Tom Gray's Thoughts on Throats and High Velocity

[mtngun]

I've been reading through a CD of old Fouling Shot newsletters and came across an article on high velocity cast, by Tom Gray in FS #115.

Tom was using a 30 caliber wildcat with 1-13" or 1-14" twist, loaded to 2600 fps (no mention of what bullet he was using). Tom said the 1-13" could only go 2600 fps without losing accuracy, but the 1-14" could go 2650+.

Tom buys into the "bands-stripping-in-the-rifling" theory of what limits cast bullet accuracy. I tend to disagree. I think my experiments with Loverider bullets have demonstrated that even itty bitty bands are able to hold the rifling without stripping.

However, it is a fact of physics that an unbalanced bullet -- and I assume all cast bullets are imperfectly balanced -- will wobble, and the faster they spin, the more they wobble. Hence my motivation to replace my factory 1-10 30-06 barrel with a 1-12. In hindsight, I might should have tried a 1-13 or 1-14, but maybe that will happen someday.

Benchrest shooters, both cast and jacketed, commonly shoot slower twists compared to the twist in hunting rifles. Obviously, jacketed bullets are not stripping in the rifling (though it is possible for the lead core to spin inside the jacket), so the fact that jacketed benchrest shooters also use slow twists backs up my suspicion that the issue is wobble, not stripping.

Anyway, I was particularly interested in the throat that Tom used. I may eventually copy it for my future rifle builds.

[LBD]

Tom buys into the "bands-stripping-in-the-rifling" theory of what limits cast bullet accuracy. I tend to disagree. I think my experiments with Loverider bullets have demonstrated that even itty bitty bands are able to hold the rifling without stripping.

...However, it is a fact of physics that an unbalanced bullet -- and I assume all cast bullets are imperfectly balanced -- will wobble, and the faster they spin, the more they wobble

This is a very interesting topic and I would appreciate you providing a link to you experiments. When you mention "itty bitty bands", the first thing that comes to mind is how much bearing surface area are we dealing with? I don't think the amplitude of the "wobble" increases with an increase in RPM but the frequency probably does.

For now however, please have a look at my question under http://mountainmolds.com/phpBB3/viewtop ... f=11&t=436 so I can proceed with my purchase.

Thank you,
LBD

[mtngun]

I don't think the amplitude of the "wobble" increases with an increase in RPM but the frequency probably does.

If your car has a badly unbalanced wheel, it will be un-noticeable at slow speeds, but horrible at freeway speeds. Bullets are a little more complicated but the same principle applies.

Some experiments with itty bitty bands in this thread: http://www.mountainmolds.com/phpBB3/vie ... ?f=7&t=415

More experiments with itty bitty bands here: http://www.mountainmolds.com/phpBB3/vie ... ?f=8&t=427

[mtngun]

Update: the 1/2 degree per side leade, as per Tom Gray's design, seemed to be a failure in my Contender 357 maxi experiment. Perhaps it would work better in a bolt gun with long bullets.

Anyhoo, I'm thinking outloud about different leade angles, and wondered how long the leade would be for a given angle?

Let's assume that the leade begins at 0.360" diameter and tapers down to 0.347", or 0.0065" taper per side. If the angle per side is "theta," the length of the leade would be:

Length = 0.0065" / tan(theta)

If theta = 0.5 degrees then length = 0.75"
If theta = 1.0 degrees then length = 0.37"
If theta = 1.5 degrees then length = 0.25"
If theta = 3.0 degrees then length = 0.12"

That makes it easier for me to imagine what the leade would look like.

I'm tentatively planning to try a 1.5 degree leade on my next TC chamber, because the length seems reasonable and because it's perhaps the most common leade angle, so it would seem to be a "safe" choice.

[mtngun]

More musings on the optimal throat angle. This post assumes we are nose-sizing and loading the bullet Ardito style.

Generally the Ardito-style bullet is seated 0.100" - 0.150" deep, so let's set the full diameter portion of the bullet at 0.150" long.

You want the front band to engrave slightly, so let's set the front band diameter at groove + 0.002".

For a given bullet, you want the throat angle to line up with those two points on the bullet:


Here are some concrete examples:
6x45, 60 gr. spitzer, .243" x .237" barrel, top of front band 0.35" from base, matching throat angle = 0.9 degree per side

30 cal, 130 gr. spitzer, .308" x .300" barrel, top of front band 0.48" from base, matching throat angle = 0.7 degree per side

30 cal, 180 gr. spitzer, .308" x .300" barrel, top of front band 0.741" from base, matching throat angle = 0.39 degree per side

357 mag, 150 gr. spitzer, .356" x .348" barrel, top of front band 0.36" from base, matching throat angle = 1.1 degree per side.

A longer bullet will require a longer leade which will require a gentler throat angle.

In practice, there will be tolerances in machining the reamer, in cutting the chamber, and in cutting the nose die, so there is no point in worrying about getting the angle perfect. When in doubt err on the steep side because you can always cut a short throat longer but you can't cut a long throat shorter.

This is just a theory based on my recent experiences using the Ardito method of sizing and loading. Feel free to form your own conclusions.

[mtngun]

Today's Musings: to freebore or not to freebore ?
.
It's my understanding that the proven Ardito throat did not necessarily have a freebore. Instead, a very gentle leade began immediately after the chamber neck. Alignment is ensured by the long tapered nose of the bullet mating with the long tapered leade. I can understand how that would work well with the long bullets typically used in CBA benchrest.

But .... what if the bullet is short, like my 155 gr. 357 bullet? You can still taper part of the bullet nose to match the leade, but not that much because the short bullet does not have that much real estate. Meanwhile the base of the bullet is supported only by the case neck, which is less than ideal.

Another problem with my short 155 gr. 357 bullet in a Contender is that Contenders do not like to "jam" the bullet into the leade as Ardito did with his bolt guns.

An alternative alignment system is to have a freebore section that is close to bullet diameter. As long as the freebore is concentric with the bore, and as long as the bullet is a snug fit in the freebore, then the freebore should guide the bullet straight into the rifling. One advantage of freebore guidance over Ardito guidance is that freebore does not require a jam fit, merely a snug slip fit.

Of course it is possible, if the bullet is long enough, to combine both systems, a snug fitting freebore followed by an Ardito style tapered nose. That is how the Grey throat at the beginning of this thread works.

Getting back to my short 155 gr. 357 bullet -- I'm wondering if it might be better off as a 1-diameter bullet in a snug fitting freebore? That should guarantee good alignment yet not require a jam fit.

[mtngun]

In summary, there are 3 ways to guide a cast bullet into the rifling:

1) a bore riding nose.

2) the Ardito / Eagan system of a long tapered bullet jammed against a long tapered leade

3) a snug slip-fit freebore

These 3 methods can be used alone, or can be combined.

Each method has been proven to work satisfactorily under certain circumstances, but not necessarily under all circumstances.

None of this is new information, yet I've never seen it summed up before.

[mtngun]

Harold Vaughn on Rifle Throats

I thought I had posted a review of Harold's book "Rifle Accuracy Facts" but a search turns up nothing. Oh well, the short version of my review is that I enjoyed reading it because it was thought provoking, I admire the hell out of Harold for taking a scientific approach to shooting and wish more people would do the same, his personality is easy to poke fun of because he comes across as arrogant and hardheaded, and I question many of his conclusions.

Anyway Harold generally preferred a 0.75 degree per side leade angle. Why? Apparently because most factory rifles have a 1.5 degree angle and he figured that dividing by two would make it twice as good. Seriously, he admits he has no proof that 0.75 degrees is better:

The 270 test rifle used in this book has a Douglas Premium barrel with a throat half cone angle of 0.75 degrees, which is about half that of a factory throat. This should ease the entry of the bullet into the throat and reduce bullet deformation. The only proof of this is that this rifle starts out being a little more accurate than a normal factory rifle, and the shallow throat angle plus a chamber centered on the bore axis are the main difference.

However, he also said that he preferred a 2.5 degree leade in his 6mm benchrest gun because "the short bullets seat too far out in the case with a shallow angle throat." I can go along with that logic -- it makes sense to match the length of the leade to the length of the bullet.

To sum things up, Harold went through the same though process about throats that I have been going through, and came to similar conclusions, and admitted that he wasn't really sure.

[mtngun]

FYI here is one of the more popular 6mm PPC chambers used in jacketed benchrest. Of interest to this thread is the freebore at only 0.060" long and the 1.5 degree per side leade.

The diameter of the PPC's abbreviated freebore is only 0.0003" larger than the bullet. I guess you can get away with that tolerance with jacketed bullets but doubt it would work with cast due to diameter variation.

Anyway, this concept of a short snug freebore followed by a 1.5 degree leade is something that I tried in my Contender 357 ..... yet it shot poorly. Maybe cast bullets require a different throat than jacketed bullets? I'm still trying to figure that out.

[mtngun]

SNUG SLIP-FIT FREEBORE ?

I'm currently testing a snug slip-fit freebore in my Contender 357 (jury is still out on it). The concept has been around for a while but only a few people, like Veral Smith, have advocated its use for cast bullets.

Here's the SAAMI chamber for the 300 Weatherby Mag, calling for a 0.3084" diameter x 0.35" long freebore, followed by a one degree per side leade.

I was surprised that the Weatherby freebore calls for only 0.0004" clearance. Cast bullets seem to need more clearance due to their imperfect dimensions and residual bullet lube.