Calculating Ballistic Coefficients
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Calculating Ballistic Coefficients
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Topic: Calculating Ballistic Coefficients (Read 3284 times))
bluethunder
Plinker
Posts: 164
yes
Calculating Ballistic Coefficients
«
on:
September 29, 2014, 02:57:00 PM »
Had some fun at the range today acquiring data that could be used to calculate ballistic coefficients. My chronograph deserves hazardous duty pay because I shot bullets over the screen at 100 yards. I'd never done this before but the chronograph survived. The rifle, an Extreme Big Bore Hunter in .30 cal. with a T.J.'s 30 inch 1 in 14 twist barrel. The gun was regulated from an 88 cu. ft. bottle at 3,600 psi.
Bullet #1:
It's RCBS 165 gr. Silhouette bullet. You can see it at this link:
http://shop.rcbs.com/Products/Bullet-Casting/Bullet-Moulds/Silhouette/Bullet-Mould-308-165-SILH-541.aspxhttp://shop.rcbs.com/Products/Bullet-Casting/Bullet-Moulds/Rifle/Bullet-Mould-30-180-SP-541.aspx
Cast in 98% lead and 2% tin it weighs 170 grains.
Velocities 3' from the muzzle were: 862, 854, 851, 852, 866, 868, & 863 Ave. 859 fps.
Velocities at 100 yards were: 810, 816, 817, 821, & 814 Ave. 816 fps.
This bullet lost a total of 43 fps during the 297' flight between the two recording points.
Bullet #2:
It's RCBS 180 gr. bullet. You can see it at this link:
http://shop.rcbs.com/Products/Bullet-Casting/Bullet-Moulds/Rifle/Bullet-Mould-30-180-SP-541.aspx
Cast in 98% lead and 2% tin it weighs 188 grains.
Velocities 3' from the muzzle were: 834, 842, 841, 836, 830, 830, 841, & 835 Ave. 836 fps.
Velocities at 100 yards were: 788, 801, 788, 791, & 784 Ave. 790 fps.
This bullet lost a total of 46 fps during the 297' flight between the two recording points.
The JBM Ballistic calculator had multiple drag functions that you could choose before inputing you date. I choose the G1.
The 170 gr. bullet calculated to a B.C. of .257
The 188 gr. bullet calculated to a B.C. of .227
I then entered the same data into Airguns of Arizona's B.C. calculator. Their B.C. calculator did not give you the option of choosing a drag function.
AOA's number for the 170 gr. bullet came out to: .238
AOA's number for the 188 gr. bullet came out to: .216
What was probably the most eye opening of the test was that even though the 2 bullets have virtually identical nose configurations, the bullet with the lower sectional density produced the highest B.C. This should not happen.
My explanation is that the 1 in 14 twist is barely stabilizing the 170 gr. bullet and stability is even worse with the 188 gr. bullet. The bullet holes on the target page are very slightly out of round and out of round in different directions. This is bullet yaw. The 170 gr. bullets only show a small amount of yaw but the 188s show more. It's my theory that the slightly greater corkscrewing of the 188 gr. bullet through the air is hampering it's ability to produce it's optimal ballistic coefficient. I probably would not have even noticed the yawing except that I have several hundreds of rounds through the rifle at 220 yards and the yawing is much worse with both bullets at that range. So I knew what to look for. I'm also disappointed that the B.C. turned out this low. I thought they would be higher.
My new .750" O.D. stainless steel match grade Shilen has arrived. It has a 1 in 10 twist. Once I have it installed on the rifle I'll repeat the test and see if the 188 grainer can out perform the 170 grainer like it's supposed to. The 1 in 10 twist should definitely get rid of the stability problems.
Stew
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rsterne
Member 2000+fps Club
Bob and Lloyd
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Posts: 26926
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Calculating Ballistic Coefficients
«
Reply #1 on:
September 29, 2014, 08:04:13 PM »
You are right on that marginal stability on a bullet will degrade the BC.... In fact the JBM twist calculator tells you what the reduction in BC is for an SF less than 1.5....
I just downloaded the latest version of ChairGun 4, and ran your numbers using the G1 Model (drift in brackets for 10 mph crosswind).... For the 170 gr. (859 down to 816) I got a BC of 0.275 (41.2" @ 500 yds).... and for the 188 gr. (836 down to 790) I got 0.240 (47.7" @ 500 yds).... using a 99 yard difference in the two velocities.... Here is the G1 Model....
Incidently, I ran both bullets through at various velocities, and the minimum drift at 500 yards was at 900 fps (47.20") for the 188 gr. and at 890 fps (41.09") for the 170 gr.... At 1000 yards, the minimum drift would occur with a higher muzzle velocity to keep the average velocity in the best BC range.... ChairGun only goes to 500 yards, but if you use the JBM calculator you can find the optimum MV for 1000 yards by trial and error....
Incidently, I doublechecked your numbers using the JBM BC calculator, using the G1 Model and the default atmospheric conditions and got a BC of 0.279 for the 170 gr. and 0.245 for the 188 gr.... Those are pretty close to ChairGun.... AoA's calculator probably uses an airgun BC model for waisted pellets....
Bob
«
Last Edit: September 29, 2014, 08:26:30 PM by rsterne
»
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
bluethunder
Plinker
Posts: 164
yes
Re: Calculating Ballistic Coefficients
«
Reply #2 on:
September 30, 2014, 09:40:22 AM »
Thanks Bob for the research. And thanks for catching my mistake at the JMB site. Now I don't feel as bad about the poor B.C.. I re-entered the data using my altitude at 850' elevation, Temp. 75 degrees F., 28.00 inches pressure and 50% humidity and I got virtually the same numbers that you did.
The T.J. barrel is 30 inches, the new Shilen is 32 inches. Have you ever seen any data to say what the theoretical velocity change will be when adding the 2 extra inches of barrel? I have already accepted that I could lose as much as 40 fps going to the faster twist. I'm just wondering if I can get any of the velocity back by installing the longer barrel.
Stew
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Ohio
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26926
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Calculating Ballistic Coefficients
«
Reply #3 on:
September 30, 2014, 11:04:08 AM »
To do a relatively precise calculation I need to know the efficiency, which means the psi drop in your 88CF tank over, say, a 10 shot string (or more) so I can calculate the amount of air used.... I can use the velocity and weights you have to get the FPE.... Even more important is to know the volume of the plenum, ie the volume of 3600 psi air the valve can draw on for each shot.... In this case, that would be the reservoir volume.... The regulator can't maintain the pressure during the 1-2 mSec it takes for the shot to occur, so effectively you are working with only the air between the output of the regulator and the valve seat (and that assumes the air can get into the valve faster than it can get out).... On my regulated PCPs I strive for 1 cc of air for each FPE in the plenum.... less than that and your average pressure during the shot plummets, limiting your power....
Without that information, my best guess is about 20 fps gain going from a 30" barrel to a 32"....
Bob
«
Last Edit: September 30, 2014, 11:20:21 AM by rsterne
»
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
bluethunder
Plinker
Posts: 164
yes
Re: Calculating Ballistic Coefficients
«
Reply #4 on:
October 01, 2014, 08:03:40 AM »
Thanks Bob for the educated guess. I'll know soon enough through actual testing. If I can get back half the velocity that I'm going to lose due to the faster twist rate with a longer barrel I'll be a happy camper.
Logged
Ohio
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26926
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Calculating Ballistic Coefficients
«
Reply #5 on:
October 01, 2014, 01:17:40 PM »
Have you ever tried plotting the velocity vs. hammer spring preload?.... or calculating the efficiency (FPE/CI) of your setup?.... If you can give me the information I asked for in the previous post I can do that for you.... You may be wasting a lot of air and gaining no velocity if you are using a gun set up for 4500 psi at 3600.... Check out this thread....
http://www.gatewaytoairguns.org/GTA/index.php?topic=74919.
Bob
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
bluethunder
Plinker
Posts: 164
yes
Re: Calculating Ballistic Coefficients
«
Reply #6 on:
October 04, 2014, 03:58:55 PM »
Thanks for the link. On Extreme Big Bore Hunters the only way to adjust the hammer blow is to either cut coils or change out the spring all together. Brent knew that I was tethering the rifle at 3,600 psi. so he sent me his "light" hammer spring. Maybe at some point I'll purchase some other hammer springs in a variety of poundages and see what happens.
Logged
Ohio
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26926
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Calculating Ballistic Coefficients
«
Reply #7 on:
October 04, 2014, 04:23:04 PM »
It astonishes me that most high-powered PCPs don't have a way to make the most basic and important adjustment available for tuning.... hammer spring preload....
Bob
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
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Calculating Ballistic Coefficients