JSB 22 cal 18.13 gr Ballistic Coefficient

Have tried the drop method and get wildly varying numbers. Did that today in fact with Cricket mini and Mutant. One at 855 FPS, the other at 900 FPS. Did BC three times with Cricket and twice with Mutant. 45 and 85 yards. BC varies between .027 and .068. I have no faith in this method. I much prefer using velocity at muzzle and a specified distance. Seems to be much more accurate. 

On some calculators it has a box to enter twist rate. Anyone know what I should enter for an FX smooth twist barrel. It is a .25 on an mk1 wildcat.

I use 0.031 at 860fps muzzle velocity and it matches my trajectory almost exactly, for my gun. (It is a little different over/around 90-100 yards, but mostly works.) 



Tsmith, I heard the smooth twist X is 1 in 30 twist. Not sure if that is the same for the regular ST barrel, but it may help.

I agree with @centercut Mike. You can do it with 1 chronograph it just takes a few more steps. Set up your chrony at the muzzle and take the average of say 10 or 20 shots, then move the chrony to various or the end distance and repeat. BC will vary depending on twist rate & velocity (among other things).

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Just make sure to put a shield plate in front of the crony , for some odd reason they don't work very well when you punch a hole in them, and good ol Murphy seems too to like to poke his head in when you least expect it 

On some calculators it has a box to enter twist rate. Anyone know what I should enter for an FX smooth twist barrel. It is a .25 on an mk1 wildcat.

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Seldom are things as simple as they seem.

FX smooth twist barrels and FX smooth twist X barrels are very different. Then within these two there are differences depending upon calibre and in the first ST depending upon which of more than 18 iterations made and pellet weight and velocity.

Example: ST in .22 calibre had a nominal actual barrel twist rate of 1:16". But the more interesting "rating" relates to the emerging spin rate of the pellet, which, because the pellet skims through that very short "rifled " section of barrel, can vary widely depending upon the pellet weight, its velocity, and the barrel iteration. Thus, without defining these parameters, when I tested my .22 ST barrel the spin rate averaged 1:64". When Fredrik Axelsson repeated my test he got 1:40+" with a later iteration barrel than mine; and Dan Brown got 1:160" with an earlier iteration.

Then when I tested my .25 cal ST barrel with JSB Kings it produced 1:75" but with a heavier pellet it was 1:90".

However, because all these still shot with great accuracy to very long range because of the flare stabilisation of diabolos, then it is of little consequence. 

Enter the FX smooth twist X barrel and we have the pellets emerging at the same spin rate as the barrel twist rates - which Fredrik varies according to accuracy tests done in house. Initially such rates have been established using the most popular JSB pellets within each calibre. My STX .25 barrel is 1:28" nominal and again because of flare stabilisation it shoots all three JSB pellets to sub moa at 100 yd. Barracudas, because of their head and skirt profile are not quite so stability tolerent but still shoot sub moa to 50 yd and further. 

I guess I have probably lost most readers' interest by now but the really interesting part is what the changing down-range pellet spin rate interacting with flare stability (or lack), remaining static stability, pellet tractability and the forces of gravity and wind do to dynamic stability.

nvelkhunter - if you enter bc 0.034 as a GA profile into your Chairgun programme, you should find it close to the money in giving you workable trajectory tables at least to 100 yd and probably much further. ... best regards from OZ, HarryyrraH.

Great post Yarrh-thanks for sharing that. Question: how did you ‘measure’ the effective twist rates? I would be interested just to understand how you did that...

I’m assuming that twist rates on rifled or polygon barrels are more closely matched to the barrel’s twist rate as manufacturered?

Thanks for sharing the knowledge with the community!



Sean

Great post Yarrh-thanks for sharing that. Question: how did you ‘measure’ the effective twist rates? I would be interested just to understand how you did that...

I’m assuming that twist rates on rifled or polygon barrels are more closely matched to the barrel’s twist rate as manufacturered?

Thanks for sharing the knowledge with the community!



Sean

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Sean, We can basically assume that for fully rifled barrels, regardless of rifling profile, the initial pellet spin rate is the same as the barrel displays. My measurements relevant to that show within half an inch discrepency, which by assumption can be considered as a reflection of the accuracy of the method percentagewise. Eg., ( 0.5 ÷ 17 ) × 100 %, or say roughly 3% for a 1:17" barrel rifling.

To respond to the first part of your question - pellets with signature marks, such as a careful ink mark on the pellet head from near centre to near expected rifling marks, are shot through three screens separated at precisely measured distances with gaps preferably less than and more than the nominal barrel twist rate.

The screens must be located such that their physical orientation is the same - and is known in regard to vertical and horizontal position. The screens should be thin enough but fine enough to be cleanly perforated by the pellets without upset.

The screens are then examined to see the marks made by the ink. The pellet rotation indicated by the second and third screens, compared to the first, is resolved in degrees. 

Then with the known measurements between the screens, a little trig and maths resolves one rotation into a rate per inches of travel as in 1:X".

Of course the right or left twist of the rifling has to be taken into account. Any error will be related to the accuracy of the measurements, but as stated above, my data for full length rifling has been within about 3% of manufacturer's stated value . So an original smooth twist barrel that gave me an average of 1:64" could be say +/- a couple of inches.

I hope that is somewhat clear in concept and helpful in seeing where I was coming from. ... Kind regards, Harry.

Interesting and clever method-thanks Harry! What do you use for screen material (cardboard?)? I'm not sure where to start with twist rate expectations for a LW Polygon barrel, or a RAW .30 cal barrel-any ideas?



Sean

0.045 BC at 930 fps matches my actual DOPE from 200 to 238 yards. At 273 yards the BC drops to 0.038.

This is if you believe chairgun of course, and manipulate the BC to match the actual DOPE numbers.

Interesting and clever method-thanks Harry! What do you use for screen material (cardboard?)? I'm not sure where to start with twist rate expectations for a LW Polygon barrel, or a RAW .30 cal barrel-any ideas?



Sean

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Sean, very very light cardboard or strong paper - white of course to pick up the ink/biro - stretched tightly and all three screens precisely aligned so that when you are drawing and subtending angles for measurement the pellet rotation to screen 2 and 3 can be assessed from the line of the pellet ink mark on the first screen. Have fun, Harry.

Rh:

Using your M#2 velocity figures 880, 761, 699, 583 fps for 0 yd, 0 to 40 yd, 0 to 60 yd and 0 to 100 yd respectively, and taking account of apparent changes of bc over each of these intervals the Chairgun bc calculator in tools gives me bc's of 0.0349, 0.0321, and 0.0289 respectively @ altitude 100 ft, 75dF, 80%RH (the given data).

Then plugging each of these bc's respectively into the Chairgun GA gives me actual M#2/predicted velocities of 40 yd 761/761.9; 60 yd 699/700.7; 100 yd 583/586 fps.

Those are very close GA predictions.

Two points from this: 1. From studies I did in the past, I know bc changes as pellets progress down range for one or a number of reasons that relate to static and dynamic stability and resulting tractability. My studies indicated that for a pellet settling well, the bc tends to improve with range and time. The GA somewhat takes this into account and highlights the fine print of using a bc appropriate to range and other relevant variables. Example - for this particular rifle and pellets, one could use a bc of 0.0289 as the average over the 100 yd etc.

Point 2. It would appear that for this rifle and pellets, under the conditions of wind, temp, RH, altitude etc., the bc was getting worse as range increased, not better. I would read this as indicating the stability of the pellet was changing for the worse. It would only be conjecture to draw any conclusions as to the reason though. It could be as simple as down-range wind upsetting pellet stability or: some gun pellet interaction which disallowed for good dynamic stability. 

I have not computed for shot M#3 - and hope at this late hour midnight here in OZ that I have not made any gross errors, Make of it what you will. Best regards Harry.

I found this site shows BCE for tons of pellets 

https://hardairmagazine.com/ballistic-coefficients/