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More thoughts on fitting a bullet to the throat

Posted: Sun Jan 04, 2015 7:50 pm
by mtngun
Some thoughts on various throat fits.
GLOVE FIT: taper of bullet matches taper of throat/leade. In theory this is the holy grail of cast bullet fits. It is sometimes successfully used by benchrest shooters, i.e., the Ardito throat. However, I find it is a problematic fit. As any machinist who works with tapered collets knows, the taper-within-a-taper is either locked up solid or else it is loose, there is no in between.

Benchrest shooters get around this problem by having light neck tension, seating the bullet long, and letting the rifle finish seating the bullet as the round chambers. If you then try to extract the unfired round, the bullet is stuck in the throat. Neither the light neck tension nor the stuck bullets are practical for any sort of real life shooting.

BULLET TAPER LESS THAN THROAT TAPER: this is what I attempted to do with my 35 caliber 0.8 degree taper bullet in a 1 degree taper throat. The slight mis-match ensures that the front band will be the first to make contact with the rifling.

BULLET TAPER GREATER THAN THROAT TAPER: this ensures that the first full diameter band will be the first to make contact with the throat, while the tapered bands will not make contact.

2-DIAMETER (BORE RIDING): If the throat angle is steep, as is common on many SAAMI chambers, the bore rider will probably be first to make contact with the rifling. But if the throat angle is shallow, the front band may make contact with the throat before the bore rider makes contact with the rifling.

FREEBORE: if the rifle has a long freebore, it may be better to have a one diameter bullet to fit the freebore, snuggling up to the rifling, and not worry so much about fitting the leade. I've heard that Veral Smith prefers this type of throat?

If the freebore is so long that it is not possible to snuggle the bullet against the rifling -- typical on many dangerous game rifles -- you may actually have better luck with a 2-diameter bullet, even though the bullet will have to jump farther to contact the rifling. This seems contradictory, but it may be that the 2-diameter bullet distorts less when it slams into the rifling.

I consider myself a student of this subject, still learning stuff every day. There is no substitute for trying different things and seeing what works or what doesn't work. Conventional wisdom is sometimes wrong -- for example, I've found that it's a royal pain to achieve a consistent fit with "glove fit" bullets, and they don't necessarily shoot any better than other types of fits. I'm still in search of the perfect bullet and throat combination. :)

Re: More thoughts on fitting a bullet to the throat

Posted: Sat May 16, 2015 6:26 pm
by mtngun
Yet another possibility that I didn't think of before -- lately I have been using "Loverins" (I use the term loosely to refer to bullets that have a long body, a short nose, and lots of little bands and grooves) that have a step down between the front band and the ogive. Usually the bullet is sized 0.002" or so larger than groove diameter.

Problem is, the groove+0.002" sized front band hits the tapered freebore before it can reach the rifling (see drawing below). It's simply not physically possible for a full diameter front band to be engraved by the rifling when chambered.

When I was last shooting Loverins in a factory 30-06, I got around that limitation by making the front band drop out slightly undersize, say 0.307", while the rest of the bullet was sized 0.314".

The original Guy Loverin designs had a taper on the top half of the bullet, which probably allowed the bullet to be seated out enough to engrave.

Alternatively I could size the nose in a taper-less-than-leade-taper die (see previous post for a picture) to accomplish the same purpose. I tried that once or twice and didn't have much luck with it at the time, but maybe I should try it again?

Re: More thoughts on fitting a bullet to the throat

Posted: Sun May 17, 2015 6:59 pm
by mtngun
To the best of my understanding, this is a typical "Ardito throat," named after benchrest competitor John Ardito.

Ardito loaded a bullet that had the same taper as the throat. He may have nose-sized the bullet in a die cut with the same reamer as the throat. He seated the bullet so that only the gas check was in the neck, with light neck tension, then let the rifle finish seating the bullet as the cartridge was chambered (that would only work in bolt actions, not single shots). No one doubts that Ardito's technique can shoot well because he proved it in CBA competition.

As with many benchrest loading techniques, if you attempted to extract an unfired cartridge, the bullet was apt to stick in the barrel, so it's not really practical for real life. :(

To sum things up, the Ardito throat may have some special qualifications:
-- it may require a bolt action that can jam the bullet into the throat.
-- it may require a fairly long bullet.
-- it may require minimal neck tension.
-- you may not be able to extract an unfired cartridge without the bullet getting stuck in the barrel.

I tried what was essentially an Arditio throat in my Contender 357 max barrel, with a 1/2 degree leade that began immediately after the case neck. It shot poorly and I abandoned it. A Contender does not like to jam a bullet into the throat, I was using lots of neck tension, and I don't want to shoot a cartridge that can't be extracted safely.

My "bad" Contender 357 chamber had what was essentially a variation on an Ardito throat, with the tapered leade beginning immediately after the chamber neck at a sloppy 0.363"+, and a leade angle of 1 degree per side (if I remember correctly.) Using a single diameter bullet, it shot better than any other 357 throat that I have tried so far.

So I may continue to experiment with Ardito throats, but I'll probably use a 1 to 2 degree leade, and I won't jam the bullet into the rifling.