Bullet design and twist

[madsenshooter]

It baffles me. I mostly shoot 30 cal rifles that have a 1/10 twist. When I try shooting really long bullet, lets say the Eagan MX3-30G, which is 1.279" long, I find that the best accuracy is around 1600fps. Accuracy gets worse the faster I go, and going to a slower burning powder doesn't help things, it does its best at the same speed. On the other hand I have a shorter bullet, the Eagan MX3-30AR, that's only 1.046" long, one would think it would stabilize better with a slower twist, according to jacketed theory. But that doesn't really fit into this discussion anyway, I only have 1/10. But this bullet gets magical at 2000-2150fps 1in groups with several cloverleafs in them. Then there's that bugger of a bullet, the squib, Belding and Mull's version. Only 1.047" long, the same weight as the Eagan 30AR. It's magic point in a 1/10 seems to be 1550fps. No wonder I'm baffled. I've had all 3 of these bullets up and down the scale with different powders, they are what they are, I just don't understand the why of it. With a 1/12 twist or slower, with the 30G, Eagan recommended a minimum velocity of 2000fps, I can get there with a 1/10, but there's no accuracy to speak of. I'm talking about target shooting, not getting close enough to maybe hit something the size of a deer. Must one be an engineer to understand bullet design? I've gone the harder alloy slower powder route also. Oh, crud, it's too late to be pondering these things. Good night all!

[geargnasher]

I don't know about everyone else, but the more I shoot cast, the fewer absolutes or even usuallys I find. Opposite of the doing of most things, where patterns emerge after a while. I'm learning to take each component as a rule unto itself, and never assume anything.

I really don't have any answers for you, you've done the work yourself and as you said the results are what they are. In my guns the boolits might behave differently.

Gear

[Larry Gibson]

Best cast bullet designs to push the RPM threshold will fit the case neck with the bottom of the GC at the bottom of the case neck. The length of the bullet should be such that the front driving band is at or just touches the leade. The front of the bullet outside the case neck should be a tight slip fit into the throat. The nose in front of the driving band should be short and preferably not over one caliber in length. The lube grooves should be just a thousanth or two less than the bore diameter and only as many as needed to hold the minimal amount of lube for no leading. The idea is to have a pefect fit with as little of the bullet unsupported (the nose and lube grooves) as possible so as little obturation , set back or slumping occurs during accelleration.

What .30 cal cartridges?

Larry Gibson

[madsenshooter]

I shoot 30/40 Krag mostly, some with like new throats, some a bit long. With the Krag, even the like new ones are long in the throat, there's an unrifled portion. Also a couple 06s, my Madsen and a Garand, the K31, and a Mosin that I haven't been able to find brass for yet. Eagan's are tapered bullets, so they seat out pretty long in the Krag. The majority of my Krags have .301/.310 bore/groove dimensions, none larger, 2 smaller. The smaller one's don't get shot much, saving them for after I wear out the others!

[Multigunner]

My old Herter's book from the 60's mentions a custom Springfield .30-06 with a 1:9 twist being very versatile so far as a wide range of bullet weights went. The Mausers used a similar Twist, not exactly on the dot and could handle a wide range of bullet weights.
I saw the book here some where recently, I'll look for it and see what it says on this.

Bullet weight is secondary to length when it comes to rifling twist. A fairly long lighter weight bullet like the MkVII at an increased velocity was just as accurate in a 1:10 bore as the heavier MkVI it replaced.
Rotational speed and spin drift depend on velocity. When velocity drops at longer ranges the bullet is still rotating at about the same RPMs as when it left the muzzle so the effects of spin drift increase with range.

The 03 Springfield with 150 gr bullet had a very slight bullet jump to the left on leaving the muzzle, probably due to the increased velocity over the 220 gr.
The bullet traveled a very small fraction of an inch to the left of the bore line, with spin drift compensating at somewhere near the six hundred yard mark, then spin drift would begin to move the line of flight further to the right as range increased.
I suspect this is why the 03 battle sight was set at over 500 yards, I've seen differing figures on the exact yardage. The spindrift compensation would put the bullet POI dead center to the bore line in the horizontal plane at pretty much the same point as the battle sight POA with battle sight centered to the boreline. That would make bore sighting to adjust the sight much more productive with less windage adjustment needed later to fine down the sight.