Power Plenum FX Impact Tuning for 50grain Pellets

[sgrayner]

After watching Giles’ video on the new Power Plenum FX Impact, I wanted one and wanted to shoot the heaviest JSB .30 pellets I could push through, the 50.15 grain Exacts. The rifle arrived from Utah Air with the regulator set at 110bar. I assume that was a good setting for the 44 grain JSBs. I’d watched tuning videos from Meathead Marksman (Uncle Hodge), Steve Scialli at AEAC and Ernest Rowe and was starting to get an idea on how to proceed. I started out opening up the valve adjustment to full open, adjusting the regulator to 120bar, closing up the hammer spring adjustment screw to max, and starting the shoot-chrony-graph of opening the hammer spring screw to balance hammer force with plenum pressure. I quickly figured out I was going to end up well below my 880fps tuning goal. I cranked up the regulator to 130bar and was getting 920fps with the hammer maxed out and the air valve full open. Went through the hammer balancing again, and finished that with the chrony at about 900fps. At this point the air noise was reasonable, but I was still hearing metallic pings from the valve. Started tweaking the air valve gradually down. The pings went away and I stopped closing the valve at 880fps. I just finished running a couple mags through and everything looks stable at an average 881fps, max spread of 7fps and a standard deviation of 2.4fps.

Utah Air was a pleasure to deal with and Josh helped with my tuning confidence. A couple of things to check on arrival, though. The barrel wasn’t fully seated (the new barrels don’t have the set screw dimple). Easily noticed this when the magazine pushed through to the left side of the rifle. Easily fixed. After a few mags of tuning, the pellet probe block set screw had backed out. I caught it before any disasters happened. I hadn’t checked it on arrival, but maybe one should do so. Other than that the rifle works perfectly and I couldn’t be happier with how it adjusts.

Steve

 

[sonny]

nice! How many shots on reg?

 

[sgrayner]

Hammer spring adjustment addendum:

I ended with a balanced and optimized hammer spring adjustment at 14 turns off of max closed. "Turns" is an approximation of how far one can turn the screw with a 1.5mm hex wrench in the available space before reaching the metal of the receiver (about 1/8th turn). "1.5mm hex wrench" is an approximation as well apparently. I went through four "1.5mm" wrenches before I found one that would actually fit the adjustment screw. The final winner was measured at 1.45mm by caliper. Here's my chrony numbers after each "turn".

0 924

1 903

2 913

3 921

4 899

5 912

6 914

7 891

8 909

9 912

10 910

11 907

12 910

13 898

14 900

15 891

16 888

17 886

18 882

19 879

20 874

21 871

22 869

The hammer spring was too strong on turns 0-13. From turn 14 on, there was a gradual and predictable decrease in speed with each turn off of max. I'm pretty sure every rifle is going to have a different optimum hammer with every plenum pressure. The above is provided as a guide and this procedure is certainly only one of many ways to get there.

Steve

 

[sgrayner]

Shots on regulator is still unknown. It got dark outside so I'll get back later. Also, I opted for the 580cc bottle, not the 480cc standard.

Steve

 

[Vana2]

Nice! Glad that everything went smoothly for you!

Yeah, it's a good to go over anything mechanical and check that the hardware is secure and all is in order - I used to fly (single engine) planes and you always do a walk-around and check things - still do that, it's often enough that toys get left on the driveway behind the car.

Per your comment on the hex wrench, I got a (medium quality) set for Christmas and found that the ends of the hex wrench had small burrs on them that needed to be removed - might explain why some didn't fit.

 

[DShepard]

Mine came today. It's a .25/700/580. It was set st 110 bar also pushing 25.39's@900fps. If initial impressions are anything to go by, this gun will exceed my expectations. Especially moving up from a Marauder. 

 

[igolfat8]

So how are the 50’s grouping at 100 verses the 44’s?

 

[MonsterMaxx]

While you will get a little more thump from the heavier pellet, the 44gr flys better at longer distances (beyond 50yds.) Frankly I'm not too impressed by 50yd groupings. Stretch it out to 75-100yds and let's see what happens.



W/ my impact I was pushing 1000fps and it was pretty obvious the pellet was tumbling at longer distances - paper all torn to shreds instead of a nice hole and I could almost see it in the air. So I slowed it down in the 880fps range and it tightened up. The 44.75gr did quite nicely at 85yds a couple weeks ago - 5 shots under a Quarter. I was never able to replicate that with the 50gr. Even Ernest said the 44.75 flys better at longer distances.



Then came the 700mm barrel and slug liner. I have JUST started tuning, the weather was miserable and windy, but at 85yds, the groupings were in the dime size. Pushing a 61gr @ 925fps. The slugs make nice round holes in paper. I am definitely on the right track now.

 

[sgrayner]

Filled my 580 bottle to 250bar and was only able to get 64 shots on regulator (130bar). A little disappointing. I was hoping my tune was a little more air-efficient. I'm glad I opted for the larger bottle!

Groups on the 50gn pellets were excellent at 35 yards...pretty much one-hole. I'll get hold of some 44 grain JSBs for comparison. I also plan on a 100 yard accuracy check soon.

Steve