Author Topic: Recasting 155 BBT  (Read 959 times)

sixshootertexan

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Re: Recasting 155 BBT
« Reply #15 on: April 29, 2019, 06:39:06 PM »
Ok the next round is with bullets weighting 150.4, 150.5, and 150.6. I used the RC Oil on all of these. 3 shot groups. Wind was not as bad as last time out but still gusting 10-15 mph.

150.4  811, 813, 813
 

150.5 814, 810, 810


150.6 814, 821, 818

CCS 2300, CCS 2400, Custom Built Regulated .25, Custom Built PRod Clone, .308 Bullet shooter, XS46U .177, Custom regulated .177, 850 Hammerli .177

sixshootertexan

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Re: Recasting 155 BBT
« Reply #16 on: April 29, 2019, 06:48:30 PM »
Ok this was the biggest spread I had in weight. 149.24 - 150.88. 2 shots of each.

149.24  811, 814
150.88  815, 822

CCS 2300, CCS 2400, Custom Built Regulated .25, Custom Built PRod Clone, .308 Bullet shooter, XS46U .177, Custom regulated .177, 850 Hammerli .177

sixshootertexan

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Re: Recasting 155 BBT
« Reply #17 on: April 29, 2019, 06:55:11 PM »
I know this is the 135gr bullets but I wanted to see what they are doing with the new spring and higher velocities.  5 shot groups.

132.7 gr one shot lube

857, 858, 857, 858,866  Avg859, ES 9, SD 3


132.7 gr  RC Oil

855,852, 854, 858, 859  Avg 855, ES 7, SD 2
CCS 2300, CCS 2400, Custom Built Regulated .25, Custom Built PRod Clone, .308 Bullet shooter, XS46U .177, Custom regulated .177, 850 Hammerli .177

rsterne

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Re: Recasting 155 BBT
« Reply #18 on: August 10, 2019, 10:34:39 AM »
Ricky.... a few questions.... Were those targets all at 100 yds?.... Is your barrel a 20" twist (and are you sure, did you check it)?.... Have you ever noticed any instability/tumbling with the 155 gr. BBT at any distance you have shot them?.... What is the highest velocity you have shot them at?....

I am intrigued that you can stabilize that bullet in such a slow twist rate.... According to the Kolbe Twist Calculator, even a flat based bullet of those dimensions should have a 16" twist rate, and would be really marginal in an 18".... and the boattail needs about a 4" faster twist than that.... I'm well aware that it may overestimate the correction needed for a boattail…. but for that long a bullet to work in a 20" twist blows me away....  ???

Bob
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sixshootertexan

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Re: Recasting 155 BBT
« Reply #19 on: August 11, 2019, 06:07:10 PM »
Yes 100 yards.
TJ barrel 1-20"
I don't remember shooting the 155's over 100 yards.
The posted velocities in the previous posts is all the data I have on them.
Looks like an average of 815 fps.
Never really notice tumbling or yaw. 

IMHO I think sometimes people rely to much on hard data. I know most of those calculators are for PB'ers. They may be right for them but I think air power just has a different effect on the bullets. I also think that most people are lazy and just want a rifle bullet combo that works without any effort. Just sale my a rifle and bullets that can hit the target at 100 yards.  I like to tinker and to learn new things. I finished this rifle in May 2015, it's been 4 years now and I'm still tinkering with it. Not as often as I'd like too. I think I almost got it where I want it. The rifle is now legal in Texas to hunt with since I changed the spring. The last mod I did (barrel support) I shot a target for the NUAH match and shot my best group ever with the 135gr BBT's. Going to try the 155's next.
Hope this helps you Bob. I know it ain't much but that's want I think.


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rsterne

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Re: Recasting 155 BBT
« Reply #20 on: August 11, 2019, 08:59:13 PM »
I'm not doubting you, jut trying to understand.... The Kolbe Twist program is one of the few that works Subsonic, and while I think it overestimates the twist correction for boattails, even a flat based bullet the length of the 155 gr. BBT should not stabilize in a 20" twist....

However, if you have checked the twist of your TJ's barrel and know that it's 20", I'll just record your results as proven and reliable....

Bob
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sixshootertexan

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Re: Recasting 155 BBT
« Reply #21 on: August 12, 2019, 08:47:10 AM »
I have double checked my receipt and it says .308, .590OD, 1-20, 4 l-g.
I have never doubted TJ's (forgot his name) word until now.
I just used the cleaning rod and tape method to check the twist.

Wow it's a 1-14".

Wonder how he missed using the wrong mandrel. Maybe he had them in stock and miss labeled it. I don't know but yes it's a 1-14". That's a huge difference from a 1-20" twist.
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rsterne

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Re: Recasting 155 BBT
« Reply #22 on: August 12, 2019, 09:17:36 AM »
Ricky, thanks for taking the time to check it, I REALLY appreciate it !!!!

Stuff happens, but I guess in your case this was a fortunate event, because you ended up with a great shooting rifle.... I know that bullet works in the Texan, which is also a 14" twist (reported to be that, anyways)…. The fact that it works in a 14" is still a VERY valuable piece of information, because the original Koble prediction was for an 11" twist.... I have suspected for some time that the Kolbe program calculates too great a loss in stability for my Rebated Boattails, and this bullet in a 14" twist is absolute proof of that....  8)

By extrapolation, since the 154 gr. BBT is 3.0 calibers long, this information should be scalable to the other calibers where I used the same proportions.... Twist rate in inches is proportional to caliber, so if the 154 gr. BBT is stable in a 14" twist, we should also see similar stability from the following 3 caliber long BBTs in proportional twist rates....

7mm (.284 cal) 121 gr.... 13" twist instead of 10"
.257 cal 90 gr.... 12" twist instead of 9"
.224 cal 60 gr.... 10" twist instead of 8.5"

While I can't draw firm conclusions for BBTs of other proportions, it certainly points the way to being able to use them in slower twist rates than the Kolbe Twist Calculator predicts as well.... Now it is time for me to try and figure out a "revised" base dimension to input into that calculator to come up with a better prediction for the minimum twist rate required.... Again, many thanks....

Bob
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