Excitement

[sholo]

Wouldn't mind improving on those strings though. It doesn't sound like it's hammer bouncing. Thoughts, comments, anyone?

You could get yourself some 1/4" Teflon rod and make some TP's of various sizes? Velocity will drop a bit (how much will depend on how small you make TP) but your string should flatten out, gaining a few shots in the process.

SeanMP posted a good thread on TP sizes for his '78. His is .177 mind you, but the principal is the same: http://www.canadianairguns.com/showt...of-a-QB-on-HPA

Hopefully Sean or Bob will chime in with a more accurate analysis than mine for you!


Todd

[Doc Sharptail]

You must be an early riser. Thanks for the input.

After sleeping on things for a bit, there's still a lot to do.
I will try increasing fill pressure a bit more. I may need to re-think how much I flow the valve as well. I'm very close to where I want to be...

I'm no where near the 200 bar that Bob mentioned, and by the look of things, won't have to go quite that high. The tube I'm using is 3K PSI rated, so there's still a bit of room to work with.

I'm almost certain that the next build is gonna be .177, and the learning curve's still a bit steep.

Regards,

Doc Sharptail

[SeanMP]

Hi Doc

Looking at your #'s it seems pretty evident that there is too much hammer strike and not enough fill pressure. All your seeing is the very heel of the curve. And seemingly your blowing a ton of air down an empty barrel

I would amp up the fill pressure to above 2200 at a very minimum and see what happens. Also the shot count seems incredibly low...Bob may be able to decipher that from experience.

But for sure the efficiency will come up if you increase the fill pressure. I'm guessing that you have one of the chromoly HPA tubes? If so you can feel comfortable going up to rated pressure if you want to. I've done lots of math and real world hydro testing on the ones I make and there is lots of ceiling.

[Doc Sharptail]

Yes, it's one of the chromoloy tubes Eric had in the store a while back.

I'll be working my way up slowly. I had read elsewhere that they could withstand 4k PSI. I have no intention of going anywhere near that. I wonder what I'd look like after pumping manually to 4K?

Regards,

Doc Sharptail

[SeanMP]

Well. this is anecdotal and I'm not suggesting that this is a safe working pressure but my hydro-static pump will go up to 8k. After I machined a similar unit I installed the real threaded plug and a dummy valve and tried to blow it up. The relief valve on the pump popped and the tube was fine.

[rsterne]

Yes, those strings indicate you need less hammer strike or more fill pressure.... I know you're using a CrMoly tube tha is good for 3,000 psi, but has the valve attachment been addressed for those pressures?.... At a minimum you need to make sure that the valve is right up against the steel backing block so that all three screws are taking the load.... That means, of course, that you can't remove any of them (ie take off the breech) while the gun is under pressure....

Bob

[Doc Sharptail]

Thanks for all the input, guys. It helps a lot. You guys are an invaluable source at times like this. I'm treading lightly, for sure.

Bob and Sean:

The back of the valve butts up snugly against it's blocking ring. I'm pretty sure buying this tube was my smarter moves. I'll be keeping my eye out for more as finances allow.

Tonight's short session was a huge improvement- I actually got a string.
I pumped the gun up to 1800 PSI. First shot was no different than last night.
On the 2nd shot, Kapow! After the dust and smoke cleared, a tiny mushroom cloud wafted up out of the pellet trap.
With ears ringing, I picked my jaw up off the floor. I'll let the numbers tell the true story.

HPA 2078

Predator 16 gr

1800 PSI

1.) 759.0
2.) 982.5
3.) 985.5
4.) 981.1
5.) 975.9
6.) 729.4
7.) 724.5

I didn't want to go any further with the fill pressure- that's nearly a 226 fps difference between the first, and 2nd shot. I'm wary of approaching a little too close to valve-lock.
I'm going to have to get some control over that hammer spring, before I proceed any further with the fill pressure.

That little 4 shot string tells me I'm headed in the right direction.

Regards,

Doc Sharptail

[rsterne]

What is your valve stem seal made of?.... If it's the stock soft material, it was likely stuck on the seat slightly on the first shot.... Even Delrin can show a slighly low velocity on the first shot, but only 20ish fps or so....The 6th shot, I have no idea..... How much volume do you have in the reservoir?....

Bob

[Doc Sharptail]

Bob-

The volume escapes me right now. I did know it when I ordered the tube in January last year- it was one of the selling points for me. Spent the last hour searching my machine for the volume, and am drawing a blank. Maybe Eric remembers?

At any rate, I'm hoping to tear the gun down a bit tonight after work, if I've got enough steam left. May end up getting the volume the old school way with measuring tape and caliper.

It's a completely stock valve stem and seal. I'm more than a bit curious as to what's happening inside that valve, and also want to polish up the stem shaft a bit. It could use it.
Also considering reducing the air flow a bit- I've got another valve here to work with. ~Pretty sure the upper 9's are a bit faster than what the Predators were designed for, anyways. From what I've read around, mid 8's seems to be the sweet spot accuracy wise for them.

Regards,

Doc Sharptail

[SeanMP]

Ya it does look like something is sticking. I was almost thinking hammer or rolling sear.

The airgun elf is delivering the delrin stems to a certain little birdie tomorrow. Fastenal was kind enough to ship a whole bundle of thoroughly rusted drill rod which had to be sent back and then the wait for the original manufacturer to deliver unrusted material.

The RVA's for QB's are as you know a real head scratcher. Mainly because everything that anyone has done including the one Todd and I made up require the end user to drill an accurate hole through the breech. The only solution I can come up with is a supplied end plug the has an adjustment hole at a slight angle so that the original screw configuration can stay the same.

Would that appeal to anyone?