Tuning Avenger velocity pogo issue (+-100 fps)

[scotton]

So I finally sat down to run a string over the chrony and I have an issue.

Avenger 22 shooting 16gr Air Arms Fields

Chrony Alpha indoors with LED lights on the shades

Reg set at 1500 psi, hammer spring about 3.5 turns in, gun filled to 4350 psi. When I first set this tune up a couple of days ago it was shooting 880 fps, but I only checked about 5 shots.

First 11 shots looked like this:

915,906,899,898,903,896,900,887,875,873

Ok, I can live with tossing the first shot away.

Now things get ugly with the rest of the shots:

787,786,880,883,931,784,887,788,883,783,883,872,783,798,795,793,795,789,872,795,797,805

Everything sounded the same, so I thought maybe the chrony battery is low. So I put in a new battery and kept going:

868,872,862,869,860

Ok, must have been the battery. But wait, there's more:

792,868,859,791

So we are now at shot 43 and the gun is showing about 2300 psi still in the tank. I decided to stop wasting pellets and come get some advice.

My first thought is that I'm running the reg too low. My second thought is that I need less hammer spring if running the reg that low. My third thought is that I need a new chrony (although it hasn't acted up before).

My plan is to turn the reg back up to 1800-1900 PSI and test with some Air Arms Heavy 18 grains at about 880 fps. If that works and I don't get the weird velocity dips, I should have my answer.

Feel free to comment while I go do this.

 

[Odoyle]

You need a weaker valve spring.

 

[scotton]

Hello from the future. I forgot to hit submit when I went and tried my fix.

So I bumped the reg up to about 1600 PSI and shot from 3000psi to 2000 psi (I had to stop because I don't like to shoot in the dog room while the dogs are trying to eat).

881,884,876,877,872,872,872,870,869,872,866,868,868,868,868,868,862,867,871,868

19 shots, 871 avg, es 22, sd 5.3

I'm thinking the first few shots were the reg settling in, but no weird dips or jumps.

I'll run a full string tomorrow, but it looks like the reg wanted that little 100 PSI bump to work right. I'll need to see if I can turn the hammer spring down a bit to get it to 860 fps so I can stay in the 30 FPE division of the extremely lucrative benchrest silhouette matches.

 

[scotton]

You need a weaker valve spring.

YOUR BUTT NEEDS A ...

Sorry, that was my standard reaction to being told I need a weaker something.

I think I got it figured out, I was apparently trying to find the lower limit of the reg. I agree that a weaker valve spring would help if I really need to shoot the 16 gr pellets at 880 fps, but I've got options that will keep me from figuring out how to change the valve spring, which sounds like something I don't want to mess with currently.

 

[scotton]

Oddly, I haven't noticed any vertical stringing when shooting groups that would indicate random 100 fps dips. Makes me wonder if maxing out the air tank to 4350 had an effect. Although that wouldn't explain why the dips didn't start until shot 11. Or maybe it would. I'm getting tired head, I'll check it again tomorrow at the new setting.

 

[Odoyle]

Some regs tend to act crazy at high pressure past 3000psi in some cases.

 

[FuzzyGrub]

If the problem returns, keep an eye on the plenum pressure. Watch it during a shot cycle, and from shot to shot. Were the dips in velocity, consistent with a quick follow-up shot?

 

[scotton]

When shooting a string I slowly count to 15 between shots to try to give everything a chance to normalize. I've had regulated guns that do not like to be rushed.

 

[FuzzyGrub]

The intervals were not on mag changes, either. If the plenum pressure is stable, then another area to look at is the hammer. A sticking hammer can cause that. The precurser to Avenger, had the issue of gaulling in the aluminum channel. Avenger uses same hammer, and think same passage design. I think most of the gauling issues were from power hungry users, who were cranking everything they could out of the HS. It just one of those areas to check out, if problem returns.

 

[scotton]

Thanks for the info. I have a match Saturday so I'm going to leave it alone for now. I have been know to fall into a pre-match tuning cycle that ends in low scores, tears and hair loss.

 

[tracker1955]

I think the reg in these is similar to the ones in the Nova Liberty, they came from the factory set at aprox 1800 psi. and have the same size plenum. I agree that your reg pressure was too low and was causing issues, the higher tube pressure might cause creep but not make a declining jumpy reg like you appeared to have been experiencing. Most all 22 Libertys came shooting JSB 18.13's at or just under 850fps. Hope you've got it sorted out.