Kral Jumbo project. Some observations.

The stock P15 spring is like 8.5mm diameter. I chose it because I only had to drill out my hammer a little. The spring I put in my buddies Orion was a .36” diameter with .051 or .052” wire. I clipped it short enough to allow the hammer to free flight but can’t remember how short I went. It’s in his gun tuned perfectly so I can’t talk him into taking it apart to measure. I had to turn down the spring guide just a little so it would fit into the spring. I have a Cometa Orion bullpup mini. I went with an external SSG on it with just a slight mod to the stock.

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Thanks much! Time to pay my Cometa a visit. 

That bullpup of yours also looks comely ... I checked some websites ... they go for around 600 EUR ... NO! NO!

Seriously, the structural issue I see with this model is the valve O-ring, which is hard to replace. I found a desmontaje video, as well as the exploded diagram. I understand you need to remove the cylinder cap and pull out the reg (if there is one). Then it gets tricky: insert some kind of huge split flathead screwdriver tool into the cylinder from the front all the way to the valve block and unscrew it. That part pressures the seal against the inside of the cylinder, so it needs to be removed to replace the o-ring. If only I knew the specs of that non-standard tool I would feel less apprehensive for when the day of o-ring reckoning ultimately comes ! Anyhow, I am going off topic ... 

Hello Vetmx, I'm new to pcp's, but I was wondering, would it be possible to lengthen your probe just a little bit to squeeze out a little more TP porting so not to be as anemic. I'm guessing since you cut the barrel down that you have access to lathe and or mill?

I am a .25 adept because my property is windswept. It is certainly true that the Kral speeds even in that caliber are nothing to sneeze at, especially out of a 53cm barrel with the ports open to 4.5mm. (This is the maximum with the stock power adjuster; need to replace the adjuster with a bigger diameter unit if you want more.) No pellet skirt issues in my experience (3 Krals) with that porting, as long as the dremel work in the barrel port is somewhat oval and slanted towards the breech end, not the muzzle. The ordinary JSB Kings travel at an honorable 255-260 m/s out of that set up.

That said, I am always itching to step up to the 30+ grain King Heavies, and would then like to see them go faster than the 230m/s that is in store. 250-260m/s will keep me satisfied. I am hoping that is feasible with 5mm ports. It is true that this step up comes at the cost of some effort. Especially, a bigger power adjuster pin as illustrated by TheBanker4. Is it worth the trouble? Well I would certainly tend to give it a go based on Troy's reassurance about skirt issues ... Unless I'm wrong  

Update: I went into my Cometa Orion 6.35 to address what I thought was hammer bounce. Turns out it most likely was action noise instead. Will post more under the SSG topic.

(Tangent: I also came across the Cometa (Airforce International in the US) Lynx bottle gun. Could this be a Kral Jumbo competitor? Looks like it has similar features, as well as superb accuracy.)

@Vetmx: whilst messing around I also discovered that the Cometa/Airforce Orion 6.35 barrel has the same OD as my SPA Artemis P12. The Cometa barrel by all accounts has excellent accuracy, cold hammered, and with a 1/2 UNF muzzle thread. My p12 is reasonably accurate at 60m (ragged hole with some fliers). However the P12 remains somewhat noisy, even after sliding the shroud forward and putting in 6 baffles from an Aliexpress oil filter. Sadly the P12 barrel does not have a 1/2 unf thread -- hard to put a can on. I can get those Cometa barrels reasonably; all I would need to do to put one onto a P12 is cut off around 4cm (still to be measured accurately) at the breech end, and polish etc. You mentioned in this Jumbo modding topic that you shortened your Kral Jumbo barrel at the breech end as well. Is that all there is to it -- do you think I would mess up the Cometa barrel by simply shortening the breech end without any further mods except for gentle tapering & polishing ? P12 does not require a barrel TP and this type of work I can do with my simple tools ... Questions questions ...

My .25 Jumbo finally arrived. Vetmx you were right -- this gun shoots pretty rough out of the box, more so than the PB. Pretty noticeable ping too, and a hefty bark. I gave it the works:

- Sanded and oiled the stock. It still needs a few more coats of walnut oil over the next few mos.

- Polished the trigger parts, adjusted set screws etc. Very nice/light 2 stage now. The 2nd stage screw is extremely sensitive.

- Put on a moderator. This also made it into a truly "jumbo" length gun. This gun really needs the look of its shroud. I modded the end piece with the 1/2 unf that the barrel slides into. It now vents into the shroud -- less shock on the shroud when firing. Put felt around the barrel. Some have complained that the shroud diameter exceeds the thickness of the breech block. For me this does not detract at all, in fact it allows venting without drilling holes.

- opened up the ports to 5mm. There was enough clearance between the barrel port and the pellet skirt to make an oval enlarged port. Also machined a new power adjuster out of an 8mm allen bolt. See the picture below. 

- installed a paintball reg plus adapter piece (30 usd in total). Breaking the thread lock required serious heating with a small Campinggaz torch. You really have to keep going. I found there is little or no risk of burn marks on the components because the powder coating is very sturdy. But i did replace the o rings just in case they had burn marks.

- lightened the hammer and put in twin springs. Lapped the barrel.

The gun sounds and looks more civilized now. Easy cocking, ping gone, quieter report. Next steps: put in a stronger reg. The supposedly 2200 (150 bar) psi Chinese reg only delivers around 1600 psi -- shots are underpowered at 240m/sec (JSB exact King,) This big gun calls for more. Also need to scope the gun and do an accuracy test.

DOES ANYONE KNOW how to increase the power in a paintball reg by changjng the bevel washer configuration ? ?

 

The larger power adjuster with 5mm port. Fashioned out of an 8mm allen bolt in my power drill.





The washer stack out of my paintball reg. The left one is around 1400 psi, the right one 1800 psi, tops. I would be really appreciative if someone could explain how to reconfigure this to get to 150 bar/2200 PSI.









Kral family, all in .25. The Jumbo is not a small gun, but it needs the moderator for quiet shooting and I don't have the skill to shorten the barrel...

You don’t reconfigure, you add shims. A .010 shim will generally jump you up 150psi at first. I had a .015 shim jump a reg from 1900psi to 2200psi. It seems the higher the reg pressure gets, you have to use even thinner shims because it really starts to jump up. I use .010, .015, .005 and .002 shims when tuning a bottle reg. Go one shim at a time. If you go too far be prepared to blow a burst disc and have to change your underwear.

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Thanks -- that would make her laugh about my addiction for a change ...

Wonder where I can get those shims?

My Jumbo .25 is now pushing the 25 grain JSBs at 820 fps. (250m/s, around 51J.)

The good news: I am using a paintball reg with a 125bar set point only. The piston is too short to add more shims, so I am awaiting its cousin with a higher set point. BUYER BEWARE: what was advertised in Aliexpress as 2200 psi (around 150 bar) was in fact much less! Other than that, those regs are well built and the price unbeatable.

So there is much more power left in this Jumbo yet, and more tuning to be done -- an SSG included. Its report is on par with the FX Royale 300 although the Kral seems to be hitting the valve harder. It has become a very civilized and accurate shooter though.

If I love it better than my FX, that is because I am a tinkerer. If I were in the hunting business and had to count my modding time, the FX would be the better and cheaper gun hands down ... It is shorter too, despite the 600mm barrel. The long Czech built moderator I put onto the Jumbo has something to do with that ...

Note: the top and bottom are both FX Royale -- the top is the misleadingly named fx300 (.25 barrel, 400 cc cylinder) and the bottom is the fx 400 in .22. The Jumbo's similarity with that FX is presumably entirely coincidental ...

Very interesting. It may explain why my P12 suddenly fell out of love with the Baracuda Match after I changed the hammer spring, regulator setpoint, and valve spring preload. Shot count increased dramatically and speed did not suffer. However shot accuracy with the lighter JSBs became excellent, whereas the former infatuation with the heavier Baracuda reached a sudden end. I thought it was the barrel, but that did not make sense -- nothing had changed there.

Another example was the Artemis M16 (aka diana Outlaw). An amazingly accurate 60-80m shooter in .22 with the Baracuda Match 21.14 grain. Then the stock regulator started cracking up and I replaced it with a Huma. M16 promptly started spurning the Baracudas, nothing doing. lt now laps up the JSB 15.89 grain instead.