slimjim wrote:

tcknight wrote: Factory = 1:9.25" twist, 22" Krieger = 1:12" twist, 23" Caliber _ 0.243 Inches Bullet Weight _ 75 Grains Bullet Length _ 0.942 Inches Barrel Twist _ 12 Inches/turn muzzle velocity _ 3620 fps Temperature _ 68 degrees Fahrenheit (59 is standard) Pressure _ 29.92 inches of mercury (29.92 is standard

Slimjim:
Remember, I know just enough about this stuff to be dangerous. I quit reloading in the ‘80s because I would rather be hunting than messing around with the minutiae of cartridge loads, and factory ammo became just about as good as my reloads. As long as it goes BOOM, groups in an area I can cover with a fingertip, and explodes only after penetrating fur, hide, and the first rib, then it is “the bomb” and I’m good with it . Having said that, I still like to keep up so:

I see what you are saying but something is wrong either here, or with my original calculation. Before selecting the barrel, I calculated that the 75 grain Hornady HP should stabilize from a twist as slow as 1:14.1. This was based on bullet length, velocity, bullet diameter AND Specific Gravity. I went with a 1:12 based on advice and the possibility that atmospheric conditions may be enough to destabilize this bullet from a 1:14" twist.

I found the stability factor from the 1:12" twist barrel should be 1.6 (if memory serves me and damned if I remember how I calculated that. Dan Lilja recommend no less than around 1.5). You are showing 1.15. One, or both of us, is getting something wrong. One error on your spreadsheet above is bullet length. The Hornady 75 grain HP is a measured 0.87”. Using this calculation with this bullet length, the stability factor would be 1.47. Also remember that Greenhill's formula was for flat based bullets and most formula's today have been modified to account for boat tail designs. I’m wondering if the underlying calculations of the spreadsheet above include a modification for boat tail designs? Remember this Hornady bullet is of the flat base design (as is the Remington 80 grain CoreLokt I also shoot in this gun) so Greenhill’s formula, only modified for higher velocity would be most appropriate. Since the 58 grain V-Max is a boat tail design it would be closer in length to the bullets I shoot but I don’t have any to measure.

Contrary to most who like the splatter factor of the new varmint bullets, I prefer the old hollow point designs in varmint bullets as I dislike “cratering” and prefer to preserve the pelts of the predators I hunt. Hence, I shoot only flat based bullets, which lend themselves well to slower twist barrels.

Finally, if this spreadsheet is correct, and this bullet has a stability factor of 1.15, then this makes the spreadsheet and/or the whole Stability Factor (SF) argument suspect, because, this gun shoots these bullets VERY well at only 200' above sea level and in all humidity and temperature ranges.

From the Shilen website:
6mm/.243
- 8" Special for VLD bullets over 100 gr.
- 10" For bullets up to 120 gr. and VLD under 100 gr.
- 12" for bullets up to 85gr.
- 13"* for bullets up to 75gr.
- 14"* for bullets up to 70gr.
- 15"* Special for bullets up to 70gr.

Shilen confirms why this tube is handling the 80 grain CoreLokts well. Maybe we are getting too technical here? When trying to find the very best barrel to replace my sorry factory tube, I was able to talk with Dan Lilja at length about barrel selection. He also suggested, based on my bullet and cartridge selection, to use a 1:12” twist to get that very small increase in accuracy this twist rate would gain. (The only reason I didn’t purchase a barrel from Dan was that he was so far behind and I needed it a little quicker. My next barrel will come from Dan).

Something to consider.

Tim

tcknight wrote:

I see what you are saying but something is wrong either here, or with my original calculation.

Tim, you are correct. I picked the wrong bullet length from jbmballistics.com. I show the proper calculations below and you are indeed correct regarding stability of your bullet with your barrel choice.

Caliber _ 0.243 Inches
Bullet Weight _ 75 Grains
Bullet Length _ 0.87 Inches
Barrel Twist _ 12 Inches/turn
muzzle velocity _ 3620 fps
Temperature _ 68 degrees Fahrenheit (59 is standard)
Pressure _ 29.92 inches of mercury (29.92 is standard)

Stability Factor (Sg) = 1.44

I'm still learning and found the GSC website informative. The discussions and comments from this topic have help increase my understanding a lot. Its amazing how what seems to be a small change in bullet length can have such a significant impact on stability. When I bought my last gun, I first picked the bullet I wanted to hunt with (.277 110gr TTSX), then the case to launch it with (.270 Win), then the rifle (Tikka). A year ago, I didn't know about all the different aspects of bullet shape and rifle twist and was fortuneate to have bought a good rifle that matched up with the bullet(s) I wanted to use. I wasn't so lucky with my .308 with 1:12 twist. I had trouble shooting boat-tails much heavier than 130. Now I know rifle twist was a contribution to its less than desired performance. I'll be better prepared before I buy my next rifle.

Thanks for the detail and keeping me straight.

Sooooo !!! Would you say a stability factor of 2.123 is to high and should not merit usage ???

chambered221 wrote:

Sooooo !!! Would you say a stability factor of 2.123 is to high and should not merit usage ???

That looks like the Stability Factor for your 6mm

Caliber _ 0.243 Inches
Bullet Weight _ 55 Grains
Bullet Length _ 0.787 Inches
Barrel Twist _ 10 Inches/turn
muzzle velocity _ 4138 fps
Temperature _ 68 degrees Fahrenheit (59 is standard)
Pressure _ 29.92 inches of mercury (29.92 is standard)

Stability Factor (Sg) = 2.11

"should not merit useage?" Not at all according to GSC or even the stability spreadsheet. As I said before, I'm still learning and I don't have experience with a rifle set-up like your 6mm. GSC has a .243 bullet close in length to your 55gr B-Tip.

GSC .243 62gr HV

GSC recommends:
Bush and dangerous game hunting - S/F greater than 1.6 is required. There is no upper limit to the S/F here. If this bullet does not meet the requirement, use a shorter bullet.
General hunting to 500m/yds - S/F greater than 1.4 is required. There is no upper limit to the S/F here. If this bullet does not meet the requirement, use a shorter bullet.
Long range beyond 500m/yds - S/F greater than 1.1 is required but more than 1.5 is not desirable. If this bullet S/F is too high, use a longer bullet. If too low, use a shorter bullet.

The spreadsheet only states that accuracy may improve if the stability factor is less than 2. "Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may gain some accuracy by going to a slower twist barrel."

The reason I initiated this topic was to discuss the benefit of faster than normal spin rates. If your 55gr B-Tip bullet is maintaining its integrity at that high an RPM the information suggest that accuracy outside of 500 meter would be degraded (bullet doesn't nose down along its dropping trajectory) or accuracy might improve if you went to a longer bullet or slower speed.

slimjim wrote:

tcknight wrote: I see what you are saying but something is wrong either here, or with my original calculation. Tim, you are correct. Thanks for the detail and keeping me straight.

Hey, you know more about it than I do. I just love talking 'bout it (when I'm not hunting )

Thanks for helping and keeping up your end of a good conversation.

I just wanted to post thes pictures of an exit wound from a 100g barnes from a 25-06 from @200 yrds as you can see the base or tail is facing out I know I've said this before and 1 picture shouldn't make anyone's case but then again this is reality not just what somebody thinks will happen.

Is this such a goal-seeking bullet that makes a 180° return when you miss the target?
Honnest: I'm happy to see such a picture. Where was that deer shot? Did the bullet only went trough the neck or did it deviate via the spine?
I once shot a wild boar (pig) in his neck and the bullet folowed the spine till it left the body about 10" before the tail.
I think when you want to preduct a bullets' flight inside an animal when it hits bones, you have to use a very large case and a very strong bullet.

fnuser, pretty good pics! What did the bullet look like after removal? I'm assuming it hit the spine or that was pretty poor penetration.

it hit the eye on the other side entered the skull and this is a picture of the EXIT

slimjim wrote:

General hunting to 500m/yds - S/F greater than 1.4 is required. There is no upper limit to the S/F here. If this bullet does not meet the requirement, use a shorter bullet. Long range beyond 500m/yds - S/F greater than 1.1 is required but more than 1.5 is not desirable. If this bullet S/F is too high, use a longer bullet. If too low, use a shorter bullet. The spreadsheet only states that accuracy may improve if the stability factor is less than 2. "Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may gain some accuracy by going to a slower twist barrel."

I somehow missed this on their site....thanks for the added info !!!

I did know a 12 twist would be better suited for the bullet. When I bought the gun I ran nothing but the 85gr. Sierra HPBT !!!

Good information SlimJim.

fnuser wrote:

exit wound from a 100g barnes from a 25-06 from @200 yrds

fnuser, can you provide some specifics.

what is your twist rate?
what is your muzzle velocity?
do you remember how cold the day might have been?

I couldn't see any "bands" on the bullet. What is its date of manufacture? Was it a ballistic tip? I ask because Barnes has changed the metallurgy to promote better expansion. If you have bullet length that would be good but I may be able to get it if you have the bullet specifics.

fnuser wrote:

it hit the eye on the other side entered the skull and this is a picture of the EXIT

Is the tip of the bullet deformed or mushroomed in anyway?

twist was 1-10 but I think you are missing my point stability inside an animal is improbable to predict.