fx impact x .25 Cal heavy over 880fps anyone?

For comparison, I have a 25 cal FX Impact X 700mm, Gen 3, with the latest AMP regulator. The fastest I can get the 34gr JSB MK2 is about 890fps average with the regulator set at about 148/150 bars, for about 60fpe. I think I have it maxed out here. Anything more would probably require power mods/upgrades from Ernest.



On a stock 25 cal FX Impact X 700mm, I just don't see how it can shoot 34gr JSBs above 900fps without modifications. Since you have a 25 cal X 600mm, I am going to guess that the 34gr JSBs would max out around 860fps?

I own a gen 2 impact orig st barrel huma reg set about 145 bar h/s not maxed or valve stop and get 880fps and had it over 910 but bact it off a bit.Thats .25cal 34gr JSBs

I can get 880fps from my Crown ST-X 600mm with the 34gr.

Have you tried tweaking the hammer adjustment screw? Maybe MAX with no slack is too much tension?

Just a thought.

I'm no master tuner but i think Bob O is correct. i started with max h/s and valve stop and as i backed the h/s the speed increased same with the valve adjustment.

It's not an Impact, but two days ago I got 890 fps with 33.9 heavys MK1s. I adjusted the regulator to about 155 bar.
My crown is a 3rd gen one with the AMP regulator. It was creeping about 5 bar but the accuracy was very good, so I can't complain.

Gen2 impact with upgraded .25 700mm X barrel shooting 34gr jsb at 918fps max. Hammer screw dial max, valve adjustment barely showing 4th line, and Reg at 154/155 bar.

You should be able to exceed 880 fps with a 500mm barrel, you don't *need* a 600 or 700 mm barrel...



Mods required to hit 880+ fps on 500mm barrel with 33.95gr (jsb heavy):

85%" bore porting from valve (.212" / 5.4mm)

.25" / 6.4mm Throat 

1/2 CC per FPE of plenum

150 bar of regulated pressure



Or alternatively you could do

90% bore porting

6.6 mm throat

1 CC per FPE of plenum

135 bar of regulated pressure



The above requires custom parts / work that most don't want to endure. Adding barrel volume reduces some of the above requirements.

You should be able to exceed 880 fps with a 500mm barrel, you don't *need* a 600 or 700 mm barrel...



Mods required to hit 880+ fps on 500mm barrel with 33.95gr (jsb heavy):

85%" bore porting from valve (.212" / 5.4mm)

.25" / 6.4mm Throat 

1/2 CC per FPE of plenum

150 bar of regulated pressure



Or alternatively you could do

90% bore porting

6.6 mm throat

1 CC per FPE of plenum

135 bar of regulated pressure



The above requires custom parts / work that most don't want to endure. Adding barrel volume reduces some of the above requirements.

Click to expand...

I believe all Impacts have a barrel transfer port larger than 90% bore yet they still respond well to elongating the port and opening the pellet probe even larger.

There can be port alignment issues but they're easy to solve by loosening the barrel set screw, rotating the barrel slightly, snug the screw back down and test over a chronograph for max velocity.

I just purchased a 600mm .22 barrel for my Impact. Elongating the barrel transfer port and pellet probe along with lapping the barrel with 1200 grit compound allowed me to lower the regulator by 25 bar and keep the same velocity. 18.5gn H&N at 1010 fps on 120 bar.

Stupid trick time. I've heard, but not validated that sometimes when you insert the barrel, there's a teensy bit of play side to side in how you position the barrel. Sometimes, if you twist the barrel to one side, the transfer port lines up better and allows more air to go through to the barrel. Might give it a try to see if it might be a factor.

Now, having said that, I find my best accuracy in the lower 800's like 830-ish. I'll take accuracy over power any day.

Keep us posted on a solution though

Now, having said that, I find my best accuracy in the lower 800's like 830-ish. I'll take accuracy over power any day.

Click to expand...

You should try polishing the barrel. I haven't setup to shoot off a bench yet but I'm hitting right where I aim on trees at 50 yds or so. I'm pretty sure I'd hit a dime steady.

What's your procedure on that h-i?

What's your procedure on that h-i?

Click to expand...

For barrel polishing I depressurize and remove the barrel, take the shroud off and remove the breach o-ring. If you're doing any port work do that first so the polishing can remove any sharp edges. Give the barrel a thorough cleaning and dry the bore. I use a carbon fiber cleaning rod that turns on ball bearings in the handle. If your bore doesn't look too dull you can go straight to 1200 grit non-embedding lapping compound on a bore mop. A jag with tight fitting patches can also be used but it's a little more difficult keeping the patch on if it exits the muzzle. Add enough lapping compound to cover the bore mop and work from the breach. The mop will be hard to get started in but keep pushing and it will go. It helps to clamp the barrel in a padded vise to free up both hands. The choke will stop the bore mop before it fully exits the barrel so keep stroking about 80 to 100 strokes. After about 20-30 strokes you may need to add more compound. When you're done the inside will have a mirror like finish. Now clean it again until you've removed all the lapping compound and reinstall the breach o-ring.

If you want to elongate the port, I start with a chamfer tool in a drill on the smaller calibers since the barrel transfer port is smaller than the port that feeds it. Open it up with the chamfer tool stopping just short of the rear o-ring slot. With a cordless rotary tool and a carbide burr at medium or low speed start making the port pear shaped with the small end toward the muzzle. Don't make the port wider, just elongate it by about 10% and blend the chamfer in so the port funnels. After that remove the pellet probe and open it up a little larger than the transfer port and blend the sides of the hole smooth. You can't go wrong on the pellet probe as long as you keep the face of it square (where it pushes the pellet). Open it up as much as you dare. A new barrel inlet (brass part) and pellet probe costs around $30 from FX USA if you ever wanted to return to original or you made a mistake.

You should be able to exceed 880 fps with a 500mm barrel, you don't *need* a 600 or 700 mm barrel...



Mods required to hit 880+ fps on 500mm barrel with 33.95gr (jsb heavy):

85%" bore porting from valve (.212" / 5.4mm)

.25" / 6.4mm Throat 

1/2 CC per FPE of plenum

150 bar of regulated pressure



Or alternatively you could do

90% bore porting

6.6 mm throat

1 CC per FPE of plenum

135 bar of regulated pressure



The above requires custom parts / work that most don't want to endure. Adding barrel volume reduces some of the above requirements.

Click to expand...

I believe all Impacts have a barrel transfer port larger than 90% bore yet they still respond well to elongating the port and opening the pellet probe even larger.

There can be port alignment issues but they're easy to solve by loosening the barrel set screw, rotating the barrel slightly, snug the screw back down and test over a chronograph for max velocity.

I just purchased a 600mm .22 barrel for my Impact. Elongating the barrel transfer port and pellet probe along with lapping the barrel with 1200 grit compound allowed me to lower the regulator by 25 bar and keep the same velocity. 18.5gn H&N at 1010 fps on 120 bar.

Click to expand...

I don't believe OEM FX valves are full 90% borted from valve seat to pellet base....Even if the barrel port somehow itself is 90% of bore, the valve throat is not big enough to flow that, nor is the entirety of the valves flow path based on my limited findings. I highly doubt the OEM pellet probe allows for 90% of bore porting flow..


The plenum is certainly not 1/2 - 1 cc of volume per FPE. 

Thank you h-i!

Click to expand...

I don't believe OEM FX valves are full 90% borted from valve seat to pellet base....Even if the barrel port somehow itself is 90% of bore, the valve throat is not big enough to flow that, nor is the entirety of the valves flow path based on my limited findings. I highly doubt the OEM pellet probe allows for 90% of bore porting flow..

The plenum is certainly not 1/2 - 1 cc of volume per FPE.

Click to expand...

The OEM valve is 5.5mm, everything past that is much larger aside from the .177 and .22 barrel transfer ports. The barrel transfer port in the barrel inlet is what I was referring to. I believe those are about 92% on all calibers but the pellet probe area is smaller. I thinned the pellet probe and removed the floor of it on my .30 so it's horseshoe shaped. I have extras to experiment with.

Click to expand...

I don't believe OEM FX valves are full 90% borted from valve seat to pellet base....Even if the barrel port somehow itself is 90% of bore, the valve throat is not big enough to flow that, nor is the entirety of the valves flow path based on my limited findings. I highly doubt the OEM pellet probe allows for 90% of bore porting flow..

The plenum is certainly not 1/2 - 1 cc of volume per FPE.

Click to expand...

The OEM valve is 5.5mm, everything past that is much larger aside from the .177 and .22 barrel transfer ports. The barrel transfer port in the barrel inlet is what I was referring to. I believe those are about 92% on all calibers but the pellet probe area is smaller. I thinned the pellet probe and removed the floor of it on my .30 so it's horseshoe shaped. I have extras to experiment with.

Click to expand...

Right, you're applying this % to your .22 cal, and not the OP's .25 cal... 5.5 mm is 5.5/6.35 = 86% of 25 cal, and for a 30 cal its 5.5/7.62 =72%...and that is just the valve, not including the throat with stem, or barrel with probe..which is considered part of the porting. I believe both the throat and probe area on fx valves don't have equivalent cross sectional area of the valves exit @ 5.5mm.


My point(s) remain valid for the op, for a .22 cal however, well, you're on the right track yourself with modifications, and the plenum volume in most FX's is likely sufficient enough for the energy most .22 caliber shooters aim for...however if one were want to build a slug shooting .22 cal that puts out 60-80 FPE, you certainly would want to consider a custom plenum between 30-80 cc's of volume. FWIW...

-Matt

Right, you're applying this % to your .22 cal, and not the OP's .25 cal... 5.5 mm is 5.5/6.35 = 86% of 25 cal, and for a 30 cal its 5.5/7.62 =72%...and that is just the valve, not including the throat with stem, or barrel with probe..which is considered part of the porting. I believe both the throat and probe area on fx valves don't have equivalent cross sectional area of the valves exit @ 5.5mm.

My point(s) remain valid for the op, for a .22 cal however, well, you're on the right track yourself with modifications, and the plenum volume in most FX's is likely sufficient enough for the energy most .22 caliber shooters aim for...however if one were want to build a slug shooting .22 cal that puts out 60-80 FPE, you certainly would want to consider a custom plenum between 30-80 cc's of volume. FWIW...

-Matt

Click to expand...

I think the best way to increase plenum space on an Impact is to make a taller rear block with additional plenum space under the valve. It's hard to get enough flow from an add on.

For a .216" (5.5mm) porting with a .118" (3mm) valve stem you would want a throat opening of .25"~ or 6.4mm to accommodate the valve stem going through it. Bolt probe (for a .25 cal) would have to be equally .118" (3mm) or smaller in diameter, or .118"/2=.059" (1.5mm) walled for a flow through. 



-Matt