FX Slug BC - Revised at Reply #49

[Dairyboy]

I did this last night a comparison with both traveling at 900fps.

[rsterne]

The tests at HAM were done with the same gun with no tuning changes, so the slug had a much lower velocity (860 vs. 940).... I used the numbers for both in ChairGun and came up with the following charts.... Green is for the 16 gr. pellet and blue for the 22 gr. slug.... As you can see, the pellet actually has less drop out to 100 yards, where they are the same POI.... Chart was generated using the same barrel angle, so zero distance was not a factor....



Windage is a different story.... The slug has half the wind drift at virtually every distance....



The article at HAM covered the velocity and energy differences, so no point in duplicating that here....

Bob

[wahoowad]

As you can see, the pellet actually has less drop out to 100 yards, where they are the same POI....

Interesting. I was expecting the pellet to drop more, especially at that distance

[subscriber]

If you look at the curve for a BC = 0.10 (the blue line), you will notice that the drift over 200 yards in this wind strength is 20" at a MV of 900 fps.

Bob,

I don't know if there is a standard assumed cross-wind for such calculations, but could you share the wind speed you used with us please?

[rsterne]

Sorry, I was being lazy.... I went back and checked, and I did that chart using a 10 mph crosswind....

Bob

[subscriber]

Thanks Bob.

[Danny]

author=rsterne

My upcoming articles in HAM will deal with this topic.... but here is a summary.... The drag of ALL projectiles has a huge increase in the Transonic range (Mach 0.8-1.2)….

Bob

Great info and another advantage for slugs.

As velocity gets closer to the speed of sound the projectile's "braking" increases.

Less aerodynamic designs are effected more at lower sub-mach percentages.  A diablo might have the same braking effect at Mach .7 as a slug has at Mach .8.

Therefore, even though a lighter diablo leaves the barrel faster, 10 feet from the barrel it might already give up that speed advantage.

Slugs aren't just better for long distances.  They also shoot flatter and retain more energy at closer distances as velocities increase above 6-700 fps.

Their disadvantage can also be an advantage as diablo's high drag makes them travel less  and their lower terminal velocity makes them carry much less energy at further distance.

[skorec]

Which are   limits   of length of slugs + FF  in your point of view for 3000PSI/24”barrel   ?

[rsterne]

That pressure and barrel length limits you to an SD of about 0.17 if you hope to see 900 fps.... That in turn limits the BC.... Smaller calibers can have a higher FF for a given SD.... but all calibers are limited to slugs about 4 calibers long.... and the twist rate depends on the length (in calibers)….

Bob

[rsterne]

I just received the detailed LabRadar data from HAM on the FX Slug and Pellet testing.... This included the local atmospheric conditions at the time of the test, which was 35*F (a cold day), 50% Humidity, 244 ft. above sea level, and 29.5 in.Hg local barometric pressure.... This allowed me to input the actual test conditions into ChairGun so that the results are properly corrected to ICAO conditions.... The G1 model was used for the slugs and the GA model for the pellets....

They shot 10 pellets and 10 slugs, washed but unsorted, and recorded the LabRadar velocities every 10 yards from the muzzle to 50 yards.... I calculated the BC using the average velocities at the muzzle, and both 40 and 50 yards, and also calculated the BC for EACH shot at both distances, and then averaged the BCs, to see if there were any significant differences.... Although there were significant differences from shot to shot in the calculated BCs, there was no significant difference in the resulting average BC using the two methods....

One thing that stood out was that in the slug testing, two shots were outside the typical variation at 50 yards.... one had a high BC and the other low, compared to the same two shots at 40 yards.... At 40 yards the spread in calculated BC for the ten slugs was 10%, while at 50 yards, with those two shots included it was 35%.... Interestingly, the average BC only changed by 1% with them deleted, but they were such a statistical oddity I decided that was the best approach....

Once those two pieces of 50 yd. data were removed, the difference between simply averaging the velocities or calculating the BCs and then averaging those was well into the 4th decimal point.... Regardless, I decided that the best way to calculate the BC for both the slugs and pellets was to average all four numbers, the two 40 yard BC averages and the two 50 yard BC averages, and report that number.... Here are my results....

16 gr. FX Pellet, with MV = 942 fps.... BC (GA) = 0.035
22 gr. FX Hybrid Slug, with MV = 860 fps.... BC (G1) = 0.071

The variation between the highest and lowest of the four BCs used to calculate the above was 2.7% for the slugs and 3.5% for the pellets.... That means that the numbers above should have a total range of error of about 0.001 for the pellet and 0.002 for the slug.... Therefore the BC (G1) for the FX slug in this test, conducted at a MV of 860 fps, is 0.070-0.072....
 
Bob

[canadian_shooter]

 Thanks for the update, Bob.

[JBinCO]

So just checking my understanding here
The BC on box is too high as is the pellet trajectory on box and the slug trajectory is too flat on box
Is that all true?

[AlanMcD]

Bob,

Interesting results - thanks for that analysis.  Interesting that the variability in BC was higher on the pellets than the slugs - that is precisely what I think one would expect, given the low BC to start with, and sensitivity to the Form Factor . . .

I have done some testing for BC on my pellets as shot, but have always worked with averages as I only have one chronograph to use at two distances.  In every case, I have always seen much higher variation in speed downrange than at the muzzle, and the only thing that I can think of to explain it would be variability in BC (especially given that the variation at the muzzle was pretty tight, on purpose).  I think it is an under explored factor on achieving the best accuracy possible at a distance (and in my mind, part of why head size is typically so important to accuracy, given the impact of the "smeared" lead area on the head).  Thanks again.

JB - the illustration on the box really had no data at all, and the illustration was just that - an illustration . . . no real world comparison provided.

[rsterne]

Alan, in much of the testing I have done, I actually found LESS variation in velocity downrange than at the muzzle, with some obvious outliers which are likely damaged pellets.... I think what is happening is that the weight variation favours a higher BC for the heavier pellet, which should start out a bit slower at the muzzle.... and the result is that they even out somewhat downrange.... Weight variance actually has little to do with vertical stringing, there have been several threads dealing with that....

The diagram on the box should be taken with a grain of salt.... it is advertising, nothing more.... The BC claim of 0.080 could well be true at SOME velocity, under some other conditions.... Overall, it is within 12%, so not that far afield, IMO.... The trajectory difference between 0.071 and 0.080 is negligible, and the wind drift would be proportional to the BC.... If you shoot in other conditions (hotter, higher altitude, etc.etc.) you could easily have an EFFFECTIVE BC different by more than that....

Bob

[FXUSA]

Hopefully some helpful info: The BC listed on the Hybrid box was determined after extensive testing at velocities higher than what HAM just shared. We have found that higher velocities absolutely affect the BC (I'm sure Bob can confirm). Our recommended sweet spot to shoot the Hybrids is 940-950fps. As Bob has explained properly other factors do play into calculating BC and it's impossible to list on a box the actual BC of what you'll get out of your rifle. Our goal of listing the G1 BC was to give everyone a rough starting point for their testing. We've had guys shooting them above 1,000fps with great accuracy and even higher BC than 0.80, and some shooting more in the 840fps range with lower BCs but still good results (much like HAM showed). Instead of just listing best case scenario, we felt 0.80 was a good healthy average many should be able to obtain.

We realize that many distrust the Pro Shooters / YouTubers we work with, so we'd say go pick up a box, run tests in your guns, and share the results. We made sure they'd be readily available when we released them to the market so everyone wouldn't have to pontificate of what they could or couldn't do, but simply go find out for yourself. We think you'll be pleasantly surprised.

Thankful for everyone's constructive talk. Our goal is just to keep moving forward our sport and push the boundaries of what airguns can do.

- Jonathan

PS - Hope you guys understand why our last post about Hybrids before the actual release date was holding back some info. We wanted to release news about a product and have anyone who was interested be able to act right away to pick them up. Something we hope can happen more and more in our sport instead of months of waiting. Thanks again everyone.

Alan, in much of the testing I have done, I actually found LESS variation in velocity downrange than at the muzzle, with some obvious outliers which are likely damaged pellets.... I think what is happening is that the weight variation favours a higher BC for the heavier pellet, which should start out a bit slower at the muzzle.... and the result is that they even out somewhat downrange.... Weight variance actually has little to do with vertical stringing, there have been several threads dealing with that....

The diagram on the box should be taken with a grain of salt.... it is advertising, nothing more.... The BC claim of 0.080 could well be true at SOME velocity, under some other conditions.... Overall, it is within 12%, so not that far afield, IMO.... The trajectory difference between 0.071 and 0.080 is negligible, and the wind drift would be proportional to the BC.... If you shoot in other conditions (hotter, higher altitude, etc.etc.) you could easily have an EFFFECTIVE BC different by more than that....

Bob

[subscriber]

Great info, Bob

Obliquely related to this, is a thought that occurred to me, shooting my new P-Rod:

The P-Rod gets only 19 shots from 2800 to 1800 PSI at a 4% ES, due to its tiny air reservoir.  While evening out velocity is great, and what regulators are supposed to do, the thought occurred to me, that if you shot an unregulated PCP from 3000 to 1000 PSI, the degree of pellet skirt flaring (or ballooning, if you like) would be more at the higher pressure, and diminish though the shot string.  As such, even if a clever valve could achieve a low velocity spread, 2 feet from the muzzle, the difference in skirt shape over that applied starting pressure range, could change the shape of the pellets; and thereby their drag and BC.

My thinking is that higher pressures would flare pellets skirts more, and come closer to converting a diabolo pellets into a more cylindrical slug.  Certainly, pellets shot out of pumpers at very few pumps show an almost out of the can diabolo shape.  This can be seen to change when shooting the same can of pellets at a higher number of pumps.  The most obvious thing is the length of the bore diameter "land" created at the rear of the skirt, for a given actual bore (choke) diameter.  When the skirt engagement is longer due to higher firing pressure, the skirt is blown into a more rounded shape, than the original straight cone shape.

Now, to make the above relevant to your HAM BC testing; was this data collected with the PCP tethered to a large air tank; or was the reservoir pressure allowed to decay over the 20 shot test?  If the PCP's built-in tank was large, then my point is probably moot.  I am certainly thinking about actual peak firing pressure in the most general sense, as a BC modifier that show at longer ranges...

[rsterne]

Jonathan, thanks for your comments about how the BC of the FX Hybrid slugs changes with velocity.... This is, of course, an indication that the G1 drag profile is not a very good match for the shape of the slug, something we always face with pellets and slugs alike.... Hopefully over time we will end up with better drag models, that better match our projectiles.... That will allow far better calculations for trajectory and wind drift.... After all, a slug launched at 1000 fps travels through the 940-950 fps range you measured the BC = 0.080 at.... and further out, through the 860 to 770 fps range that HAM measured the BC = 0.071 at.... as it travels downrange.... Perhaps eventually FX will supply a complete Cd drag model for their slugs like Lapua do for their bullets.... That would be the ultimate....

Subscriber, you are quite right that pellets skirts change shape depending on the pressure they are launched at.... I'm sure that has an effect on the Cd and hence the BC.... I am not involved in the BC testing done by HAM, but quoting from their website....

all HAM airgun Ballistic Coefficient testing has been undertaken by the same experienced, dedicated testers, using the same FX Impact air rifle, set to full power, with all FPS measurements recorded using the same Labradar Doppler Radar system.

The Impact is a regulated PCP, so all shots should be subject to the same pressure, within the mechanical limits of such a gun....

Bob

[FXUSA]

Correct, the G1 profile was the best of the available models but not a perfect fit. Agree that a better drag model for airgun slugs would be very helpful as we grow our sport.

Jonathan, thanks for your comments about how the BC of the FX Hybrid slugs changes with velocity.... This is, of course, an indication that the G1 drag profile is not a very good match for the shape of the slug, something we always face with pellets and slugs alike.... Hopefully over time we will end up with better drag models, that better match our projectiles.... That will allow far better calculations for trajectory and wind drift.... After all, a slug launched at 1000 fps travels through the 940-950 fps range you measured the BC = 0.080 at.... and further out, through the 860 to 770 fps range that HAM measured the BC = 0.071 at.... as it travels downrange.... Perhaps eventually FX will supply a complete Cd drag model for their slugs like Lapua do for their bullets.... That would be the ultimate....

Subscriber, you are quite right that pellets skirts change shape depending on the pressure they are launched at.... I'm sure that has an effect on the Cd and hence the BC.... I am not involved in the BC testing done by HAM, but quoting from their website....

all HAM airgun Ballistic Coefficient testing has been undertaken by the same experienced, dedicated testers, using the same FX Impact air rifle, set to full power, with all FPS measurements recorded using the same Labradar Doppler Radar system.

The Impact is a regulated PCP, so all shots should be subject to the same pressure, within the mechanical limits of such a gun....

Bob

[ballisticboy]

Jonathan, thanks for your comments about how the BC of the FX Hybrid slugs changes with velocity.... This is, of course, an indication that the G1 drag profile is not a very good match for the shape of the slug, something we always face with pellets and slugs alike.... Hopefully over time we will end up with better drag models, that better match our projectiles....

Bob

Production of a drag model for slugs which will fit over a wide Mach number range will be difficult due to the variations in the size of the large flat areas (meplat) at the front of many of them coupled with the detail shape at the edges of the meplat. Very small amounts of rounding on the edges can have a large effect on the drag coefficient as well as the aerodynamic stability. I produced some drag curves for wadcutter pellets for Chairgun some time ago which showed the difficulty of producing a single reference drag curve for projectiles with large flat front facing areas as the shape of the Cd curves in addition to the Cd values changed as Mach number increased.

Individual drag curves similar to those used in artillery fire control systems is the way to go but their creation could be problematical. Years of experience using tracking radars of all kinds have shown me that using fixed head doppler radars can easily give large errors in Cd values due to improper setup of the trial instrumentation and radar internal software which modifies the output to smooth out the data. We ended up always using a very large tracking doppler radar with a fully tracking head which is obviously out of the question for pellet makers or researchers. Aerodynamic prediction software is available, some of which will work for slugs and bullets, which can give initial values but again the very large meplat values used on many slugs can give gross underestimates of drag as the software was not designed for such blunt shapes.

[Thane]

I would prefer to see a pellet of similar mass (Baracuda Match 21.14gr) to the FX Hybrid  shot at the same velocity, not just the same gun settings.