Bullet bearing surface

[Rich Coyle]

Can anyone address the above question?

 

[Jud96]

I've been down the path of measuring bearing surface lengths accurately in order to determine what freebore I needed or if a given freebore would work with the desired bullet. I found the most accurate way to determine where a bullet will engage the lands in a typical 1.5 degree leade angle is by taking your calipers and setting them .002 below actual bullet diameter and slide the blades of the calipers down the bullet until they stop. Rotate the bullet or calipers to lightly etch it the bullet. Then measure from that mark to the bearing surface/boattail junction. This gets you within .020 of the actual number.

I found that Berger'a data can unintentionally be misleading because some bullets with a long tangent ogive, like their 6mm 108BT, appear to have a pretty short bearing surface. However, because of the long tapered ogive, that bullet will engage the lands sooner then a VLD bullet with an equal length published bearing surface. In short, you'll end up needed more freebore for the tangent ogive bullet vs. the VLD even though on paper the bearing surfaces may be very close. Sorry for the long winded response, but this is something I've struggled with in the past but feel I have a good way of determining the bearing surface length now.

 

[Beluebow]

All I know is when you start sending bullets at MACH 2-4 everything matters.

Quoted from Bryan Litz Applied Ballistics for the long range shooter 3rd edition.

"Gyroscopic stability is affected directly by the twist rate of our barrel, velocity of the bullet."

 

[QuietTexan]

Can anyone address the above question?

TL: DR; Read the Applied Ballistics for Long Range Shooting by Bryan Litz. Longer bullets generally needs more spin for science reasons. As with all generalities, there are exceptions. (Edit: yup, same book Beluebow just referenced ^^^)

If you want to read a longer answer:

There's a chapter where he goes over the use of rifling to spin a bullet to overcome the tendency of the center of pressure (cp) to try to move behind the cg (center of gravity). Since the cp is ahead of cg in a pointed rifle bullet, this tendency means the bullet is inherently unstable and requires an outside force to overcome that instability - angular momentum imparted on the bullet by the rifling is the force used. The more distance between the cg and cp, the more leverage cp has to move behind cg, the more force needs to be imparted to keep cp in front, so more spin (aka faster twist rate) is needed to impart more angular momentum on the bullet.

The distance between cg and cp cannot be reduced to a single variable because their locations depend on the specific values for the lengths of the bearing surface, boat tail, nose, and the weight of the bullet. Bullet diameter (aka caliber) has to be constant, so starting from that point while you can hold multiple additional variables constant (ie boat tail and weight) at least two variables of bearing surface and nose length would both have to change to stay within function boundaries of cartridge dimensions (you can't feed a 9" long bullet with 6" of pencil lead material for a nose).

Hence the generality that longer bullets require more twist - to keep weight the same and increase bearing surface the nose is shortened, closing the distance between cp and cg, requiring less twist. A shorter bearing surface will general have a longer nose to maintain a given weight (ogive profile of tangent, ogive, hybrid, etc coming into play) so requires more twist because cp and cg are further apart.

I'm sure there's a way that you could mess with material density and bearing surface/nose/meplat, etc to design two bullets with identical cp and cg points that would require the same angular momentum (and thus same twist) at drastically different lengths - but to me that's a solution to a problem that doesn't exist because it's not difficult to deal with different bullets needing different twists. Once a minimum angular momentum is achieved there doesn't seem to be a lot of draw backs to additional spin for a pretty decent range of spin rates.

Eventually precession of the meplat becomes an issue where the bullet essentially becomes unstable at too high of an angular momentum/ rate of spin and settles on random trajectories other than intended, but that's also nearer to the point that the bullet might might tear itself apart from internal imbalances than to the point of being sufficiently spun to usually be an issue.

So back to using a twist of a minimum, but still being able to use an inch or two faster twist without causing problems. This is why you can't shoot 125gn flat base 308 bullets meant for a 13 twist 30BR out of a 7 twist 300 RUM - the bullet would most likely be torn apart by the angular moment imparted by such a fast spin, but even if it stayed together the bullet would precess to the point it tumbled and either blew apart or went drastically off course when it exited the barrel. Precision would be impossible at this point.

You can see the proof of this concept in copper alloy monolithic bullets, and how at any given length they require more spin to be stable than an equal length cup-and-core bullet. They're lighter at any given length because the density of the material is much lower than lead, so the center of gravity is naturally further rearward and that requires more angular momentum to overcome the higher leverage from the longer moment arm (distance between cg and cp). Since they're made from a single alloy there's no jacket/core boundary with inconsistencies to destabilize the bullet, and the dimensional tolerances are set by CNC machining equipment and not dies in a press - those lead to a mono bullet holding together at higher angular momentums without failing. This also places a much higher importance on the very tip of the bullet at the meplat, ensuring it's uniform to minimize nutation. The way Cutting Edge and Hammer machine their bullets, there apparently has not been a point reached where a very uniform 2-4mm hollow point meplat nutates to the point of a bullet tumbling at any twist rate.



At least as experimentally tested by ButterBean. I've pushed Hammers over 4k FPS but not in the most aggressive twist for a caliber. I have a 36" 6-twist 308 barrel in 30-338 LIMP, so maybe I'll push a 101 Hammer Blackout one day.