FX Impact - accidental tuning discovery - extra FPS

[frank320]

I removed my FX Impact X barrel to clean, and as I was putting it back, i realized that once the barrel goes into the slot, there is some minor "rotation" play. Since this aligns the transfer port, i figured every small rotation will ever so slightly align/mis-align the transfer port between the barrel and the port.

So, I shot over the chrony as i experimented with tightening the barrel at both ends of the "rotation" and realized there is a "better" alignment end. The counter-clockwise end gave me an extra 8fps than the clock-wise rotation end consistently on an average of 10 shots over chrony. I think, for my FX Impact X 700mm barrel, setting the barrel counter-clockwise before tightening the barrel screw aligns the transfer port better than the other directions.

Your barrel might be different, but i gather it is worth trying it out for your own Impact/Crown to get more fps and consistency. Don't even need to remove barrel, just loosen the barrel retaining screw a little, rotate and shoot over chrony to check fps, find the most FPS "setting" and re-tighten. That's it.

 

[BeemanR7]

I suspected this after cleaning my barrel and then shooting over the chrony. My FPS was down about 7 FPS for no reason other than that the barrel was not perfectly aligned with the TP. I haven't yet taken the time to locate the perfect position, but, it will be worth the effort when I do.

 

[ackuric]

Good tip/find. Likely applicable to many makes/models if you're out to squeeze every last FPS out of your rifle.

 

[frank320]

Good tip/find. Likely applicable to many makes/models if you're out to squeeze every last FPS out of your rifle.

Thanks. I figured it would help with efficiency too. Mis-aligned transfer port wastes energy with inefficient airflow.

 

[Bob_O]

You guys are pulling the entire barrel from the breech block to clean?

 

[Funshine]

Thanks, that is good to know. I have the exact gun, however, I do not remove the barrel to clean as I made a pull through that works great.

 

[frank320]

You guys are pulling the entire barrel from the breech block to clean?

I am because I am using bore-snake and the cylindrical brass weight can't fit thru the breech housing.

 

[frank320]

I just ran a few chrony strings across the lower power settings on my impact with the better aligned port - got an average of 15-20 fps increase at the Min/1/E power wheel settings. Goes to show how much more efficient a properly aligned transfer port is to power and efficiency. 

 

[Funshine]

frank320, what cal do you have? I have a .25 shooting JSB's 33.95gr. What is your most efficient setup when your gun is in "harmony" 

 

[frank320]

I have an FX Impact X 700mm 25 cal tuned for high power/FPS so i can reach over 125 yards. I had AOA tuned it for 865 fps when it came to me. I have since tuned it a little higher. Regulator currently set at 145bar. Shooting 34gr JSB MK2 at 895fps at Max power wheel setting - I was at 885fps this morning until this accidental transfer port discovery gave me an extra 10fps or so.

I get a 10/15 fps drop at each successful lower power wheel setting from Max to 3, and a steeper drop from 3 to Min of about 25/30fps per lower power wheel turn, which is nice.

I cannot say if my Impact is at the most efficient(probably not due to high FPS/power), or in harmony as per Ted Bier since I lack the tuning experience and 10,000s of pellets these guys go thru. But am getting absolute 1-holer at 30yards in my basement and have no problem hitting 2.5inch steel plates out to 130 yards or more.

 

[Bob_O]

You guys are pulling the entire barrel from the breech block to clean?

I am because I am using bore-snake and the cylindrical brass weight can't fit thru the breech housing.

Just curious as to why guys do things differently, as I have changed the way I do things after hearing new ideas......why not just pull the liner? That's what I'm currently doing when I don't want to use a pull-thru.

 

[ackuric]

Good tip/find. Likely applicable to many makes/models if you're out to squeeze every last FPS out of your rifle.

Thanks. I figured it would help with efficiency too. Mis-aligned transfer port wastes energy with inefficient airflow.

I concur, having the best possible path for air flow is critical! A slightly offset port or misalignment will definitely cause a slight hiccup there

 

[ackuric]

frank, have you ever shot below set point? I am curious if your velocity has a rise to it and if so how much. That would indicate theres more left in your current tune just your spring/hammer combo is maxed. According to my calcs 895 fps is plenty under your plateau and your gun should be operating fairly efficiently for its power level. However if there is a huge rise (3%+) in fps as you come off reg, I would advise lowing the set point down a tad. and re-tuning to your desired FPS.



Not sure of the porting on impacts. But a 27.5" barrel @ 145 bar shooting 33.95's should yield between 950-1000 fps plateau (depending on port size), provided there is adequate plenum space to keep the pressure up enough while the valve remains open. I believe Ernest achieves around 1k~ fps with his 700mm barrels @ 145-150 bar. Ideally you wanna tune around 2-5% below the plateau to increase shot count when coming off reg, which provides a slight bell curve no worse than the % below your plateau, until fps drops below nominal. 

 

[frank320]

ackuric,

Interesting and valuable information. I will shoot over chrony keep track as it goes off reg the next time. It would definitely be nicer to be able to shoot at lower reg pressure and still get the speed and power I am getting now...

Many thanks.

 

[Funshine]

I currently have mine tuned at approx 865fps with the JSB's MK11's 34gr. I am far from Maxing out my hammer. Reg is at 125 (fx gauge), valve stop is at about 3 strokes, and the Power is at "4" . I can get approximately 85 plus shots going down from 250 to 125 bars more or less. The gun hardly makes just a slight sound as Ted explained in his harmony settings. It has been very accurate so far. With the 25gr's I dial the power down to "1" and it shoots quite accurate, too. Initially from AOA, I got 900 fps with the 25's, but I adjusted it for the 34's. I got around 985fps with the 34's at the beginning of my settings (at Max). However, I tested and tried until I set it to 865 ish. I should say that past 950fps, I could feel a recoil on this gun. 

 

[frank320]

Wow, that is a good tune. I can't trust my FX gauges, the bottle gauge on mine differs from my reg pressure by over 30bars - yes, you read that right, 30 bars! I have a feeling my reg pressure is closer to 140, but even will that, I am getting about 65 shots from 250bar before it falls off the reg.

I will follow ackuric's advice and monitor over a chrony the next time it falls off the reg to see if i can lower it. 

Separately, i much prefer shooting, pesting and plinking with my son than to sit in front of the chrony to extract the perfect tune, if you get what i mean. As long as my compressor does not fail me, efficiency is secondary...

 

[Funshine]

I tried moving the barrel, but there was not too much play and the fps remained the same. frank320, you may want to play with the hammer adjustment screw behind the power wheel. At max, my adjustment is about 17.10 mm and at power level 5, 4 etc. it will increase the distance

.

 

[Revoman]

You guys are pulling the entire barrel from the breech block to clean?

I am because I am using bore-snake and the cylindrical brass weight can't fit thru the breech housing."

I fixed that by inserting a brass beaded chain end on my bore snake to enable the tight turn at the breech. I opened up the brass end, cut it off to just longer than a couple of beads and pressed it back over those couple of beads. When I drop it through, I just have a bent paper clip to fish it out of the magazine opening. I then use a plastic shotshell hull cut straight on one end and on an angle at the other end and push the beaded chain through that to partially hold it against the magazine opening so the 'rope' won't mar the bluing. It works great.

 

[BDX]

I tried moving the barrel, but there was not too much play and the fps remained the same. frank320, you may want to play with the hammer adjustment screw behind the power wheel. At max, my adjustment is about 1710 mm and at power level 5, 4 etc. it will increase the distance

.

Funshine, I was totally thrown for a sec when I read 1710mm (17.1 cm)...What!!?? and I started thinking "now wait just a second here...Funshine's FX has got a helluva lot more adjustment than I do!...I've been shorted!"

George 

 

[Funshine]

BDX, I think that I typing too fast and missed the decimal point. I corrected it 17.10mm. Thanks for the heads up.