Author Topic: .25 Flash – "Korean Cliff" Tuning (Read 257 times)

UCChris · « **on:** October 12, 2019, 03:13:58 PM »

Hey all, tinkering with my Hatsan Flash .25 again. I have a couple shot strings to share and some questions for those that know far more than I do. First, here are my shot strings. The first shot string is with the internals polished and new seals throughout the gun. I don't know what the hammer spring was set at; the important part being that the hammer spring stayed the same between the two strings. The second string is identical to the first, but with the air tube and transfer block opened up to 3/16" to match the barrel port. You can see that I have a "Korean Cliff" going on. I know that I need to back off the hammer spring (I am grabbing some long handled wrenches today), but my question lies in what I can do with pellet weight. These strings are with the standard Kings but I am really wanting to try the 33.5 NSA slugs. I don't know if they'll be accurate in the Hatsan choked barrel, but that's beside the point right now, since I can use the JSB King Heavy if the slugs aren't accurate. I am filling to 3000 psi, so I can't increase my fill pressure to get a better curve.

So, after being way too long winded. My question is what can I expect to do with 33.5 grain slugs and messing with the hammer spring. I'm going for 5 shots in a 2% spread, not sure what the max velocity I can get out of it is.

rsterne · « **Reply #1 on:** October 12, 2019, 04:19:13 PM »

I can't read your graphs very well.... but it appears your second string starts at about 940 fps, and does indeed show a declining velocity…. and about 50 FPE of energy.... Changing to the 33.5 gr. slugs should increase your energy slightly, but at much lower velocity of course.... I would think you would be lucky to hit 850 fps (54 FPE)…. The heavier slugs should also flatten out the beginning of the shot string a bit.... but whether you would get 5 shots within a 2% ES is the question.... Right now you maybe have 4 shots within 2%, and no bell curve....

Increasing the hammer spring may increase the power slightly.... but remove any chance of getting a bell curve.... All I can suggest is try it with the 33.5 gr. NSA slugs with no changes and see what you get....

Bob

UCChris · « **Reply #2 on:** October 12, 2019, 04:26:46 PM »

So sorry, I thought you could click on the graphs and enlarge them.

UCChris · « **Reply #3 on:** October 15, 2019, 08:16:18 AM »

Alright, I got a bell curve out of it. I was still getting a cliff even with the hammer spring backed out as far as possible. So this is with 3/16" porting, hammer spring shim removed, and 4 turns on the hammer spring. This is with JSB 25.39 Kings, 3000-2400 psi, 165CC tank. That puts me at an efficiency of 1.08, which is .02 less than out of the box. Not bad for a considerable fpe increase via porting.

887
889
890
896
892
894
893
891
893
890

That's 10 shots (one magazine) with less than 1% spread. I need to shoot a longer shot string to see how many shots I get before it drops off. I have a Huma reg showing up today and I'm going to have to decide to regulate the gun or not, depending on the results of a 20 shot string. I had originally bought the reg to make a hyper-efficient 12 fpe Flash, but maybe it will be good in this rifle. It is the .25 regulator.

UCChris · « **Reply #4 on:** October 15, 2019, 11:53:25 AM »

I did a 20 shot string and came up with this:

Here are the numbers

894
890
887
893
891
897
875
890
873
870
888
873
876
854
849
856
835
831
818
810

I think it's pretty clear that I'm starting somewhere in the first 1/3 to 1/2 of the power curve with a 3K fill. Since I can't fill higher to get the first part of the curve, I have either back off the hammer spring until I get a full curve, or I could explore regulating it, right?

UCChris · « **Reply #5 on:** October 15, 2019, 12:52:34 PM »

Just put the regulator in set at 130 bar. Here's my shot string:

808
795
792
793
793
806
791
789
787
786
796
787
785
780
779
772
758
753
747
727

This is from 3000-1600 psi, with an efficiency of .71, so the current hammer spring tension is definitely on the plateau. I will bump the reg to 160 bar and see what happens.

rsterne · « **Reply #6 on:** October 15, 2019, 01:07:40 PM »

Your 20 shot string in Reply #4 starts in the sweet spot and then declines.... That is why you had a low ES for the first 10 shots.... Increasing the setpoint pressure will increase the velocity and move you closer to the knee of the regulated power curve, increasing the efficiency.... Whether that is the entire solution or not, you will have to try to find out....

Bob

UCChris · « **Reply #7 on:** October 15, 2019, 03:03:05 PM »

So I have been adjusting and shooting and adjusting more. This is what I've come up with.

Regulator @ 160 bar
No hammer spring spacer @ 2.5 turns out
3000-1900 psi

819
810
804
813
813
810
816
803
811
811
813
814
816
798
802
812
805
804
798
798

AVG: 809
ES: 21 or 2.56%
SD: 6.32

I think the ES and SD will shrink as the regulator breaks in, since I only have ~80 shots on it from all my testing. I'm getting an efficiency of about 1.18, which is about what I hoped for. But I don't think it's terrible for a ~130cc (after regulator) reservoir and a 17" barrel. I think a lightweight hammer could definitely get me some more efficiency, but the gun is really, really quiet with the Ronin on it, so I think it's doing okay.