Bullet stabillity question.

[Laurie]

The formula for plastic tipped bullets was also developed by Don Miller, who developed the formula for untipped bullets that the Berger calculator uses. I have found it to be a reliable indicator regarding twist rate requirements. By comparison, the Berger calculator predicts a lot of twist rates will not work with tipped bullets, when they really work great.

I suspect there is another error inducing factor around too for bullets like light 22s, sixes and suchlike. The Berger stability calculator is for boat-tail bullets and it states specifically that it is overly pessimistic with flat base types. Comparing Miller/Berger against the twist rates Berger quotes for its various FB types, they're out by 1-inch or more.

Although the 55gn NBT is 'officially' boat-tail, it's such a measly little thing I barely consider it as such and regard it as an aid to bullet seating rather than an external ballistics advantage. Likewise when it comes to stability, it'll have less effect on the bullet's needs than the 'proper' long angled tail sections you find on long-range 6mm designs.

If my hypothesis is correct, it also applies to many other lighter small calibre match models such as the 52/53gn Sierra and Hornady .224s and similar. Whenever I've run them through Miller, they're barely stabilised in commonly used slower twist barrels. At 3,200 fps in standard ballistic conditions, the Sg for the 224 52gn Sierra MK is only 1.05 in a 14-inch twist barrel, yet they're use very effectively in .222 Rem rifles with this twist rate.

 

[Berger.Fan222]

I suspect there is another error inducing factor around too for bullets like light 22s, sixes and suchlike. The Berger stability calculator is for boat-tail bullets and it states specifically that it is overly pessimistic with flat base types. Comparing Miller/Berger against the twist rates Berger quotes for its various FB types, they're out by 1-inch or more.

Although the 55gn NBT is 'officially' boat-tail, it's such a measly little thing I barely consider it as such and regard it as an aid to bullet seating rather than an external ballistics advantage. Likewise when it comes to stability, it'll have less effect on the bullet's needs than the 'proper' long angled tail sections you find on long-range 6mm designs.

If my hypothesis is correct, it also applies to many other lighter small calibre match models such as the 52/53gn Sierra and Hornady .224s and similar. Whenever I've run them through Miller, they're barely stabilised in commonly used slower twist barrels. At 3,200 fps in standard ballistic conditions, the Sg for the 224 52gn Sierra MK is only 1.05 in a 14-inch twist barrel, yet they're use very effectively in .222 Rem rifles with this twist rate.

Another factor might be the air space in the open tips of match type bullets.

The original Miller formula applies to constant density bullets, and jacketed lead bullets are close if they are spitzer hunting types. But a lot of match bullets have open areas in the front making them less dense in the air space in front of the lead core.

At some point, I downloaded a spreadsheet with yet another formula for computing the gyroscopic stability of these. It might have been here.

 

[Laurie]

Another factor might be the air space in the open tips of match type bullets.

The original Miller formula applies to constant density bullets, and jacketed lead bullets are close if they are spitzer hunting types. But a lot of match bullets have open areas in the front making them less dense in the air space in front of the lead core.

Yes, this is definitely so. In the early days of FTR, many British competitors used the 155gn Lapua Scenar mostly in 13-inch twist barrels, but some in 14-twist examples at 3,025-3,100 fps. This bullet has a huge airspace in the nose above the core. (I once opened up the meplat on one and used a bit of thin wire to find the core marking its position on the jacket. I've got a photo somewhere but just can't find it sadly.) Bryan Litz marks it up as 'minimum twist for stability': 12.7" and for 'optimal performance' 11.4 inches or faster. Yet Russell Simmonds (a later FTR world champion) won two GB FTR championship titles in a row (the majority of matches in the series shot at 1,000 yards) using this bullet and 14-inch twist barrels immediately after the FTR class came into being. That's despite a Miller Sg of 1.02 at 59-deg F and dropping below 1.00 in the conditions seen in the inaugural GB F-Class Assoc Autumn Challenge shoot in November at Bisley which saw frost on the ground for most of the morning and air temperatures barely above freezing all day for the weekend.

I'd not recommend this combination nowadays, and Russell certainly hasn't used it in a long time, but in theory it should never have worked at all.

 

[Berger.Fan222]

The top image is a screenshot of the Berger twist calculator results for the 53 grain V-MAX at 2800 fps in a 1 in 12" twist. You can see the errant predictions of instability, BC loss, and needing a 1 in 9.25 twist.

The bottom image is a screenshot of the JBM stability calculator which includes the length of the plastic tip and accounts for it. The JBM calculator predicts the bullet is stable, which is what a lot of shooters have seen with it. Further, my LabRadar confirms that there is no loss of BC for V-MAX bullets, as suggested by the Berger calculator. This is in excellent agreement with the Hornady recommendation of a 1 in 12" twist for this bullet. I really do recommend the JBM stability calculator as more reasonable and accurate than the Berger twist rate calculator, esp for tipped bullets. See: http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Analysis of the predictions for the 55 grain V-MAX in a 1 in 14" twist yields similar results.

 

[barefooter56]

I have been tinkering with the Berger Balistics Twist Calculator. I am toying with the idea of a new slow twist BBL in either Dasher or 6-47L to shoot light bullets at near light speed for close range prairie dog hunting. I have twelveteen 1-8 twist tubes to shoot the 105 - 108 class bullets. When trying to shoot the lightweights in my used 1-8 twist tubes, my tests are showing either outright bullet blowup or impending blowup. I only tested at 100 yards, and think that some of the bullets that are actually getting to 100 may not make the next 200 yards!!! So, I am thinking about a dedicated dog killer in those two chamberings since I already have the brass and dies.

Now...the balistic calculator..... Can anyone tell me what twist people are shooting the 55 gr NBT and the 58 V-Max out of. Inputting all of the data on the calculator tells me that I would need at least a 1 - 12 twist tube to "properly" stabilize these bullets. Now, for those of you who have ever read my responces to other posts about which twist to stabilize which bullet, and someone says you "CAN'T shoot xyz bullet in an xx twist bbl because the twist calculator says it is only at xxx on the chart" and I respond with.."hey try it..it worked for me". BUT, I also usually state that if my intention from the start was to shoot xyz bullet, I would buy a faster twist right from the start to be absolutely sure it works. I shoot 200 Hybrids in a 1-12, I shoot 230 Hybrids in a 1-11 twist in sub zero weather. Didn't plan it that way..it just ended up "working out". I see a bunch of 1-13.5 and 1-14 twist tubes for sale, but, according to the twist calculator I am down under 1.10 with a 1-14 twist and a 55 gr NBT!!! What bullets do they shoot in a 1-14 twist. I don't even see a 6mm bullet that could possably work in a 14 twist with any cirtanty, which,if the calculator is correct, would be at least 1.50 on the chart.

Now, I would just simply buy a 1-10 to 1-12 twist and be done with it, but I can't find one that I can use. Anyone using a 1-13.5 - 1-14 twist tube out there. There must be lots of them, because everyone who sells bbls stocks them.

Thanks,
Tod

4xforfun,
Read here :
http://www.bergerbullets.com/berger-bullets-twist-rate-recommendations-by-eric-stecker-bryan-litz/
For more information on how to interpret your findings on our TWIST RATE CALCULATOR. Always enter the correct or as close to correct altitude you will be shooting at to get the correct result !