One valve port Pneuma

[Ribbonstone]

One port Pneuma

Rotated the brass insert to better align one single valve port with the transfer port.
Had plugged 4 of the 6 ports previously, plugged the 5th today.
Slightly enlarged the  valve port aligned with the transfer port.
Left the strangled off (made a new one with a smaller dimater passage) transfer port alone.
Rest the spring tension to slightly increase the sweet spot pressure.

Test #1: 160BAR
810
815
814
816
750
800
735 (got a good bit louder)
734
725

We gotta problem...time to stop shooting and investigate.

Blown o-ring:
Was going along well, speeds right at expectations, when suddenly the velocity dropped and the gun got louder.  Pretty good signs to stop shooting.

Bolt probe o-ring giving a little burst of gas, oddly kind of directed to the right more than straight back.  Good look showed a cracked o-ring. One of those tiny little 003 sized rings.

Found my last one (and got on line and ordered 100 of the most common sizes), made the replacement. While the barrel was off, replaced the two barrel o-rings as well.

(Barrel is a good fit, but the barrel is o-ringed to the inside of the breech.  Good news is that the transfer port doesn't lock it in place, so you can remove the barrel without taking the gun all the way down. Bad news is that while most of the air goes straight through the aligned transfer ports, there is still a good bit that races around the tiny space between barrel and breech, sealed by those two o-rings.)

Pneuma .177
2500psi - 1550psi
8.4gr. JSB

791
789
795
798
804
808
807
809
808
811
810
807
808
810
815
817
818
821
819
822
823
827
820
821
823
825
822
821
829
827
827
827
922
820
821
819
825
824
824
822
824
818
817
819
820
820
818
817
807
807
808
803
802
803
798
799
800
795
793
785

Results:
10 feet per second = 827-817fps = 33 shots
20 feet per second = 827-807fps = 46 shots
30 feet per second = 827-797fps = 54 shots

[rsterne]

Without actually doing the average, I figure about 817 fps over 54 shots?.... That's 12.5 FPE x 54 = 672 FPE total.... 950 psi drop = 66 Bar x 11 CI = 726 CI total.... Efficiency is 672/726 = 0.93 FPE/CI.... How does that compare with previous?....

Bob

[Ribbonstone]

Not quite as good as the best previous setting...but its always shown its best efficiency with heavier (10.6gr) pellets.

Looking it over, its not so much the 10fps lower speeds, but that 5 ports blocked fell off the curve at 1550psi...when it was 4 ports blocked, had more of a bottom end and didn't go offthe curve until 1400psi.

Four ports blocked.
2500-1400psi
8.4gr. JSB

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

[rsterne]

That is not that unusual.... When a gun breathes better the peak of the power curve is typically at a lower pressure.... You have two sources of restriction?.... the blocked holes AND the strangled transfer port?.... Probably some complicated interactions occurring that are difficult to explain/understand....

On the second string (just posted).... avg. vel ~827 over 51 shots?.... That's 12.8 x 51 = 651 FPE total.... 1100 psi drop = 76 Bar x 11 = 834 CI total.... 651 / 834 = 0.78 FPE/CI.... With 5 holes plugged the efficiency is quite a bit better.... That was what I found as well.... The best efficiency on the Pneuma seems to be with only one port open and that port lined up with the transfer port.... I never tried filling the groove with JB weld but understand that is even better for effiiency.... It makes sense, extra transfer port volume just wastes air and reduces the highest pressure reached during the shot.... High peak pressures generally give high efficiency....

Bob

[Ribbonstone]

Makes sense...and any one string can be off by a couple of shots, so they are basically equal in count and not enough energy difference to worry about. Probably leave it as it is (5 ports plugged). 

[darryl]

this thread is a help. I'm still trouble-shooting. Did some brief testing yesterday. Start fill was 145 bar / 2000 psi: ten shots only (JSB 18.1 with tension set to minimum.
867
854
847
836
829
821
815
802
790
780
That's an average of 27fpe with a .22. What does this brief test say? I haven't a clue. No holes plugged yet but I'll buy what I need tomorrow.
darryl

[darryl]

btw, the 145 bar fill is the best for accuracy as the rifle is now. start any higher and I just waste air and pellets until the pressure drops. I may sell this rifle.

[rsterne]

You are only getting half the shot string as it's starting at 145 bar.... I understand your frustration because of the lack of accuracy at higher pressures.... I wish I had a suggestion....

Bob

[darryl]

hi Bob,
Maybe I will have to sell it. But if I get to plugging ports... in for a penny; in for a pound? Can it be using that much air? Your results are much desired. Have you had accuracy problems with your rifle before or after plugging holes? I suppose if I get no results I can always order a new valve for the tank then sell the rifle.

[rsterne]

The AT-44 I worked on was a friends.... I was more interested in learning the ins and out of power rather than accuracy.... I didn't see any significant difference in group size at different pressures, however.... It was a .22 cal....

Bob

[darryl]

I'm baffled.

[rsterne]

Just a thought.... try a different pellet, something with a thicker/harder skirt.... The JSBs are pretty soft.... Is it possible that when the pressure is over 145 bar they are expanding more and that is hurting the accuracy?.... It could be even a combination of the pellet and the shape of the leade....

I dunno....

Bob

[lauchlin]

I'm baffled.

Have you pushed a pellet down the barrel

[darryl]

it is a tricky issue with this rifle, Bob. I have tried several different pellets, including the hard CPHPs, and H&N FTTs.
lauchlin, I've had the barrel off and pushed pellets through from the breech towards the muzzle. The choke seems snug, but I am not a judge of what is correct in that regard. I do know that the JSBs (5.52mm) had rifling marks that did not appear to be very deep, nor were they very difficult to push through. The H&Ns @5.53mm felt about the same. IN both cases the rifling marks seemed uniform when I looked at them under an eye loupe. Again, I'm not trained to know what is good or not, but the marks looked pretty uniform. I even gave the barrel a light polish and I can say I saw no improvement.

But what's going on that the accuracy improves at the lower pressure, whereas even with the hammer spring set to minimum tension the accuracy goes south when the fill is above 145 bar? That's what I need to sort out. Notice in my string how the FPS drops off from 867 FPS:
-13fps
-7fps
-11fps
-5fps
-8fps
-6fps
-16fps
-12fps
-10fps
Why isn't this drop  more uniform? Do I have a leaking O-ring somewhere? Even if I do, can that affect poor accuracy at higher fill pressures? A mystery wrapped in an enigma.
darryl

[Ribbonstone]

IF there is a leak when firing, would feel the air at some point along the breech or loading port.  Could be...worth checking (and could check by sprinking suspected areas with baby powder).  Would make a kind of sense having a leak only past s certain pressure hitting the transfer port/barrel o-rings/bolt probe.

The barrels on these guns are o-ringed to the breech, and those o rings do contain pressure when fired.  If the FRONT o-ring leaks, you'd never feel it from behind as it will leak FORWARD, so don't just look at the bolt probe and breech/air tube area around the transfer port...its one of the guns that can leak forward.

Could also check that the breech screws are down tight...getting hit by pressure tends to lift loose breeches and promote leaks (and inaccuracy).

This is one of 5 pellets fired into soft dirt (differnt gun..a BAM51 with some inital accuracy problems)...the big gouge along the skirt was not made by impact, was made by a burr at the transfer port inside the breech. 


Might also fire a few pellets at very low speed (only fill it to 500psi or so) and capture them in something soft for examination.  IF there is a burr at the breech or other major defect in the barrel, the pellet will show it.

[darryl]

Rib, I do feel air along the stock somewhere. I'll have to track it  down. Last week I noticed that the breech screws were loose so I tightened them, but did not notice any air until today. A new leak? Whichever, I'll track it down, and see what that yields. I'll also try the pellet capture to see what one looks like. Hey! Maybe we're getting some place!
thanks man,
darryl
edit: So now I have two problems: weird accuracy and an air leak. I bet once I get it sorted out it'll be a great little rifle...IF I ever get it sorted out. I forgot to mention that I did some shooting with it today and saw that same inaccuracy phenomenon at higher fill pressures. I saw pellets from one magazine hitting two separate POIs, one higher and one lower. Once the pressure dropped I could put ten shots in one ragged hole. What is the higher pressure doing? Maybe I need to capture some pellets from a high pressure firing and examine them for evidence?

[Ribbonstone]

Faster they go, the harder to capture without turning them into shrapnel.  Need a whole lot of soft stuff to stop one without screwing it up too bad to be of any diagnostic use.  Water works reasonably well, but is kind of messy (line up plastic bags of water...takes more than you'd think.)  About 18" of soaking wet news print works; it usually screws up the nose a little bit, but the rifling marks are still clean (and SOAKED news print...dry it acts like wood).

Investigated the rear end, and found a leak.  Makes a kind of sense that it would leak the most when shot with high pressure (and probably leak differently from shot to shot).

Worth looking hard at the muzzle end of the barrel as well...crown could be disrupting the pellet's exit. Higher the pressure, the higher the exit pressure, so the more air that could disrupt.

[Pipewelder]

I had a leak at the transfer port. When I took the gun apart, there was only one seal on the transfer port. I was feeling a puff of air on my hand when I shot. But the accuracy was fine.

[darryl]

This morning I removed the barrel again, and shoved a few 5.53mm H&N Ftts through it. No unusual markings on any of them. Not the same as shooting and recovering, but the uniformity of marks was somewhat relieving to see. Inconclusive, maybe, but a good sign. I alos looked at the port hole on the barrel, and it looks fine.

One thing I've been hesitant to speak of until now is that I thing I was sold a used or refurbished rifle. My reason for that is two things that I've noticed, but let slip through the cracks: the outer barrel o-rings weer in bad shape, nicked and with sections missing; this rifle has four barrel-retaining set screws two per hole, and one if them is a phillips-head screw. That's not represented in the parts. I intend to call the seller whom I won't mention here. If nothing else, he'll know about it,and I will have had my say.

Pipewelder, where did you see that the TP is supposed to have two seals? I'd like to check that against my setup.

Also as I took my rifle down this morning, a small thin o-ring fell out and I haven't a clue where it goes. I'm looking at the drawings, but I can't seem to place it. The inner barrel o-ring is in place as are the outer ones. Oh well. One more little bit of intrigue with this rifle.

[darryl]

a surprising turn:
#6 o-rings from the plumbing department fit on the barrel. I happened to have some, and so replaced the nicked ones. Installed the barrel, then removed it again, and these new o-rings were nicked too. Found a burr I had not seen before, at the rear hole for the barrel's set screws. Carefully removed that burr, put new o0rings on, and inserted the barrel again (after a light coat of marine grease). Removed the barrel again. No nicks.

On a lark I filled to 180 bar, and got thirty shots off with a final tank pressure of 75 bar. Most interesting is that the grouping was much improved.
The picture isn't the best, but it's the first one that shows promise of better things to come.  Each group is one 10-shot magazine. I used H&N FTTs with 5.53mm the pellet head. I knew these pellets were a bit tight in the choke before I started. I still feel a bit of air, so maybe I need to check around the TP now, but I'm miles ahead of where I was earlier. Next I'll try some 5.52mm JSBs.
darryl
PS indoor range is only 9.5 yards, but it's a start. I was scattering pellets at that range before.