Krait low speed for first shots after sitting a few days

[dens228]

My Krait was doing great, consistent and accurate. Several weeks back I lowered the spring tension which actually added FPS and made the rifle much more quiet. Quiet was what I was looking for as the gun sounded like the hammer was striking harder than it needed, which turned out to be true. Now for the last week the gun shoots quite a bit slower for the first several shots and settles back in where it should be.
The thing I noticed as I keep an eye on it is that the reg is very consistent and returns to the same pressure after each shot, it's also on the correct pressure before that first shot.
This has me thinking,..........hammer spring tension.
Maybe I lowered it too much to overcome any setting in after sitting a few days??
Thoughts before I start messing with it.

Edit.........meant to put this in the PCP section.

 

[JimD]

When you got increased velocity from lowering the hammer spring setting it means it was badly tuned as you are thinking (way too much hammer spring). When the first shot velocity (or first several shots) is low, I believe that means the hammer spring is a bit low for the regulator setting. So you went from too high to too low. If you don't mind the first few shots being lower in velocity it won't hurt the gun to be tuned this way but if you want the first shot to be consistent with the rest you need to give it a little more hammer spring. I will use my newest gun as an example of what I am talking about.

My Caiman came to me with a regulator setting of 100 bar (it does not have a gauge but I watch when it falls of the reg and I have also had it apart and looked at the Huma regulator). With the wide open air path it has, that can give me a velocity of as much as 940 fps with 18 grain pellets, it's favorite. Initially I turned it down to around 875 fps because I think that's fast enough and because I want them to stay stable. When I did that, my first shot velocity would be lower than the rest of the string (20 fps or more). The gun shot well, I just had to give it several shots to settle in. More recently I've given it a bit more hammer spring and it is now shooting at about 900 fps. I shot a few shots last night and the first shot was less than 10 fps lower than the highest velocity. I did a 20 shot string with it at these settings and got an ES under 8. So it likes this tune better. I have not shot a 30 yard challenge score better than I shot with the 875 setting but I've shot another 199 with it tuned this way. I'm hoping for some time when the wind is calm to see if I can't get a 200, the challenge is both the wind and me, I am pretty sure the gun can do it. I prefer to have the gun shoot the first shot at the same velocity as the rest of the string.

5% under the peak is the "normal" recommendation for velocity but I've found sometimes my guns want a little different velocity. But I think it is quite important to establishing a good tune to know what your peak velocity is for a given regulator setting and then trying hammer spring settings a bit below and also possibly slightly above that value to see what the gun likes best. My Avenger seems to want the hammer spring to be slightly above the point that gives the highest velocity. I think it is a little bit more accurate tuned that way. But my other guns want the hammer spring turned down slightly from the peak. Not always 5%, my P35-25 wanted about 3% at its previous tune. I think it is normal to set the hammer spring to give a velocity a little below the peak.

 

[dens228]

Thanks. Later today I'll start increasing Hemmer tension a bit. I may be able to lower the reg a bit and stay in my desired speed range.

 

[Dairyboy]

3-5% below max fps is generally the best range for a regulated PCP

 

[Vetmx]

My Krait was a weird gun to tune. One it was sensitive and two I could never get the balance I was looking for. Balance being first shot or shots consistency, then consistency from full bottle down to reg set point. What I’ve experienced over a wide variety of guns is you have to do what works for you in the situation the gun is used for. If you are a ES junky, then you might find numbers in the 4-5% range that let you sleep at night but the catch is you are dumping your first few shots in the dirt. You have to or they will ruin your beautiful shot string. Or you operate in the 2-3% zone, maybe have a little larger ES but the gun is consistent with its cold velocity and accuracy doesn’t noticeably suffer.

Some platforms make life easier but for now in airgun land, they can still have quirks. Dual regs let you get away with murder when it comes to the 3-5% rule. But, you’re gonna have to deal with temperamental barrels. Or you go the balance valve route and deal with a complex little valve that can have issues from time to time.

 

[JimD]

If the Krait has a pivoting hammer like my Caiman I agree the hammer spring adjustment is "sensitive". I struggled to find a setting that gave me about 875 fps. Much smaller rotation of the screw was needed than I am used to. My other guns also do not have a grub screw to lock in the hammer spring adjustment. It's an interesting design and it is used effectively in the Caiman to keep it really short.

With respect to ES and standard deviation I have found my guns have lower values when tuned to a place they shoot accurately. My Caiman had ESs a little under 20 at it's previous tune but the last string I measured at it's current tune was under 8 fps. Seems like guns like tunes somewhat like they like certain pellets.

 

[dens228]

The Huma reg on my Krait has that adjustment that needs the pins. I can use a needle nose but ordered an actual pin key to adjust it.
I really think I need less pressure at the reg as my starting point. With my pressure at a bit over 100 I can easily get into the high 900's with the hammer spring adjustment. Too much pressure as I don't want to shoot that fast. My groups start to open up around 930 or so. Once that tool gets here I'll start the tune over with lower pressure and go from there.

 

[JimD]

I have not seen that sort of regulator, my huma has a simple straight blade screwdriver head on it. It sounds like you need to decrease velocity significantly over 5% and if that is true I agree you need to decrease the regulator. I was thinking of it before I got good results at about 900 fps. My peak velocity was about 940 fps so I am at less than 5% under. I was still getting low first shot velocity when I was 5% under. I don't know anything about the Krait layout. To change the regulator on my Caiman you essentially have to take the regulator out of the gun. It is not terribly difficult, fortunately, but it is nothing like the external adjustment of my Avenger.