FX FX Impact M4 22 cal 700mm velocity issues

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Camsleadslinger

Member
Jul 20, 2024
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Perth, Western Australia, Australia
Hi all,

I’m having a peculiar issue with my M4. Previous week I ended shooting at 120bar pressure that was yielding approx 915fps with 27gr .218 H&N slugs. Fast forward to the next week and the rifle starts out at 830fps and over the next 40 shots very gradually climbs back up to 915fps. It then goes up and down maybe 10-20fps for the next 20 shots and then stabilises. Extremely confusing considering the electronic sekhmet gauge is showing a consistent 120bar. M4 has had about 500 slugs through it total.



Macro wheel 16

Micro wheel 4th line

Valve 4 lines showing

120bar

Superior STX heavy liner 1:16

I would be keen to hear if anyone else is having similar issues.
 
I had the same issue with my brother’s M3 .22 700 mm when it was tuned to 95% of max velocity. After many hours of experimenting I found that tuning to 98% gave the best results…no slow first shot(s)! But at the expense of less air efficiency/decreased shot count. Btw the m3 was shooting .218 27.5gr nsa’s at 935fps. Second reg set to 119bar and micro ~4.75, VA was used in the end to shave off ~2-3fps to improve accuracy and regain minute amount of efficiency. The only true fix for the phenomenon you’re experiencing is the custom plenum piece that a forum member here on agn sells. So sad that FX has not fixed their design flaw after 2-3 generations of impacts.
 
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My m3 had a less intense version of this with the stock hammer. After the tungsten hammer and huma regs it's a non issue but the gun has to run at higher state of tune and shooting lighter stuff gets challenging. The vast majority of these issues can be worked out, the gun is just a tuners platform, not an out of the box miracle as the youtube infomercials would have you believe.
 
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Tuning - FX Impact Mk2/M3 - slower first shot - no reg creep - quasi-scientific analysis

Hi guys, Welcome everybody. This is my first post here. I have a problem with my Impact Mk2 PP .30. The reg pressure is 145 bars. When I shoot shot after shot then my v0 is 260m/s. However, when the gun sits for a couple of hours then the first shot is 250m/s despite the exact same reg...
www.airgunnation.com www.airgunnation.com

Here is a whole topic discussing it. Same for M4.

It is essentially a design flaw that causes the O-ring to extrude on the valve stem.
 
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Buck_Wild said:
I had the same issue with my brother’s M3 .22 700 mm when it was tuned to 95% of max velocity. After many hours of experimenting I found that tuning to 98% gave the best results…no slow first shot(s)! But at the expense of less air efficiency/decreased shot count. Btw the m3 was shooting .218 27.5gr nsa’s at 935fps. Second reg set to 119bar and micro ~4.75, VA was used in the end to shave off ~2-3fps to improve accuracy and regain minute amount of efficiency. The only true fix for the phenomenon you’re experiencing is the custom plenum piece that a forum member here on agn sells. So sad that FX has not fixed their design flaw after 2-3 generations of impacts.
Click to expand...
Can you please link me to the member who makes this custom plenum piece?
 
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Camsleadslinger said:
Hi all,

I’m having a peculiar issue with my M4. Previous week I ended shooting at 120bar pressure that was yielding approx 915fps with 27gr .218 H&N slugs. Fast forward to the next week and the rifle starts out at 830fps and over the next 40 shots very gradually climbs back up to 915fps. It then goes up and down maybe 10-20fps for the next 20 shots and then stabilises. Extremely confusing considering the electronic sekhmet gauge is showing a consistent 120bar. M4 has had about 500 slugs through it total.



Macro wheel 16

Micro wheel 4th line

Valve 4 lines showing

120bar

Superior STX heavy liner 1:16

I would be keen to hear if anyone else is having similar issues.
Click to expand...
Send them to UA under warranty. Will not cost you a dime. It’s a known issue and while it is similar to the M3 issue, it does not seem to be the same. Some M4s are affected way worse. They’ll change valve rod, valve house and some o rings. There are people on here that did that and report that gun is still down after the fix with multiple 1000 shots through it since then. Or ask them to send you the parts and put them in yourself. Hopefully it works…
 
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Camsleadslinger said:
Can you please link me to the member who makes this custom plenum piece?
Click to expand...
88 turbocoupe makes the plenums on a 5 axis CNC machine look him up on eBay Mudpro1 he also has coated valve rods and his own o rings he had made
also, his plenum uses a thumb screw on the top looks factory.
i have one in my M3 never been so consistent,
 
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The machine said:
88 turbocoupe makes the plenums on a 5 axis CNC machine look him up on eBay Mudpro1 he also has coated valve rods and his own o rings he had made
also, his plenum uses a thumb screw on the top looks factory.
i have one in my M3 never been so consistent,
Click to expand...
I am going to have to look him up and thumb nail it so if my M3 starts acting up I can remedy the situation!! I followed that Thread and it kept me from buying an M4. I bought a Maverick and now a M3, pick it up today.
 
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