Pneuma/ can't adjust the hammer spring/help me understand something

[darryl]

Stephan, good luck. I'll be anxious to read about your results.

By the way, I posted elsewhere, but I thought I had them all covered. Here's the short version. I replaced both outer barrel o-rings with new ones. There was a burr inside the receiver where the forward (muzzle end) hole for the retaining screw. I removed that burr, and thought I'd try filling to 180 bar. I posted a picture of the three 10-shot groups I made. I find it hard to believe that those two barrel o-rings were the cause of my problem (all along?). These groups were shot in my basement at 9.5 yards. Before I replaced  those o-rings you'd have seen a scatter of holes, so this is a vast improvement. Now I need to test some more, and then try rsterne's hole-plugging instructions. I want to keep the power up.
cheers!
darryl

[Stefan]

well, I did test it today. got some weird results. no bell shape string, with or without regulator (I have two separate air cylinders). On 30 yards I was all over the target. I don't know the reason, since I took the barrel out and replaced the orings, with -011, but when I took it out again afterwards, there was one small nick on the one of the o rings. I was getting 1015 fps, and down on unregulated cylinder, using CPUM and CCW.
And fully screwed in, I had 495 fps, 720 fps etc, and my AT is FAC. I'm kinda desperate now. I don't have spare orings  (they are on the way). I'll have to work on it to.
Another thing - i can put a 2mm allen key through T/P, and there's more space there. Is that normal? I'm kinda wondering is T/P even there? I can't take the breech of to check. Also, I don't see any tiny orings on the hole where T/P is supposed to be (and on EPV they are mentioned).

[rsterne]

I'm going from memory here, but IIRC the transfer port diameter is about 4mm.... The O-rings should be metric, so I don't know if a #011 will work.... I don't know what you mean by "fully screwed in"....

Bob

[Stefan]

fully screwed in - full CW 

part number 608 - 7,66x1,78 mm o ring - according to chart, that's -011.
ok, glad to know that T/P is larger than the allen key that I used.
Bob, how did you center the barrel, If you ever took it out?

[rsterne]

I don't remember ever removing the barrel from the breech.... There are 2 screws holding the breech on, one under the magazine IIRC.... When I had the breech off I checked to see that the barrel port lined up with the breech port (it did).... and I adjusted the position (fore and aft) of the brass sleeve that the air reservoir screws into so that one of the valve ports was exactly vertical, lining up with the transfer port.... I eventually drilled that single valve hole to 0.166".... which leads me to believe that the transfer port was that size as well.... but I didn't record it in my notes....

Bob

[darryl]

Stephan, you may have a burr at one of the holes for the barrel-retaining screws (or elsewhere). I can tell you that I had such a burr and it nicked two new O-rings. Once I removed the burr, the accuracy of the rifle improved because the o-rings were sealing properly (consistently). The burr I found was in the hole closest to the muzzle, so it nicked both o-rings when I re-installed, then removed the barrel. The original o-rings were nicked up similarly too. Odd thing is, for a "new" rifle, when did the barrel get removed in the first place? It came to me with whacky accuracy. Anyway, good luck.
Hey I finally got the valve out of my Pneuma! now to size the holes. Maybe a 4/40 tap will do. Time to consult Grainger.

 

[rsterne]

The O-rings were probably damaged on initial installation at the factory....

I drilled the holes out and used a 6-32 tap.... The screws were wide enough to block the groove (I left the screws long enough to be just below flush on the outer edge), effectively helping to limit the transfer port volume.... I think 4-40 would allow air to pass the sides of the screws....

Bob

[darryl]

I will email Hatsan about the issue with damaged barrel o-rings. I've been in contact with it anyway. It should be made aware of the damage being done.

I'll have to think on the 6-32, Bob. Are you saying the 4-40 would be a loose fit? I did try a 3/32" drill bit,and it seemed a perfect fit in the port holes. But I'm no machinist, so I don't know quite what that tells me. Need a little help.
darryl 

[rsterne]

The correct size tap drill for 4-40 is a #43 (0.089").... 3/32" is 0.094", which is probably close enough, it's not like the screw has to hold any load.... If you look at the photo on the previous page of my drilled valve, you will note that the screws bite into the sides of the groove, effectively blocking air from passing the sides of the screws.... I don't know if a 4-40 would do that, or if air could pass the sides of it....

Bob

[darryl]

Thank you Bob,
Now I understand more clearly. I saw where the #43 was the drill size in the source i looked at, but it also conveniently listed the closest drill bit as being the 3/32 bit. I did not know there was a size difference at issue here. So now maybe I need to use some sort of thread filler to take up the slack? It seems, in using the 4-40 tap in a hole that accepts a 3/32 drill bit, the thread depth might be a bit slight. Still, we're only taking about 50 PSI (3.45 bar) per shot. So perhaps it's do-able to go with the 4-40. Either way, I need to find the tap and the screws before I proceed. Might as well locate the screws first. I very much appreciate your help, Bob.
thank you,
Darryl

[rsterne]

The screws will likely fit OK.... the thread will just not be full depth.... A 3/32" hole is slightly smaller than the "pitch" diameter.... which means the screw may just be a bit loose in the hole.... If you don't run the tap in all the way, you could still have the screw tight in the hole because of the taper on the tap.... Just go a bit at a time and then when you find the best depth for the tap, wrap some tape around it and do the other holes the same....

Bob

[darryl]

thanks again, Bob. Now to find a little time to get at it finding the proper tap. Today's a wash out for that. Wife has me tied up. (no jokes, please).
darryl

[Stefan]

OK, done with tinkering today. got my barrel re crowned, on 11 degree angle, got everything set up nicely, lubed the hammer, and internal parts etc.
spring is set on 6 CCW (because on full clock wise i have 500 fps???).
I shot with unregulated air cylinder and with regulated one.
on unregulated, got the stock 950ish fps with CPUM, with bell curve. with regulated speeds were going up and down. something is wrong with my regulated cylinder. either the regulator isn't working properly, or somethings wrong with the valve (look at the pic, that's how the spring sits inside, is that ok, and some marks on outer threads). shots were taken with front sand bag, on 30 and 50 meters. I used JSB 18.1 grain and CPUM. Barrel was cleaned previously. It was really cold, so I didn't take time to check for POI shift.

[rsterne]

IIRC, the valve stem should be able to enter than hole when driven open by the hammer.... It looks to me like that spring being offcenter would prevent that.... That could be the reason for your low velocities....

Bob

[Stefan]

Did more tinkering. Also, a new regulator is on the way, Altaros offered to change the one that I got for free (nice gesture).
I made a new hammer spring - same force, but thicker wire and less coils. Made a 1,5 turns from CV. Shot from 200 to 100 bar, 40 shots.
822 842 856 858 872 893 892 904 910 911 920 928 941 949 950 949 948 953 953 957 964 963 963 961 962 962 961 954 947 948 945 942 931 923 919 909 903 903 899.
First 11 shots and last 5 were out of 4% limit. So from now on on unregulated cylinder with CPUM I will fill to 175 and go to 110 bar. That should give me:
High 964
Low 923
Avg.FPS 950,13
SD 11
AVG FPE 28,752
that gives me aprox. 16.95 BAR-cc/FPE
avg FPE/cuin per shot 0.97

also, even it was really cold, 25 F, I also tried it for accuracy. But these are not the best results, my hands were freezing. Front sandbag rest.
First 4 photos are at 33 yards, from 200 bar down. Photos 5-8 are at 57 yards, down to 100 bar (measured with Halo rangefinder)













I also added this air stripper to my AT, and made this little metal part to cover that ugly "uncut" place on barrel that was left when I took the front sight off.

[rsterne]

24 shots within 4% is pretty typical for the AT44.... Here is a (24) shot string (to 96%) I got with 14.3 gr. JSB Express.... stock AT44-10....

914
921
917
929
930
940
938
932
933
937
944
950
942
939
935
943
942
937
942
937
923
920
917
912

avg. velocity = 926 fps
avg. energy = 27.2 FPE
efficiency = 0.99 FPE/CI
pressure range 185 bar down to 125 bar (60 bar used)

Looks like you've got it sorted pretty well....

Bob

[darryl]

Guys, I think I'm OK with where my rifle is now. I don't think I'm good enough to post a legible image of the last spreadsheet I put together
I learned a lot about this rifle and PCPs in general. thanks for your assists, Bob.
darryl
btw, here's another attempt. still small-ish, but you can see the curve is fairly flat. there are 23 shots of the 30 within the 4% range. Not sure if I can improve it by starting with a lower fill pressure. 

[rsterne]

Looks GREAT!!!

If you only want the best 24 shots (or 20), then move your fill & refill pressures accordingly....

Bob

[darryl]

Thanks Bob! A slightly lower fill pressure maybe? One sure way to find out.

[rsterne]

It looks to me like if you fill to 175 bar and stop at about 125 bar you will have 20 shots above 830 fps (3%).... For a 30 shot string you could try a fill of 185 bar and shoot down to 110 bar.... That looks like 30 shots within about 5% which is pretty good....

Alternately, you could try 1/2 turn further out on the hammer spring preload.... You should get a whisker more velocity but may need a slightly higher fill pressure to compensate.... I'd try for the flattest string you can get with a 200 bar fill and see what you can get....

Bob