308 F-T/R Load Tuning - Difficult Bullets

[DumpHollow]

I am starting this thread as so as not to hijack the wind drift thread.

The question is for Greg and any others who would like to weigh-in. Greg closed his post on "wind drift..." with:
"There are at least two recent 30 cal bullet offerings with exceptionally high BCs for which many F-TR shooters have had extreme difficulty developing consistent loads/precision. At this point, it's very difficult to pinpoint the exact cause for this behavior, but the number of people that have experienced it suggests it's not an anomaly. In my mind, only time will tell how easy to load/tune the Blackjack bullet is with regard to an anticipated use in F-Class. shooting. "

I have not tried to load much more than the Berger 185 Jug and the 185 Hybrid. As these have served me well with little pain in tuning, I have not tried any of the heavier bullets available. Specifically, which heavy 308 bullets is Greg referring to?

 

[XTR]

200 Jack from KP ballistics in Canada, the 200SMK from sierra, and the 198 Flatline from Warner, there may be others, but those three come to mind...

 

[dcali]

A rule of thumb is that the longer the bullet is in calibers, the tougher it is to get it to shoot well. Both the jack and the new SMK are pretty long bullets. Let alone solids like the flatline. At the end of the day, you have to balance BC with accuracy potential and go with what makes sense for your application.

The trouble for bullet makers is that shooters want to hit silver dollars at 1000 yards. A tall order!

 

[Ned Ludd]

Fred - Wade nailed it with the first two he listed. I haven't ever tried the 198 Flatlines, or spoken directly with anyone that has, so I wasn't aware they might also exhibit aberrant behavior. As I mentioned, I cannot definitively state the exact cause(s) for this behavior, I can only state that it has been observed by a number of different shooters. I can imagine a few possibilities where the design of the bullets themselves might be the problem. For example, where are their center of gravities located relative to a bullet like the 200.20X, which does not appear to have the same issues? Obviously, those are both noticeably longer bullets for their weight than the 200.20X, but even using a 9-twist barrel does not appear to alleviate the precision issues. The bottom line is that using a bullet that is difficult to tune or even that may group poorly just for a few extra percentage points of BC simply isn't worth it for most shooters; they will move on to a different bullet relatively quickly. Understanding why these bullets exhibit this behavior is the more difficult question, but would likely be beneficial for bullet design in the long run.

 

[dcali]

The cause (largely) is aerodynamic jump and initial yaw/yaw rates. The physics are just worse for long, skinny bullets. No way around it other than figuring out some way to make and launch them perfectly.

 

[DumpHollow]

Thanks for the replies. I am EASY button shooter and will eventually venture to the 200.20x in my 10 twist TR rifle.

 

[Ned Ludd]

Thanks for the replies. I am EASY button shooter and will eventually venture to the 200.20x in my 10 twist TR rifle.

"EASY button"??? How does a person get one of those?

I think you will be very pleased when you tune in the 200-20Xs out of your 10-twist barrel. In my hands, they tuned in quite readily at ~ 2640 fps from a 30" barrel with Varget, starting at approximately .009", then up to .015" off the lands.

The cause (largely) is aerodynamic jump and initial yaw/yaw rates. The physics are just worse for long, skinny bullets. No way around it other than figuring out some way to make and launch them perfectly.

This is certainly a good possibility. However, it's not clear (to me at least) that aerodynamic jump and/or yaw rates are the definitive cause(s) for the behavior of the bullets mentioned. Obviously something is causing it, but I do not have sufficient information to do anything more than guess at the cause.

 

[DumpHollow]

That's EASY. Listen to sources that you can trust...

 

[Medic505]

Thanks for the replies. I am EASY button shooter and will eventually venture to the 200.20x in my 10 twist TR rifle.

When you do this and if your freebore will allow it, try the Berger 200 Hybrid. Much like the 185 Hybrid, simple to tune. Don't overlook a good bullet because of BC numbers on a box.

 

[DirtySteve]

Litz Mentioned in one of his books what caused the longer bullets to be more difficult to stabilized. I will attempt to find the information. But as I recall it had to do with the amount of spin, center of gravity on the bullet combined with the air pushing on the tip of the bullet. Maybe also some thing about spin dampening. I will try to get the accurate info so please try to avoid jumping on me at this point if I am mistaken about any part of this. I have several books to skim through for the info.

EDIT: Ok, I found the factors that make longer bullets more difficult to fly true.
The CP(Center of Pressure) is moved further back on long bullets as compared to short blunt bullets. The faster the bullet travels the more aerodynamic force and accompanying torque is applied to the Center of Gravity of the bullet. Air Density also comes in to play as the denser air will increase these effects. (Chapter 10 of the Third Edition of Applied Ballistics for long range shooting)