Pneuma re-think

[Ribbonstone]

.177 Pneuma got re-thought and down-tuned.  Basically turning it into a 12-foot pound gun.

Starting from fresh (an how p’d away an unexpected day off):


Fiddled around with the striker spring setting to get it running well at 2900psi.  Not usually a big fan of higher-pressure settings, but past experience has shown that short quick “blips” of high-pressure air are quieter than longer blasts of lower pressure air.

Pneuma has a valve attached to the end of the air tube (the tube unscrews with the valve).  The valve has ports all around.

As issued, only the heaviest weight .177 pellets were able to shoot well.  Anything under 11gr. tended to have a nasty transonic “crack”.  16.1gr. EunJins made it to 914 fps for just at 30 foot-pounds.  13,4gr. JSB’s made it to 985fps (28.8 foot pounds) but didn’t shoot all that well.  The useful (3% of max) shot count was short-short.

Make the top of the air tube while it is screwed into the gun. Plugged all but 3 of the ports; because the tube was marked, the ones left open were the TOP posts (which best line up with the port to the barrel).  Tapped the port holes and made little plug screws.  Covered the plug screws with epoxy putty.  Epoxy was probably over kill, but if the plug screws were to back out, it would jam the cylinder into the gun. If they were to self tighten, would foul the valve stem.  Can pick out the putty and unscrew the plug screws if you need to.


With 3 ports open:

10.6gr @ 916fps = 19.75foot pounds X 26 shots.

With 2 ports open:
10.6gr. @ 845 fps = 16.8 foot pounds X 33 shots

With one port open:
10.6gr. @ 710fps = 11.8 foot pounds X 52 shots

Still not real efficient.  Suspect with more fiddling around could increase the 12-foot pound shot count by 10-20% (consider that 15% would be another 7 or 8 shots inside of 3%).


Much quieter.  Quiet enough that the LDC got removed for the time being.  Still have my home-brew LDC, but ordered a TKO. Will do a side by side once it arrives.


Quick test of accuracy. Removing the LDC didn’t help or hurt, seems to shoot about the same.  Lower speeds may have changed the favorite pellet from the 13.4gr. to the 10.2gr, but it would take more shooting to be sure.


Even though I used the Crosman 10.6’s for shot strings, the rifle never really liked that pellet.  Velocity is pretty uniform, but every so often it will just drop one low without any noticed change in velocity.

[darryl]

Man that brings back memories. A great rifle for tuning. rsterne had excellent results doing as you did. I did too except my .22 was so inaccurate that I sold it. Accuracy aside, I had a ton of fun tuning it. Your results look great to me.
darryl

[Ribbonstone]

Bare-barrel base line for comparison to mounting an LDC.  Got the scope set about right (cheap scope ping-pong) by target #7.  Even though that’s close to 30fps variation, it really doesn’t matter at short (20yard) ranges.


“Borrowed” this home-brew LDC (2.1 oz) from another gun:



Same pellets, same 20yard range, same fill pressure.]:


Even 2.1oz. of extra weight on the muzzle made itself be known.  Even at 20 yards, groups formed up just at 1/3” low (and it would have been progressively lower as range increased).  But other than adjusting the scope (which I got right about target #6) for the new barrel vibration dynamic, no harm seemed to come from adding the LDC.



In case you RE WONDERING, some sound measurements at 20 feet (20 feet because I don’t really care what I hear, but do care what the neighbors hear).


BARE BARREL: 73DbB

HOMEBREW LDC: 61DbB

Will test the store-bought LDC (TKO) once it shows up.

[grumpy]

Great write up. Very impressive shot count and uniformity.

[Ribbonstone]

Couple of ideas for the Pneuma.

Have zero experience with the repeater version (just not a repeater kind of guy) but have read of problems with accuracy with some guns.

But when I got the gun, it didn’t shoot all that well.  Had a good bit of variable un-n-down pellet impact.

Some of them seem to come with one barrel band. Some with two.  These are big fat plastic bands, with and o-ring on the inside.  Oddly mounted, they fit into circular bosses molded into the stock.

What bothered me was this: When the bands were tight, the air tube could not be unscrewed.  Had to loosen the bands to unscrew the tube.  Which told me that the bands were bearing on the tube and not the stock. 




Figured that can’t be a good thing as the air tube tends to expand/contract with air pressure, and that would be transmitted to the barrel.

A little experimentation whoed that the bands were pulling down on the air tube.

So by shimming the bottom of the bosses into which they fit, found a spot where they would fit tight to the stock without binding the air tube.  Barrel stable to the band, band stable to the stock, and the air tube not inducing any stress to the band.

(If I had found the bands were pushing UP on the air tube, I’d have filed down the seat of the barrel bands until that stress stopped.)

Any way, once I got the bands not  bearing on the air tube, firm to the stock and to the barrel,  the gun started grouping well.

This may not have helped (but it didn’t hurt). With the rear sight off, there are two more tapped holes over the barrel breech.  I dimpled the barrel and used those two extra holes for set screws to lock the barrel in place. So there are a total of 4 working set screws keeping the barrel in the receiver.

[renowntwo]

Thanks for sharing