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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Light vs heavy pellet and long range accuracy potential

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Franklink:
Requesting thoughts on the following from minds who understand ballistics much better than myself.....

JSB .20/15.89 grain vs JSB .30/44.75grain pellet.

The .20 Heavy has a better BC than the .30, both in JSB's chart and in my measuring with the speed at two distances method. Mass is part of the BC formula but the 45 grain .30s seem to be a better long range pellet choice than the 16 grain .20s (less wind deflection from the .30s than the .20s). Why the apparent discrepancy?

If it was as simple as choose the better BC pellet we'd be seeing more .20s than .30s in the long range competitions....right?

If they're both going in the 900fps realm the .30 is going to have more energy left over @ 100 yards than the .20 started with at the muzzle. is there some component (inertia?) that makes the heavier pellet want to continue on it's path better than the .20, cuz I thought that was essentially what the BC is? Does BC not take account of the mass of the pellet to an appropriate degree? 

rsterne:
BC is to a large extent dependant on the SD, which is the "weight per area".... so yes the BC takes into account the mass.... The SD for the 15.89 gr. in .20 cal is 0.057, while for the 44.75 gr. in .30 cal is 0.071.... You would therefore expect the .30 cal to have about a 25% higher BC.... Since they are significantly different shapes, that likely accounts for the BC not tracking with the SD....

The problem with the BC is that if they are not measured at the same velocity, the BC changes, because we don't have a drag model that matches the pellet shape.... I haven't actually seen the BC on that .20 cal pellet, but if you have measured both, at the same velocity (and the one that you will use), then the pellet with the higher BC should perform better in wind drift, and for % retained velocity and energy....

Bob

aluminumfetish:
Chairgun has the BC of the 5mm JSB 15.89 as .030 for whatever that is worth.

Franklink:
The background to this question was an Xtreme Field Target match we had here in AZ about two weeks ago. Large field targets out to 100 yards with a 100 fpe limit. So, fairly long range, at least for pellets.

Since inception of these monthly matches (second winter now), most of the competitors have gravitated towards the more mainstream (if that's a thing in airguns) long range pellets like the .22/25.4gr Monster RDs, the .25s, and the .30s.  Those are coincidentally the pellets with better BCs. Most of these guys are pushing 45fpe and above. I personally have used the .22/MRDs every month except at the Feb match.

So at the most recent match I used the .20/15.89 and shot a 36/48. Now, that doesn't sound great compared to regular field target, but the winner shot a 39/48 (he was using .30/44.75gr) so I was pretty close to the top. And that winning 39/48 was one of the best knockdown percentages we've seen.  I have shot the Heavy .20s quite a bit at home, out to 130 yards, and always been impressed with their performance, FOR SUCH A LIGHT PELLET. I have measured the BC  of the .20s a couple different times and they average out to about 0.048. Which is quite high for such a light pellet, actually higher than most guys report getting with the .30/44.75 (including JSB's own BC chart).

The winner was able to hold within the kill zone with his .30 but I was needing to hold multiple inches outside of the kill zone to account for wind on the long range shots.

Bob, from your comments about the SD and from real world examples, the .30 SHOULD have a better BC.  You think it is all due to shape? The .30/44.75 is a short squatty little dude while the .20/15.89 is more elongated and similar in shape to the .22/25.4gr MONSTER RD.

So this all creates some conundrums in my limited knowledge bank of external ballistics:
First, The .20/15.89 has an abnormally high bc, for it's light weight (which Bob ties back to the sectional density).
Second, the .30 is more (anecdotally) resistant to wind deflection than the .20/15.89 even though the .30 has a lower BC.

(as an aside, yes, I saw firsthand the effects of speed on a pellets BC when testing the .20/15.89. The BC is down around 0.038 when they're going 860 but up to 0.048 if they're pushed to 910-915).

rsterne:

--- Quote ---The BC is down around 0.038 when they're going 860 but up to 0.048 if they're pushed to 910-915
--- End quote ---

Right there is the key.... If the pellet shape matched the drag model you are using to calculate the BC, then the BC would not change.... A change from 0.038 to 0.048 with a velocity change of only 50 fps makes the data very suspect, IMO.... The pellet slows down as it goes downrange, so although you are launching it at 910-915, by the time it has passed 20 yards, it has slowed to less than 860 fps, so the BC is less than 0.038.... Without doing additional measurements at slower speeds, you really don't know what your pellet is doing.... With a BC(GA) of 0.048, starting at 915 fps it would be down to 685 fps at 100 yard.... Do you know what the BC is at 685 fps?....

The 44.8 gr. JSB .30 cal is one of the few pellets I have run through my LabRadar, and the BC(GA) is pretty consistent.... I used range increments of 10, 30, 50, 70 and 90 yds., and got 20 shot average velocities of 936 (muzzle), 903, 839, 785, 737 and 691 fps.... The BC(GA) as the pellet travelled downrange was very consistent, and calculated out to 0.041, 0.035, 0.035, 0.036 and 0.033.... The high value of 0.041 from 0-10 yards is probably due to the LabRadar underestimating the MV by extrapolating it from the data beyond 10 yards (where it acquires the pellet), while the pellet is slowing quicker during that time from its higher velocity, so I never look at the 0-10 yd. BC as relevant, and discard it.... Averaged out over 10-90 yards, it was 0.035.... The 90 yard data was getting a bit inconsistent (the SD was double what it was at 70, and one shot did not register), and the average BC(GA) from 10-70 yards was 0.036.... I have seen 0.037 published for that pellet in more than one place, so my results agree well with the claims.... This pellet seems quite a good match for the GA drag model, at least between 700-900 fps....

Wind is very seldom consistent in speed or direction over the 100 yards from muzzle to target.... If the BC of your .20 cal pellet is lower than the .30 cal once it drops below 800 fps, and the wind is the strongest closer to the target, then it would drift more.... BC is only a guide, as I have written before, what we really need is to be able to create a drag model for each pellet, from your own gun, and then use that for drift, trajectory and retained velocity calculations.... The only company I am aware of doing that is Lapua, but it is really the only way to create a model that does better than averaging too many variables....

Bob

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