Canadian Airgun Forum

Here's a picture of the washers I am talking about, switch it for a metal washer and a rubber washer I had laying around to fill the space inside the stock. My poi was roughly 6" high now, probably could rid of the scope shims I added when I first scoped it.


Picture of the end stems, one is fatter then the other, believe it's the old one that is fatter.


Here are 5-shot groups, (except for the one with 6}, 10 yard/30ft away. I was hunch down, leaning back and using a flimsy homemade rest so had a couple of flyers.


Hope this helps but don't see much shifting if I do my part.

The end of the stems where they are different in diameter is where the hammer hits.... There is NO airflow at that point, only through the holes below that in the photo.... As long as the distance from the shoulder on the air tube where it seats against the gun to the end of the stem is the same there should be no difference because of hammer strike.... The only difference in airflow could be a slighty difference because of the orientation (clocking) of the holes the way they tighten up in the gun.... OR an internal difference in the valve stem diameter....

I would think a much more likely reason for a POI difference would be pressure from the barrel bands.... If you can't measure a velocity difference with a Chrony, you should be able to eliminate airflow differences as the culprit.... and look for a mechanical reason instead....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.

ok. you now have run into the same things as I' m talking about,I had the bedding problem on my single shot.A thick washer at the rear bolt fixed that ,my action was warping causing the airtube to separate from the barrel.
next thing I have all the same stems not the config .you have.1 barrel band free floated,port blocking per Bob instructions ,lubed hammer.
Much more consistent and no cost. good luck..kelly

_________________
Cometa lynx,Air Arms S 200,Hatsan AT 44 W, AT 44 10 W , Weihrauch HW 80,Weihrauch HW 97, Weihrauch HW 77, Benjamin classic, Cometa Fusion , Hatsan 60s, telly, beeman p1,Hw 45 Silverstar,Crosman 2240,Crosman 22xx "the beast"

I'll make a better rest and try it again, I know I can tighten my groups if I were more steady, wish I had the proper setup to do this.
Then I will try some of the things mention here and repeat the tests just to see.
I also have a airstripper on the end that has not had any adjustments done to it, still newly gotten.

I thought this thread was about air tube differences and not about barrels bands, warp actions etc... I don't see much if any differences in poi from one air tube to the next though that was not my best grouping ever in that pictures. I just don't understand the question that was ask I guess. Like I said, I'll try to set myself up so I am more steady with my aim and see if there really is any differences by switching air tubes. I'm just curious.

I have not been able to chrony differences or freefloat the barrel yet.
I did measure the diameters and found about .021" difference in diameter. This was using a cheap digi caliper and needs verified with a 1"-2" mic.

With a very snug fit through the band and under the receiver where it screws in, thus could move things around.

The groups at 25 yards were essentially the same hole. So a longer distance makes thing more obvious.

Have a good day

Gerald
Iuka ms.

0.021" is a HUGE difference in diameter.... If one tube can be forced through the barrel bands, the other would be flopping around loose.... That would very likely create a POI difference when the tubes are swapped, but it is a mechanical problem, not an airflow problem.... That is why mechanical problems are being discussed in this thread....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.

I would love to get it clamped in a gun vise and see if the barrel moves as the cylinder is being installed. Gonna try to setup a dial indicator if I can get a rigid setup.

Im also curious if the tube diameter changes as the pressure changes. Not sure if the hoop stress is at the yield point.

I am still satisfied with how it shoots. Just wish I could get more shots at a given velocity.

Gerald

So what exactly should I do here? Measure tubes and put all my shots in one small hole. not blocking any holes like some of you are doing, basically stock rifle though I need my scuba tank refill soon.
Just asking what you all want to see. Different tubes give different results??? I just need more shooting time but can only to that when tenants above me leave which isn't often.

I haven't had a chance to measure movement with a dial indicator yet. I did bleed both cylinders empty and notice the valve was much harder to operate on the new cylinder.

The valve would stick in two distinct spots when opening and closing. Upon removing the valve, visible concentric rings were visible on the bore. Polished with a qtip coated with jewelers rouge until smooth.
Pushing both valve stems agianst each other shows both of them responding the same.

Still have to test to see if swapping makes a difference.


Gerald