Tuning Kral Jumbo pressure questions

[Dr. Kralenstein]

I wouldn't exactly compare a tire made of rubber with flexible steel belts to a hard wall steel tube designed to be a pressure vessel, but I get the sentiment. Also, that tire may be just fine being overfilled... until you start driving on it down the road and it heats up and expands. Also loading the weight of the vehicle on it is another variable. Not trying to sound hard headed and not listening, but those weren't the best two items to compare. 

And no, I don't over inflate my tires. I do however inflate to the tires recommended cold pressure, not what the sticker in the door jam says. Everyone thinks 35 psi is the gold standard, but you're supposed to inflate to what the tire manufacturer states on their specific tire. 

 

[Dr. Kralenstein]

Just looked at the bottle and it is rated for 300 bar so at least I have that safety margin covered. I honestly believe the 200 BAR fill is solely because of the valving and also, it'd much easier to fill a gun to 200 BAR with a hand pump vs 250-300 BAR. Gotta remember, it's a $600 rifle. Most buyers probably don't have a compressor. Not everyone has compressors and FX, daystate, kalibrgun, etc at their disposal. 

 

[Vetmx]

I didn’t want to get into a tire debate. I was just pointing out perspectives. 500psi doesn’t seem like a lot because of what we deal with. But 500psi is no joke when you look at it all by itself. 

 

[Vetmx]

But just like a tire, you are working that metal across pressure ranges. Not as extreme, but it’s still happening. I don’t have a clue about the long term effects of what we are discussing. But I can tell you that I’m not going to be a lab rat over a pellet gun. 

 

[Dr. Kralenstein]

I get what you're meaning. I'd just really like to squeak out a bit more power with this gun without adding 6 more inches of barrel. It's probably doing close to 50 fpe with that barrel.

 

[Vetmx]

How does the gun sound with you blasting that much air out the barrel. Are you sure you haven’t ventured into the land of diminishing returns?

 

[Dr. Kralenstein]

Sounds fine to me, I haven't shot it without the DonnyFL Koi on there though. 

Doing 840ish with 25.4 grain AAs

Doing 915ish with 20.06 FTTs

I'd just like to see 900 with the heavier pellets. It's got more in there I'm sure. I haven't even messed with the hammer or spring yet, just ported out. I'm sure my shot count will go up if I can get the hammer bounce to stop. Currently gets 40 shots in its curve. Then 20 or so more decending slowly a few fps at a time. There is more air left, I just don't go past 50-60 shots since the POI drops. 

It has a nice sound to it, minus a bit of bottle ping which I don't really care, got used to it. Can always stuff the bottle with a nylon brush. Figured that would take up too much air space on the 425cc bottle though.

 

[Vetmx]

To stay in that 40 shot sweet spot, what are your pressures?

 

[Dr. Kralenstein]

Pressure is 200 down to 150 bar then it starts to slowly creep down. I'm talking 910-915 fps for the first 25 shots, then it gets to 900-905 for the last 15 or so. Shot 40 was 898 last time I shot it with the FTTs. It gets down into the 875 range by 50 shots though.

That was actually a 3100 psi fill, so says my compressor. Not even a curve there, im sure 3200-3300 would get me a curved string.

So to clarify, the gauge on the gum reads 200 BAR, top of the green. My compressor says 3100 psi.

 

[Vetmx]

If it’s really accurate as far as you plan to shoot it, you may have found a really nice spot for a gun with that short of a barrel. You have a couple things working in your favor right now because your valve is closing fast. If you had a reg in the gun you would be set at an insane reg setting. But you also have some things that could bite you. Especially if you keep pressing onward and upward. My Jumbo regulated shoots 18.1’s at 833fps with my short barrel. Can I get it to shoot the customary 880fps? Yes. But I would be going past the when to say when stage of the build. Right now because of its dead nuts reliability and consistency, that 50fps doesn’t mean squat. I just got back from my sisters in Va today. That gun has sat in the safe almost a year and I took it for my brother-in-law to shoot. Topped it off and never even touched the scope. Spinning spinners out to 80yrds. Your shot count/barrel/power would sell a ton of those guns if they came that way. I think you need to focus on the Harlem Globetrotter inside your gun. Get it refined and enjoy it for a while to see if it’s reliable and consistent. Maybe the inner American in you will subside and you’ll realize 60fps is no longer important.

 

[Dr. Kralenstein]

You have a good point there. Realistically, I can just put the rifle barrel on if I need more power and it is doing consistent groups, especially with the AAs and polymags out to 40 yards is as far as I've taken it. But the AAs will give me dimes, and the polymags I thought weren't hitting the target til I realized they were going in the same hole. I can stretch it out to 75 at my dad's house, or go to the range. This gun doesn't really leave the house so I don't know why I'm looking for crazy hunting power anyways lol. I just have to tinker with everything I own. If I could hot rod my stove, I would.

As for the B-Balling hammer, I'm waiting on a spring assortment to get here. Hoping I can get one that fits without too much modification and will calm that bounce down.

 

[CyberDyneSystems]

Not recommending anything drastic, but I just wanted to chime in to say that the Pro 500 has a 250 bar fill rating right in it's specs.

I understand that the Jumbo and Pro 500 are literally the same hardware with a different stock and bottle size. (i've also got an ambi Jumbo stock, but prefer the original right hand only Pro 500 stock.) Of course is possible that the springs valving etc. might also be different. So based on nothing but the sepcs of my Pro 500, i am in tentative agreement that 250 bar would be completely workable with new valving.

 

[Dr. Kralenstein]

Not recommending anything drastic, but I just wanted to chime in to say that the Pro 500 has a 250 bar fill rating right in it's specs.

I understand that the Jumbo and Pro 500 are literally the same hardware with a different stock and bottle size. (i've also got an ambi Jumbo stock, but prefer the original right hand only Pro 500 stock.) Of course is possible that the springs valving etc. might also be different. So based on nothing but the sepcs of my Pro 500, i am in tentative agreement that 250 bar would be completely workable with new valving.

Thanks for posting this and clearing up what I was thinking. My jumbo when filled to 2900 psi starts at its highest velocity and after 15-20 shots it starts a slow decline. There was never a curve. Now if I go to 3100 psi, I get more shots at that high, say 20-25 shots then it starts the decline. I'm betting if I go to 250 BAR it will have a nice curved string and get me the 50 shots or so it's supposed to get in its curve. 

Once I port the rifle barrel to 3/16" and install that, I will report back with a new numbers and also from a 250 BAR fill. 

 

[Dr. Kralenstein]

Decided to run a string with the NP-03 barrel and FTT 20.06s. This is on a 3300 psi fill, the gun didnt explode lol. In fact i think it does have a 250 BAR fill by the string. I think this new wave of jumbos have the higher fill like the pro 500, they just didnt label them as such. Anyhow, i think this is pretty darn consistent for an unregulated gun. There is a couple in the string where the velocity dropped to an odd number which i think was just me wiggling the gun around since i was shooting it off hand over the chrony. When i port the 530mm barrel and install i will fill to 250 BAR and let her rip!

 

[moodyblues]

That’s a great string. It does appear it likes/needed the higher fill pressure. I filled the breaker to 3,200 yesterday, didn’t have enough spring in the gun. Tuned for MRDs at 825. It started the string at 770 and took nine shots to get to 825. I’ll probably play around with it a little more.

 

[Dr. Kralenstein]

Not sure which hammer/spring they use in the breakers, but the mega I have is a different setup then the jumbo. The jumbo has a heavier hammer and smaller diameter spring but it's stiffer. 

I wonder how it would respond with that combo installed in the breaker?

 

[moodyblues]

I think the actions are the same length so length of throw should be the same. Don’t know why I wouldn’t get the same results. I’ve seen jumbo hammers for sale somewhere, gotta go find them. I need to go get some more springs, I’ve ran out of good combos on the assortment I bought.

 

[thammer]

Jumbo bottle is stamped 200bar wp 300bar test pressure. The valve and action are the same as the Super Jumbo/Pro500 which have 250bar bottles. You can swap bottles to 250bar working pressure but shouldn't fill the standard bottle beyond 200 bar is rated for. 

 

[Vetmx]

I am currently tuning an unregulated gun with a 13 inch barrel. With my current spring, hammer weight and porting, the gun is sweet with 16gr pellets at 850fps. Starting pressure 210b. A 3.5” mod is all that’s needed to make the gun silent. Decided to push for the coveted 880-900fps. Increased hammer strike and found a higher start pressure that was necessary for a nice flat string. Long story short, I’m back to the 210b 850fps tune. The gun just feels and sounds better at those settings. I know we were talking about how far we can safely stray from what is stamped on a gun but keeping it within reason, in the end you have to listen to the gun. Yes mine got faster the more I strayed and pushed it but for now the gun is telling me it likes it a little slower than where I pushed it. I look at it like this and lets put the whole safety parameters aside. I would never crank one of my regulated guns reg to 200+ bar and expect a happy gun with longevity. So all these guns that come with 200b stamped on them but probably actually do their best work from 180b down, it’s probably for longevity and a smooth gun. When you start shooting at high pressures, that takes more spring. We all think about our poppet. But what about our linkage and sear. Those things are all seeing forces that might be above the components design. Just food for thought.

 

[Dr. Kralenstein]

Jumbo bottle is stamped 200bar wp 300bar test pressure. The valve and action are the same as the Super Jumbo/Pro500 which have 250bar bottles. You can swap bottles to 250bar working pressure but shouldn't fill the standard bottle beyond 200 bar is rated for.

Thanks for chiming in Troy, I knew you would know what the guns all came with. Just looked at my bottle and it does state PW 200BAR/PH 300 BAR. I guess the H stands for test pressure. Anyhow, that's fine, I have no plans of going that high(4500 psi) on this bottle. I'll stick to 200 BAR for 30 shots in my power curve until I upgrade the bottle. I just like the 425cc bottle length.