D34 Losing Velocity While Shooting

[Bill_in_TR]

Well I have been playing around a bit with my new chronograph and I have discovered what seems to be an issue with my Diana 34. As the shooting session continues it seems to slowly but steadily lose power.

I was getting some numbers that didn't make sense yesterday. The last string of pellets I shot was a 7.87 grain wadcutter and I got velocities that were significantly lower than I got from an 8.64 dome. It was getting a little late and I wrote it off to the lower light although it was still fairly bright I thought.

So today I went out and reshot the 7.87 grain pellets.The first 7 or 8 shots gave me the more expected velocities of around 845 to 850. Then as I continued to shoot the velocity started dropping. It was a steady drop until by shot 18 I got readings of 814 to 819.

That is a significant drop off. I can't say that I felt a noticeable change in shot cycle during the shooting. The spring in the gun is only a couple of years old from when I installed a Macari OS kit in it so it shouldn't be a spring problem I don't believe. And if it was then why did it restart today at the expected velocities after being much lower yesterday.

Anybody able to give me a hint where to look or what to consider?

[jason miller]

Are you shooting fast in the sun and heating up the compression chamber enough to transfer heat into the air inside of it, lowering its density and therefore volume?

Heat in the lubes/seal/spring might make a difference?

[Bill_in_TR]

Are you shooting fast in the sun and heating up the compression chamber enough to transfer heat into the air inside of it, lowering its density and therefore volume?

Heat in the lubes/seal/spring might make a difference?

Well it was about 85 outside.although I wasn't sitting in direct sun. And since my only purpose was to get some chrono numbers I was shooting at a good bit faster pace than I normally would. I wasn't really paying attention to whether there was any heat build up in the compression tube. If no other possible factors are mentioned I will try again tomorrow and slow down my  pace of shooting.

[El Greco]

Where your shooting parameters exactly the same? Sometimes even shooting at a slightly different angle other than 90 in relation to the sensors will change fps. Moving back half a foot will drop fps.
Lastly maybe a bad seal that was loosing air as the Piston expanded with heat?

[Mark 611]

LUBE Burning off the piston seal!

[Bill_in_TR]

Where your shooting parameters exactly the same? Sometimes even shooting at a slightly different angle other than 90 in relation to the sensors will change fps. Moving back half a foot will drop fps.
Lastly maybe a bad seal that was loosing air as the Piston expanded with heat?

One thing I am certain of is that the rifle was positioned the same way relative to the chronograph for every shot. And I am care ful to make sure I am shooting parallel to the sensors.

I am really hoping right now that Jason is right and I was just shooting at too rapid a pace.

[Roadworthy]

Actually I think Mark is probably on the right track.  I suspect it is the piston seal.  It may be related to lubricant burning off or the seal may fit a bit too tightly in the compression tube. 

[Bill_in_TR]

Actually I think Mark is probably on the right track.  I suspect it is the piston seal.  It may be related to lubricant burning off or the seal may fit a bit too tightly in the compression tube.

Alright I understand what you're saying. But if lube is burning off then why would it return to the original velocity after the rifle sits for a while. I am back outside shooting it now and it started out back at 844 fps. I am shooting at a much slower interval and so far it is staying pretty near that.

[Yogi]

Actually I think Mark is probably on the right track.  I suspect it is the piston seal.  It may be related to lubricant burning off or the seal may fit a bit too tightly in the compression tube.

Alright I understand what you're saying. But if lube is burning off then why would it return to the original velocity after the rifle sits for a while. I am back outside shooting it now and it started out back at 844 fps. I am shooting at a much slower interval and so far it is staying pretty near that.

Because over night it seeps past the piston seal into the compression tube.  Then you shoot it out the next morning.  Sounds like 810-820 fps is about right and the first several are "boosted".

-Y

[Bill_in_TR]

Actually I think Mark is probably on the right track.  I suspect it is the piston seal.  It may be related to lubricant burning off or the seal may fit a bit too tightly in the compression tube.

Alright I understand what you're saying. But if lube is burning off then why would it return to the original velocity after the rifle sits for a while. I am back outside shooting it now and it started out back at 844 fps. I am shooting at a much slower interval and so far it is staying pretty near that.

Because over night it seeps past the piston seal into the compression tube.  Then you shoot it out the next morning.  Sounds like 810-820 fps is about right and the first several are "boosted".

-Y

Well as I have been shooting here this evening. It started out shooting back up at 844 and stayed between 835 and 840 for about six shots then started to drop again. I was shooting at two minute intervals. After shot six it dropped to 822 by shot nine. I then dropped back to shooting five or six minute intervals. The velocity climbed back to 839.

[jason miller]

Weird. Did you replace the seal? Has it been in there for the whole time you’ve been running the OS kit?

I was also running my 34 with a Macarri OS kit over the chrono the other day and it was dropping, but that’s because it was a new kit I had just put in. Started out with FTT’s at ~850ish, and it seemed to settle down to 830ish after probably 50 rounds. How fast does yours shoot the 8.64’s?

[Bill_in_TR]

Weird. Did you replace the seal? Has it been in there for the whole time you’ve been running the OS kit?

I was also running my 34 with a Macarri OS kit over the chrono the other day and it was dropping, but that’s because it was a new kit I had just put in. Started out with FTT’s at ~850ish, and it seemed to settle down to 830ish after probably 50 rounds. How fast does yours shoot the 8.64’s?

When I put the OS kit in it was two or maybe three years ago and I replaced the seal at that time and the same seal is still in there. The 8.64's shot 812 for a 10 shot average. But interestingly I just looked at that 10 shot string for the 8.64's and there is no drop off there and my shots were about one minute intervals and occasionally less. So if it is a case of the compression tube heating up why didn't the 8.64's show a drop in velocity. This is starting to perplex me.

[jason miller]

I don’t know the answer. Heat build-up was/is just a guess. What other possibilities are there for change as you shoot?

[DanD]

I have a  .22 Crosman NP with a "695 fps" gas piston that does this, too- 1st shot 630 fps, then slows with each of the next several shots until it stabilizes around 590. Put it away for an hour and back up to 630 only to slow again with each subsequent shot.  I assumed it had to do with the gas piston, but maybe there's something else causing it.

[HectorMedina]

Bill

It's the lube.

Lubes are designed to work themselves into nooks and crannies and with heat, even grease will flow.

Don't worry, just reconcile yourself to the fact that all springers need between 5 and 10 shots to achieve "steady state conditions".

Now, if after the 30th shot, it is considerably lower than on the 10th, then THAT is cause for worry.

If you really want your gun to start and stay shooting the same MV, then your tune needs to be different and you should use Teflon based lubricants that are "sticky" and VERY LITTLE of them.

It CAN be done, but it is a slightly different thing that using a Vortek kit and the greases that come in it.

Keep well and shoot straight!





HM

[subscriber]

But if lube is burning off then why would it return to the original velocity after the rifle sits for a while.

Oil in the grease is creeping towards the seal when you are not shooting.  Then that lube is consumed when you start shooting it.

Another possibility is a small amount of spring creep - as in relaxation from spending time cocked between shots.  Then after "resting", spring "length" is restored.  This suggests that if you simply store the air rifle cocked for an hour, then start shooting, the velocity should start out reduced.  If that does not happen, it points to the lube creep theory.

[Bill_in_TR]

Appreciate all the suggestions folks. I will continue to play a bit with this and the D34 while I am eagerly awaiting a new 280. The rifle shoots well and until I got the chronograph I didn't even know this was happening. Right now I am spending more time with the HW30 anyway. I haven't noticed the same behavior in the 30 but that is a good bit lower powered.