Ham/Pneuma: free-float the barrel? and an air efficiency lesson?

[Stefan]

here are my groups, stock, 195 bar, distance 30 yards, using cpum.
poi shifts after shot nr. 40. I ran out of targets where there are 1 shot at 1 target. The other, are 5 shot groups  (its 1/2 inch inner circle)

[Ribbonstone]

stefan:
Looks like the barrels are good ones, although I need to clean mine very couple of 100 shots to stay on top.
rsterne:
Will try the port plugging eventually, but more tentatively. Will thread them all, but plug them one at a time.  Don't expect too much change with the first 2 or 3.

Guess I should have a goal (or I'd not have a reason to stop farting around).  IF I get to 45 shots inside of 20fps, 14 foot pounds, and keep the fill pressure under 160BAR, will stop goofing around...until then, its a work in progress.

[Stefan]

it is c/p, but it might be useful

These are shot strings with JSB 13,43 grain exact jumbo RS pellets.

Before regulator, from 195 bar to somewhere bellow 100 bar, 30 shots

966.1
956.3
951.5
946.7
942.8
939.7
935.8
933.4
931.7
914.4
924.9
906
897.3
897
891.1
869.4
878
870.3
864.6
853.5
854.2
840.4
831.6
818.4
812.5
807.2
784.5
777
767.9
759.7

After regulator, from 195 bar to 100 bar (you will notice the spot when pressure went bellow 125 bar) 30 shots

883.5
875.2
880.4
869.4
868.3
880.6
878.9
866.0
863.2
864.5
868.1
870.4
867.9
868.3
863.7
862.5
864.7
867.7
862.3
865.3
858.8
861.9
852.1
842.7
853.6
834.5
828.8
817.9
814.3
797.5

it looks like I have 22 regulated shots  

or as graph

[rsterne]

I would agree, 22 regulated shots at about 870 fps average (slight slope from 880ish to 860ish).... That works out to 22.5 FPE average and a total energy of 495 FPE.... You are using 70 bar on a 180cc reservoir (11 CI) which is 770 CI of air used.... The efficiency is therefore 495/770 = 0.64 FPE/CI which from my experience on an AT44 is very low....

As a comparison, the gun I tested with 14.3 gr. JSB Express for 30 shots produced the following string from 190 bar down to 115 bar....

889
897
914
921
917
929
930
940
938
932
933
937
944
950
942
939
935
943
942
937
942
937
923
920
917
912
904
908
903
897

Average velocity 926 fps (27.2 FPE).... total energy 817 FPE.... 75 bar used for 825 CI.... Efficiency 817 / 825 = 0.99 FPE.CI....

Bob

[Ribbonstone]

Happened to test a string using EunJin pellets.  Still down tuned, strangled off transfer port, and a 2500-1400PSI fill.


656
661
664
660
665
669
672
671
674
674
671
671
672
674
678
680
681
685
684
687
682
680
685
688
685
690
684
681
681
682
682
681
681
684
683
688
686
685
684
685
680
683
681
680
678
674
671
670
672
664
661
661
656
650

summation:
690-660 (30fps) =  51 shots
690-670 (20fps) =  43 shots
690-680 (10fps) =  29 shots

Which works out to something close to a less sucky bar-cc/fps of 16.

[Stefan]

maybe using heavier pellets might give me better efficiency. but, I was after same speed, more than efficiency. i ll do more tests. maybe i should of get a bigger string, what you think?

Bob, did you tweak that at44, and how?

[Ribbonstone]

Probably would gain some shot count.  Hard to do a direct comparison, two differnt calibers (mine is the .177) and the strangled off transfer port in mine.

But so far, in every case except a couple of PCPs strangled dwon to the 5 foot pound range, heavier pellets have earned more energy and a longer shot count.  Are slower, but still earn more energy.

Got to wondering about that and posted this on another (yellower) forum:
Got to thinking about things we all think we “know”.  Often we don't, but even if it does turn out to be true, its worth looking at those “facts” a second time...may find new thoughts on how to control those factors though reexamination.

Air hog guns like heavy pellets.  The only time I've found heavy weights to earn a bit less energy has been with a few 5 foot pound guns that are made to let loose a tiny “blip” of air at each shot.

With well designed higher powered PCP's, the difference isn't really drastic, but heavy weights do earn more energy.  With gas-hog PCPs the difference in energy AND the shot count difference is much more noticeable.

So I got to wondering “why”?  I thought I knew: back pressure.  But perhaps it is some other factor as yet unthought.

Probably has to do with back pressure.  Thinking that if I under stand back pressure a bit better, ways to use/tune for it might occur that I hadn't thought of before.

Seems like inertia.  Gas hits pellet, pellet moves.  As pellet moves, it creates more volume. Heavier pellets are harder to get to move (inertia) and accelerate slower (creating volume at a slower pace).

Main thought: Heavy pellets are harder to get moving and when they do move, accelerate slower.     So they create barrel volume slower, the valve cycle is closer to its end phase,  and they wastes less gas in trying to fill the expanding volume?

Wrong?  Got a better  simplified mental-construct?

[rsterne]

Stefan.... The AT44 I worked on started out life as a non-PAL (ie under 500 fps) rifle with a very much oversized valve stem (~0.150").... It got something like 180 shots per fill.... I was supplied with a "full power" valve stem that measures 0.100" diameter and installed that... The string I posted above was with no other changes.... I subsequently increased the hammer preload and plugged some of the holes in the valve.... but that was not the case for the string above.... I therefore believe that to be the stock setup for the Pneuma/Halestorm....

Bob

[rsterne]

Ribbonstone said:
With well designed higher powered PCP's, the difference isn't really drastic, but heavy weights do earn more energy.  With gas-hog PCPs the difference in energy AND the shot count difference is much more noticeable.

I would agree with your statement but would change the wording slightly.... Instead of "well designed" I would use the term "efficient".... Any PCP can become a gas-hog if you push it hard enough.... The trick is to decide how much power you can get before the air consumption goes into the dumper.... Here are a couple of charts for the AT44 showing what happens when you back out the hammer spring preload (increasing tension).... The first chart is for 18.1 gr. JSB Heavies....



The second chart is for 25.4 gr. JSB Monsters....



Compare the average Energy in each column to the one below for the heavier pellet.... Also compare the Efficiency....
Then look across the row at the average Energy comparison as you increase the hammer strike.... Do that again for the Efficiency....

You will notice a few important things....
 
For the "stock" setting, the 25 gr. pellet is 33 FPE compared to 31 FPE for the 18 gr.... only 7% more power....
For the "maximum" setting, the 25 gr. pellet is 42 FPE compared to only 36 FPE for the 18 gr.... 17% more power....

For the maximum setting the 18 gr. pellet is 36 FPE compared to 31 FPE stock.... 17% more power....
For the maximum setting the 25 gr. pellet is 42 FPE compared to 33 FPE stock.... 28% more power....

For any given setting, the 25 gr. pellet has higher efficiency than the 18 gr. pellet....
The 25 gr. pellet can stand a heavier hammer hit and still deliver 1 FPE/CI minimum efficiency.... (I marked those columns in bold as being maximum practical tune)....

I don't think the term "backpressure" is correct, but you have the right idea.... As you state, the inertia of the heavier pellet resists acceleration more.... If the valve stays open for the same length of time, the volume between the valve and the pellet is smaller (and at higher pressure) when the valve closes.... Less air has actually escaped from the valve (which accounts for the increased shot count with heavier pellets).... That air has a longer period of time to expand before the (slower moving) pellet leaves the barrel.... which extracts more energy, increasing efficiency.... The residual pressure as the pellet leaves the muzzle is less, and hence the report is quieter (noise = wasted energy).... All these factors add up to produce higher efficiency with heavier pellets....

The more of an "air-hog" the gun is, the more these effects add up.... and so the better a heavier pellet performs relative to a light one....

Bob

[Ribbonstone]

Thanks for that reply...makes sense.

Believe its time to start tapping and plugging holes.

[rsterne]

Make sure you mark which hole is the top one.... good idea to check and see how accurately it lines up with the transfer port as well.... You can actually change where the air cylinder tightens up in relationship to the transfer port by slacking off the screw retaining the part the valve screws into and sliding it back and forth slightly in the outer tube.... There is enough play to change the length between the transfer port and the end of the tube, allowing the reservoir to stop in a slightly different rotation....

Once you are sure you have one hole pointing straight up....  you will find that blocking the bottom three holes will not lose much velocity but will gain efficiency.... I don't think blocking just 1 or 2 will be worth it....

Bob

[Stefan]

got 30 shots with 18,1 grain jsb.

[darryl]

Stephan, what pellet head size on those JSBs? This is with a "floating barrel; right?

[rsterne]

AFAIK, the only JSB Exact series (ie round nose) pellets that are available in anything other than the standard 4.52/5.52mm head size are the regular Exacts.... 8.4 gr. in .177 cal and 15.9 gr. in .22 cal.... I have never seen the Express and the Heavies in anything else....

Bob

[Stefan]

I will have to measure it, but I believe that its 5.52 mm.
My barrel is kinda "floating" I just removed the upper o rings in barrel bands, plus I have hatsans LDC.

I didn't take photos of groups today, since my efforts were focused on shot count, and I didn't get my bench-rest out today. But it was grouping really well, and all of the pellets were going into same place (plus the shooter "factor").

To be honest, I was expecting more efficiency with this regulator. My primary goal was to get same grouping and no POI shift, and it is acomplished, but I also hoped to get better efficiency. It looks like I will have to tinker with hammer spring preload, to make it more efficient. 

[rsterne]

I have no idea why the efficiency is so low.... Since you reduced the power of the gun, it should have increased.... The only thing I can think of (and I don't know how that regulator works so can't be sure).... is that it appears the regulator seals right on the end of the valve.... If that is the case, then the valve cannot draw ANY air from the tube during the shot, relying only on the air inside the valve.... That would reduce the AVERAGE pressure during the shot, lowering the power and the efficiency.... I'm not sure of this, of course, just trying to offer a possible explanation....

Bob

[Stefan]

well it looks like this. from what I can see, the design isn't like that, to seal the valve.

I can read real pressure on the air cylinder, and I didnt have to drill a hole in cylinder body.

[Ribbonstone]

Got to plugging valve ports this AM on that Pneuma .177.


Lots of previous posts..can do a search...been farting around with this gun for weeks.

So I started plugging valve ports. De-gas the air tube, disassemble the valve.


 Are 6 ports around the head of the valve. Found that a 4-40 tap was a good fit to the factory holes, so did not drill them out...just tapped them.  Had to make  cap screws by cutting off short lengths of 4-40 screw and slotting the tip for a screw driver.

(yes..these are a PIA to make..buy them. I was being hard headed and didn't want to stop, clean up, and make a trip to the store.  Turned out to all be about 1/8” long, but exact length isn't needed...something between .12” and .15” will work just fine.)

Hole degreased  with a q-tip, fill screw degreased, a drop of red locktite (the tough one), and plug the hole. Do not run the screw in far eough to touch the valve stem, but run it in far enough to not stick up and block screwing the tank back on the gun.

Shot a shot string for each added plug screw.  BUT remember, this .177 rifle is set to run at 2400psi and is using a much smaller transfer port (using .11” rather than the factory .163”). the 12 o'clock ports were the LAST to be plugged, the ones farthest away from the transfer port the first.

On a guess, the large size issue transfer port, would probably see a difference by the 2nd or 3rd  port plugged.  With the smaller sized transfer port,  would see the changes later.  (Opinion,  factory valve and transfer port too large for a .177 unless you want to shoot only heavy weight .177's at +20 foot pounds).

All ports open:
2500-1400psi
8.4gr. JSB

850
851
856
858
866
864
870
877
875
876
879
876
879
876
874
876
874
878
885
884
882
884
884
881
883
881
882
882
886
884
882
886
884
881
884
885
888
881
875
874
871
868
862
861
858
851

Summation:
888-858 (30fps) = 42 shots
888-868 (20fps) = 36 shots
888-878 (10fps) =  21 shots

The bar-cc/fps rating is about 21.

One port blocked: no real change.

Two parts blocked: no real change.

Three ports blocked: no significant change (believe there was a little decrease, but it was so small I could spend the rest of the day proving if it existed or not)

Four ports blocked.

(can skip the  numbers and got to the summation)
800
801
814
810
817
818
818
820
819
822
819
821
826
824
825
828
827
829
830
829
830
834
832
834
832
837
837
829
831
830
829
834
834
832
835
829
829
829
831
834
831
833
929
829
831
827
821
821
819
814
808
810
809
800
798
Summation:
837-807 (30fps) =  51 shots
837-817 (20fps) = 45 shots
837-827 (10fps) = 31 shots

Down to about 12.5 foot pounds per shot.  Efficiency rating up to 16.8 bar-cc/fpe.  Still sucky.

5 ports blocked:

728
730
734
737
736
739
748
744
753
751
752
749
746
750
753
752
754
755
756
762
757
760
759
757
756
755
760
759
756
761
757
758
758
756
757
759
760
761
756
759
758
758
762
759
756
757
752
759
755
753
746
744
746
745
740
740
735
733
734
730

Summation:
762-732 (30fps)= 57 shots
762-742 (20fps) = 48 shots
762-752 (10fps) = 36 shots

Another way of looking at is is to focus on just the shots inside of 20fps.

All ports open gave 36 shots...518 foot pounds inside of 20fps
2 ports open gave 45 shots...574 foot pounds inside of 20fps
1 port open gave 48 shots...506 foot pounds inside of 20fps

Settled on two ports open.  Could adjust the striker and fill pressure, or the transfer port, but 12.7 foot pounds isn't bad for a .177.  Will eventually get back to work and get the efficiency up a bit more (and even more shots) but have decided not to reduce power any farther.

To go back to two ports open, had to remove one red locktited plug. Heated a screw driver...placed it in that dinky little 4-40 screw slot...and after a few seconds, just unscrewed it.  So small and localized, that does NOT heat the valve head enough to harm the other screws or the seals.

[rsterne]

Pretty consistent with my results in .22 cal, really....

Bob

[rsterne]

Stefan.... I can't tell from the photo how the regulator works.... Sorry....

Bob