating the peak bushing and a removable screw for easy access to the inside of the valvetube. The ebay seller does not have his custom valvetube listed on ebay yet. Message him directly on ebay if you'd like to purchase it, and to ask him questions. He's very helpful and knowledgeable.
So until FX provides us with a redesigned plenum nose, more durable valve rod o-rings, and a smoother valve rod, those of us who are experiencing this issue with our M3s and M4s should do these things:
1. Deburr the ends of your hammer spring if you see a burr.
2. Polish our valve rods (see the update to the main post for how to do this. $15 for polishing compound)
3. Upgrade our valve-rod o-rings ($12 from ebay seller mudpro1)
4. Lube the o-rings with SuperLube Synthetic Grease (around $7
https://www.amazon.com/dp/B000XBH9HI?ref)
5. If the above 4 don't fix the issue to your satisfaction, then consider buying a redesigned plenum nose from either
mubhaur or
https://www.ebay.com/usr/mudpro1 Both of them charge around $225-250 for this piece.
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