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.25 impact, 700mm superior liner, what speed?

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WildcatEric

WildcatEric

Member
Feb 3, 2018
83
4
Ky, United States
Good evening AGN,

I have a an .25 Impact X upgraded to the MKII impact, I’m running a 700mm superior liner and shooting NSA 34.5 gr Hp slugs. It shoots pretty good at 890fps, however I can’t get StrelokPro to jive well with it. The slugs are advertised to have a .096 BC and to get it close I have to true the BC down to .033 in StrelokPro to get it close at 100 but never spot on. So my questions are as follows:

What speed should they be at to get that advertised BC? Am I too slow? 
Do any of you use the same slug selection out of your superior liners? If so at what speed? 
Any information is very much appreciated! 
Thank you for your time .

Eric
 
I don't think the speed impacts the BC as the BC is:



In ballistics, the ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. 

Agree with B, double check data input first. Somehow, I always manage to foul up my data input. Sometimes building the slug info in Chairgun is easier for me. Still learning Strelock.

The BC is determined by the slug weight and shape (if I get it correctly, but who knows on this techy stuff). So if I get it right if you shoot a slug at 1000 fps at the muzzle and it slows to 900 at target, or start at 800 and it slows to whatever, the BC should calculate the same.

Good technical discussion here:

Ballistic coefficient calculation – Sellier & Bellot (sellier-bellot.cz)


 
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900 to 940 FPS seems to be ideal in my testing with power wheel set at the number 3 setting. It's better to test different speeds based on what kind of harmonics your gun has going on with your particular setup and tune, whether lubed projectiles or not, cleanliness of bore, sizing, reg set point, etc. I find StrelokPro to work quite well when all the inputted numbers are dialed in specifically for your particular setup. Use two chronographs to get your actually BC based on your tune. A little work, but the numbers are only as good as you are and your testing for its accuracy.
 
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I have litle experiance with slugs, but as far as I know I would guess the tune (speed, reg pressure etc) which gives the best accuracy will be where the slug has best bc. A problem with slugs is that they can wobble in flight, and If a slug wobble the BC will suffer. The ballistic calculator apps presume the slug will fly straigth, that is probably not always the case in real world. I shot some with the JSB knock outs in .22 before the winter, without doing much tuning. At 100 m I sometimes heard the sound " ch,ch,ch,,," of a slug in flight, and that was probably wobling. 




















 
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I have litle experiance with slugs, but as far as I know I would guess the tune (speed, reg pressure etc) which gives the best accuracy will be where the slug has best bc. A problem with slugs is that they can wobble in flight, and If a slug wobble the BC will suffer. The ballistic calculator apps presume the slug will fly straigth, that is probably not always the case in real world. I shot some with the JSB knock outs in .22 before the winter, without doing much tuning. At 100 m I sometimes heard the sound " ch,ch,ch,,," of a slug in flight, and that was probably wobling. 





What do you want to do ?
Copy








What do you want to do ?
Copy








What do you want to do ?
Copy


Click to expand...


Sorry for derailing, but notice this "WHAT DO YOU WANT TO DO ?...NEW MAIL...COPY" in few of your replays why is that ?

B
 
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Good Evening folks,

I did a good bit of shooting today got the BC to work out as advertised. However, I did have issue getting my 33.5 gr Nielsen’s faster than 860. They shoot good there, but I’d like to have them faster. I’m pretty sure I had them shooting at 890 as my original post specified, but I cranked up the reg pressure up to 160-170 bar (huma) and I couldn’t get any noticeable difference in FPS. Also the hammer spring is maxed out and I can’t adjust it anywhere. I believe the previous owner stripped it out because I’ve never got movement out of the hammer spring. Any suggestions? 
Thanks again for your interest and suggestions!

Eric
 
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F4F5BE05-EDC9-4286-90DB-59E2739A2505.1613958050.jpeg
9D3CCA0B-5851-4817-BEB3-779DF3BE048B.1613958052.jpeg

Current reg pressure and HS adjustment. 
 
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Sounds like the hammer spring may be shot. My 700 mm .25 cal is shooting 860 FPS with heavy 41 gr noe boat tail hollow point slugs with my reg way down at 115 ish bar. Also has slug pin probe and slug a liner as well as Ernest Rowe dual transfer port. I’m also backed way off on my hammer spring tension and my valve is cranked down to around the 4th line.
 
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@B 

I do have the power plenum, I really only jacked up the reg pressure to see if I could get an instant boost of FPS. The gun is stock as far as I know, it was upgraded from an x, now has the MKII fx plenum and the wika gauges. Also had a huma reg put in it functions well and is very consistent. 
Also, the hammer spring is maxed out as far as I can tell, the block doesn’t slide at all, no side to side play.

@Dannya1a 

Are you saying that the spring itself could be broken? Or the adjuster? 

 
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with the spring under that much tension for who knows how long, it could have taken a set and shortened itself. I had this problem on my jsar tuned gauntlet. it was shooting perfect then all the sudden my speed dropped off. found my hammer spring had taken a set and got shorter. Could be the same thing going on with the impact. I would pull out the hammer spring and measure it. I'm sure if you called FX or dug around, someone would know how long it should be from factory to see if this is possibly why you FPS is down.
 
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Thank you all for the quick replys, I’ll do some calling around and see what I can dig up. I may tear into myself to check the hammer spring out to verify it’s in spec. I’m sure FX is backed up with work right now, I may end up sending it in for a check up/ diagnosis, just depends on the wait time. I’ll keep the thread updated with what I find out. 
thanks again!
 
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