Kral Arms Tuning Unregulated PCP - Running out of Hammer Spring Adjustment

I’m trying to think of how to make this short. When you have an unregulated gun, you don’t decide too much. Especially your velocity. You need to get the tools out to change that. You shoot it over the chronograph from a full fill and take a look at the curve and how long the flat spot is. You use your hammer spring to try to stretch that flat spot out as long as possible. Then you only fill the gun to where it just flattens out and refill it at the pressure where it drops below what you deem acceptable. None of my unregulated guns get filled to 250b. Even the ones I modified for more velocity. Some 210b another 220b and my buddies gun only get filled to 190b.

How long is the flat spot on your bell curve with your spring cranked all the way in? Then back it off and don’t obsess over your velocity and take a look at how many shots are in the flat spot of the curve. You don’t tell an unregulated gun what you want, it tells you what it wants. If it’s not enough then you have to decide if you want to gamble screwing up the gun with your Dremel tool or just live with it.

Lastly concerning a reg. You don’t just slap a reg in an unregulated gun and live happily ever after. Your porting is too small. Your hammer is too heavy and possibly your spring is too. So after you buy a reg, you have to brush up on your machining skills to get the most out of the gun. Kral most likely tested the gun with a 20gr H&N .25 pellet for their bragging numbers.

CanMike said:

I have a Reximex Daystar. (Same as KRAL as Same Manufacturer) Im trying to tune it to get the proper bell curve, but I am runing out of Hammer Spring Adjustment. Im almost all the way in on the screw and the FPS seems to be still going up. ITs a 250 BAR fill and at Hammer Spring almost at max I start around 750 FPS and will go up to about 800 at 210/200 BAR, which is what I want.. (~800)

I assume I need to get a stiffer Hammer Spring?

From my understanding is you want to back off the Hammer Spring once you hit a plateau and I can't seem to hit it at 200+ BAR (Plateau)

From my understanding with an unregulated PCP at higher fill pressures the hammer is fighting the bottle pressure so FPS is lower at higher FILL pressure..

There is a regulator from Huma-Air made for this rifle so I am wondering if thats the best course of action.. If I have to figure out a proper stiffer spring I might as well just install a regulator.

Maybe I am missing something obvious..

I should add this is 0.25 CAL and I am tuning for the JSP Exact 25.5 grain. Rifle is rated for 850 FPS according to manufacturer specs at that grain / Caliber. So I am in the range for sure, but not sure if the hammer spring should be maxed to get to that number.

Thanks
Mike

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Based on what you said, your shot string is:
250 bar 750 FPS
200-210 bar 800 FPS

That shot string is too short to be meaningful.

Try shooting a shot string with the hammer spring at the max preload.

Keep track of the velocity and pressure.
Decide which range of velocity and pressure you like.

In my Kral NP03 I installed a stiffer hammer spring so that I can start my shot string at 250bar.
Then I used the transfer port adjuster to reduce the velocity spread, which meant that I got a flatter shot string but at lower velocities.

nobody1 said:

Based on what you said, your shot string is:
250 bar 750 FPS
200-210 bar 800 FPS

That shot string is too short to be meaningful.

Try shooting a shot string with the hammer spring at the max preload.

Keep track of the velocity and pressure.
Decide which range of velocity and pressure you like.

In my Kral NP03 I installed a stiffer hammer spring so that I can start my shot string at 250bar.
Then I used the transfer port adjuster to reduce the velocity spread, which meant that I got a flatter shot string but at lower velocities.

Click to expand...

His shot string is going to start at 210b. The key is to see how long it maintains that 800fps below 210b. If it immediately starts to drop off. He’s outa whack. But if it holds steady for a while then he’s close. You have minimal air volume with a NP03 so I can understand you trying to start your flat spot of the curve at higher pressure. He shouldn’t have to.

You probably only need to fill to 215 - 220 bar for that hammer pre load. shoot and see where the velocity is but my guess would be around 785 fps. As you shoot it will climb to 800 fps then level out until it falls again. You have to decide how much ES you want in the shot string and set your max fill accordingly and your max shots to stay inside that ES. If you want 800 fps then dont increase hammer preload just find out what the max fill needs to be to get you 790 fps for the fist shots.

Requirements of a HIGH pressure ( BAR ) setting and only getting modest performance make me question a few things ...
Valve and port sizes Upstream of projectile being launched ?
Hammer weight ?
Spring specs ?
Stroke said hammer is traveling ?
Barrel length ?

The newer kral actions, and I'm assuming the rexi guns too, have a pretty shallow spring adjustment thread now. The older ones you could crank til the spring as well into coil bind.

If you must have a 250 fill, then you will need to either swap the spring, or better yet just get a dual spring setup. It'll have less bounce and better consistency. It will take time and a tin of pellets or so to get the perfect spot you want.

I will say this though. I've noticed on the newer models with the shorter adjustment range, that they have more trouble getting into a good curve and staying accurate. The valve dwell seems like it can't get set right to be happy.

I will get a good string over the chrony, but the last 10-15 shots will start to move off POA even thought the chrony is saying it's still in the curve.

Theyre definitely harder to get a dialed in tune than the 20-22 model years.

CanMike said:

Ya I am passed the locking screw in terms of turn in.. I probably have a couple more rotations before the adjustment screw looses threads..

I don't have to fill to 250 by any means.. It just seemed natural to fill max before I started understanding the physics of the way things are working. Still learning.

Mike

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Well if the gun is meant to be filled to 250 bar by the manufacturer, they should set it up to perform that way. They should just call it a 200 bar fill gun not 250. Kind of misleading. It'll handle it safely, just doesn't perform correctly at that level.

Being a 25, I would open all the ports up to 3/16". If it's anything like my Kral Jumbo in design, it'll put out 60 fpe with the right work and pellet. I get 50 fpe with only a 16" barrel and 3/16" ports. Dual shorter springs. Outer is stiffer with a softer/smaller, reverse wound wire spring inside that spring.

It takes a ton of pellets dialing in an unregulated gun. Even once you know what you’re doing. I always do a full string with how the gun comes out of the box before I become a crankaholic with the spring. After you have that baseline, then it’s real easy to see how you influence things when you start playing with the spring. Unregulated guns are like slug shooting, there are no shortcuts.

You are putting the cart ahead of the horse. Unless someone has your exact gun and has achieved what your goal is, you have some shooting and data recording to do. If you noticed, I didn’t ask what spring or port size your gun has. It’s because you have no definitive data. What does your string/curve look like with the spring in the stock position. What does your string/curve look like with a darn near maxed out spring? Now we can start making recommendations.

CanMike said:

OKay then shot string is in my future tommorow starting at 230PSI and we will see how things go. I'll report back I'll leave hammer spring as is currently set and I will look to you guys for some advise once I complete the string..

Glad I got a tin of 350 count.. LOL

Mike

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One string won’t do it. Do another with less hammer spring adjustment also. At least one turn out from where you have it now. The second the velocity falls off over 20fps, you can stop the string. We‘ll get the picture.

Then after you go thru a tin of pellets shooting over a chrony for testing, you get to find out which pellet it actually shoots best, and then tune for that pellet.




You need more pellets.

Dr. Kralenstein said:

Then after you go thru a tin of pellets shooting over a chrony for testing, you get to find out which pellet it actually shoots best, and then tune for that pellet.




You need more pellets.

Click to expand...

One of the more satisfying parts of a PCP education. Learning how to kill two birds with one stone.

I don't do full shot strings anymore.

The whole point of the shot string is to find the minimum and maximum velocity over a pressure range.

This is what I do:
-fill the air gun to the manufacturer recommended max
-shoot 2 shots with real pellets.
-shoot 5-10 shots with no pellets
-keep track of air pressure and pellet velocity
-repeat the above

You need to make sure that you know when you are close to peak velocity.

This way you shoot 7-10 pellets and get a shot string that tells you what you need to know.

You stop when you have a velocity bell curve and the velocity drops bellow what you want.