Litz tests the BC of ABLR's...

[Ridgerunner665]

Fwiw...I've read reports that bullets fired out of tight twist barrels are stressed more at impact on an animal. Actually some people I think are pretty knowledgable say its pretty significant.

You can tell thats true by looking at low velocity (muzzle velocity) expanded bullets...JD338's ABLR photo's in the "bullet tests" section (280 Ackley) are a good excample...the ones he fired from the 7-30 Waters expanded flat...the ones from the 280 Ackley expanded and curled, rounded...and I'm pretty sure one of the 168 grain ABLR's from the 7-30 Waters tumbled on impact, and tore the nose off the bullet.

Both the 280 Ackley and the 7-30 Waters are probably 1-9 or so twist...but the difference in muzzle velocity spins the 7-30 Waters bullet at a MUCH, MUCH slower rate.

 

[andrewctillman]

Riverrider,

Sorry about that.

FA means Frontal Area.

 

[RiverRider]

Thanks for the clarification.

 

[Wis65x55]

This neophyte needs some clarification on a few ballistic coefficient questions.
1. Does increasing muzzle velocity decrease BC; I if so, how much?
2. Is the BC calculated before or after the bullet goes down the rifle barrel?
3. Just how substantial are we improving BC are we talking about when changing twist rate from 1:10 to 1:9.5 in a 280 Remington?
Keith

 

[Ridgerunner665]

This neophyte needs some clarification on a few ballistic coefficient questions.
1. Does increasing muzzle velocity decrease BC; I if so, how much? Sometimes yes, sometimes no.....it varies depending on enough stuff to write a book about
2. Is the BC calculated before or after the bullet goes down the rifle barrel? BC is a measure of the efficiency of flight....its measured in flight, not in the barrel
3. Just how substantial are we improving BC are we talking about when changing twist rate from 1:10 to 1:9.5 in a 280 Remington? Again...it will depend on the bullet being used...longer bullets require faster twists...I've always though 7mm rounds should have no less than 1:9 twist, 6mm too for that matter
Keith

My answers in red...

 

[Ridgerunner665]

At any rate...the higher spin rates do seem to improve the BC, and the more I think about it...the more I wonder why this wasn't known sooner...because it makes perfect sense.

I always thought the concept was well known. I recall advertising for a chronograph system that utilized screens at or near the target in addition to those near the muzzle, and the stated advantage was that actual achieved BC could be derived from the velocity loss over a known distance. I don't recall whether it was explicitly explained, but it made sense that the difference between theoretical BCs assigned to bullets and the realized BC calculated on every shot fired would pretty much HAVE to arise because of stability, or lack thereof. Maybe someone with a better memory than mine (which should be just about everyone) will recall more details.

I'm just a "shade tree LR shooter" (sorta like a shade tree mechanic, lol)...but I've been at it a good while...I always understood that more twist than was needed (for stability) adversely affected accuracy because of centrifugal forces and less than perfectly balanced bullets...thats why I said there has to be a happy medium in there somewhere...or bullet production is gonna evolve into rocket science...but I guess thats what happens when you have a rocket scientist (Litz) influencing the industry, lol.

The stability factor thing...I thought 1.3 was plenty, up until now...well, 1.3 is still plenty enough to keep a bullet from tumbling...accuracy is fine...the only thing you give up is some BC.

 

[Wis65x55]

RR,
I am primarily talking about 140-160gr 7mm AB or PT bullets
Keith

 

[Ridgerunner665]

RR,
I am primarily talking about 140-160gr 7mm AB or PT bullets
Keith

At what muzzle velocity are you shooting them now?

Using the stability calculator...I'll see what the SG factor is, if its 1.5 or better...you're about as good as can be expected...as to how much BC more twist would give, the only way to know is live fire testing.

 

[Oldtrader3]

BC is a shape constant which is measured by the amount of drop of the bullet over some known distance.

 

[Wis65x55]

RR,
All of these variables are bases around a 280 Remington. The 140gr Pt has a MV of 2950. I have not tested the AB yet, however I believe it would be similar because the powder charge is the same.
Keith

 

[Ridgerunner665]

The AB will likely get a little more speed I think...

140 PT stability factor at 2,950 fps...1:10 twist = 2.15 (excellent)
140 AB @ 2,950 fps...1:10 twist = 1.53 (good enough)
140 AB @ 3025 fps...1.6


140 AB @ 3025 fps in a 1:9.5" twist is 1.77

1:9" twist gets you 1.97....like I said, 1:9 should have always been the minimum with 7mm...but 9.5 and 10 will work, as they have for decades.

 

[.300winmag]

Thank's Fotis!

I'm glad you stumbled on that! I would like to see the .30 cals and the 175gr. .284, but that is great info. If I can get my hands on some 129gr. 6.5's, I may very well try to convince my Dad to use my 6.5x47 Lapua instead of his '06 for his buck tag this year. . . I think that would be an awesome cartridge/bullet combo for mulies! Assuming I ever see that darn rifle and have time to develop a load and dope on it before his hunt :|

Question, are the 6.5mm 129gr. ABLR's in QuickLoad?

 

[DrMike]

Question, are the 6.5mm 129gr. ABLR's in QuickLoad?

Yes.

 

[.300winmag]

Question, are the 6.5mm 129gr. ABLR's in QuickLoad?

Yes.

DrMike, would you mind running QL with the 129gr. ABLR with H-4350? 29" bbl, 2.800" OAL and the default case capacity? I don't want to derail this thread, so feel free to use your judgment on posting it here, elsewhere or PM.

Thank you very much!

 

[DrMike]

Question, are the 6.5mm 129gr. ABLR's in QuickLoad?

Yes.

DrMike, would you mind running QL with the 129gr. ABLR with H-4350? 29" bbl, 2.800" OAL and the default case capacity? I don't want to derail this thread, so feel free to use your judgment on posting it here, elsewhere or PM.

Thank you very much!

Which cartridge? Glad to run it for you; just let me know what cartridge I'm looking at.

 

[.300winmag]

Question, are the 6.5mm 129gr. ABLR's in QuickLoad?

Yes.

DrMike, would you mind running QL with the 129gr. ABLR with H-4350? 29" bbl, 2.800" OAL and the default case capacity? I don't want to derail this thread, so feel free to use your judgment on posting it here, elsewhere or PM.

Thank you very much!

Which cartridge? Glad to run it for you; just let me know what cartridge I'm looking at.

Oops! Sorry about that! 6.5x47 Lapua please, and thank you!

 

[DrMike]

Cartridge : 6.5 x 47 Lapua
Bullet : .264, 129, Nosler AccuBond LR 58943 G7
Useable Case Capaci: 43.031 grain H2O = 2.794 cm³
Cartridge O.A.L. L6: 2.800 inch = 71.12 mm
Barrel Length : 29.0 inch = 736.6 mm
Powder : Hodgdon H4350

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms

-20.0 86 33.20 2384 1628 31956 5842 90.8 1.716
-18.0 88 34.03 2440 1706 34090 6012 91.9 1.672
-16.0 90 34.86 2496 1785 36366 6177 93.0 1.630
-14.0 92 35.69 2552 1866 38798 6335 94.0 1.585
-12.0 94 36.52 2608 1948 41393 6487 94.9 1.539
-10.0 96 37.35 2664 2032 44171 6631 95.7 1.495
-08.0 98 38.18 2719 2118 47141 6767 96.5 1.453
-06.0 101 39.01 2775 2205 50316 6894 97.2 1.413
-04.0 103 39.84 2830 2294 53691 7012 97.8 1.374 ! Near Maximum !
-02.0 105 40.67 2885 2384 57294 7121 98.3 1.336 ! Near Maximum !
+00.0 107 41.50 2940 2476 61155 7219 98.8 1.300 ! Near Maximum !
+02.0 109 42.33 2995 2569 65297 7307 99.2 1.264 !DANGEROUS LOAD-DO NOT USE!
+04.0 111 43.16 3049 2663 69746 7383 99.5 1.231 !DANGEROUS LOAD-DO NOT USE!
+06.0 113 43.99 3103 2758 74530 7448 99.7 1.198 !DANGEROUS LOAD-DO NOT USE!
+08.0 115 44.82 3157 2855 79682 7501 99.9 1.166 !DANGEROUS LOAD-DO NOT USE!
+10.0 118 45.65 3210 2952 85241 7541 100.0 1.135 !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 10% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 10% relative to nominal value:
+Ba 107 41.50 3056 2675 72335 6993 100.0 1.217 !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 10% relative to nominal value:
-Ba 107 41.50 2780 2213 50295 7109 93.8 1.409