Home Forums Hunting & Shooting Ask The Gunwriters 458 Win Mag Oversize Throat?

Why did the designers of the 458WM spec a throat that starts out .010+ larger than bullet diameter that tapers like a funnel for an inch plus to get down to land diameter? I understand freebore and "Weatherby throating" to keep pressure down, but an oversized funnel an inch long doesn't make sense. Seems like a great way to cause accuracy problems.

http://www.saami.org/PubResources/CC_Drawings/C%20and%20C%20Dwgs%20-%20TOC%20-%20Rifle.pdf

The .458 is one of the most inherrently accurate cartridges out there. As to why the throat is so long, ownership for that distinction has not been claimed by any individual.

John

I don't know why either, but I suspect that is the reason I've never been able to get good results with cast bullets in any of the 4 commercial .458's I have owned. Cast bullets have always keyholed from my rifles. I tried every combination of bullet (from 300 to 550 grains, both GC and PB), alloy (hard, soft, straight Linotype, wheelweights, Lyman #2 etc.) and powder (from Unique to H4831) I could think of with no success in 3 Model 70's (Post '64) and one Model 700.

Jacketed bullets of 350 grains and up have always shot very well in all of them. I think that the cast bullet must be stripping the rifling after making the long jump into the relatively fast twist (compared to most .45/70's, which have always shot well for me with the same bullets).

I did get good accuracy with cast bullets years ago in a custom Mauser that apparently had a standard type throat. But that rifle liked to burp the entire contents of the magazine into the air whenever the bolt was opened quickly.

I suspect that it is designed that way for pressure control reasons, assuming the rifles will be used in a tropical climate. (?)

My 458 is very accurate with longer bearing surface bullets, but accuracy degrades with shorter bullets. 300 to 400 grain conventional bullets groups open up significantly. Have heard of more than a few with the same problem. Just in front of the cartridge case the throat minimum diameter is .4690" by SAAMI drawing. Understand the pressure problems trying to get performance out of the 3.340" oal and why it would need freebore, but a ten thou oversized funnel that would leak pressure around the bullet (especially shorter ones) doesn't make much sense to me.

Wild- I am also having problems with cast bullet accuracy.

The tapered throat is quite common, even though parallel throats tend to be more accurate.

The way I understand it, tapered throats evolved with black powder rounds as a way to allow rifles to keep shooting accurately a little longer before powder fouling built up in the throat.

They also served the same function with many smokeless powders that also fouled more than many of today's powders--which was of course complicated by shooters who didn't clean their rifles very often. This is also one of the reasons for the wide-spread belief that even the barrels on smokeless-powder rifles must be brush-cleaned frequently to maintain accuracy.

Tapered throats don't work as well with shorter bullets, or bullets seated very far off the lands. Today the trend is toward parallel throats, in part thanks to cleaner-burning powders.

Thanks JB, that explains a lot. Have found that a 350gn round nose is well clear of the brass before engaging the rifling which does little for group size. Guess my best bet is to have my WM (CZ550) reamed out to Lott or Express to get rid of the widest part of the throat for better accuracy with lighter bullets.

There is a short article on "throating" in one of the old "Percision Shooting" annuals that Dave Brennen used to put out when he had more submitted than he could publish in the regular magazine.

One of the items discussed was by Keith Francis,who was then at JGS the reamer folks still in Oregon. In the article he stated that he had modified the SAMI 458 WM throat in such a manner to eliminate much of its oversized throat to something more sensible, and he maintained it much improved accuracy.

When talking to Sarah Humbert at JGS a year or so ago I asked if they still had the "figures" for it. She told me they not only had them, but still stocked it, and called it the 458JGS Mag or something like that. If anyone is thinking of putting together a 458, I'd suggest it would well be worth getting that reamer as speced by old Keith. From what I'd read in various places he really seemed to know his stuff, and you'd know it didn't produce high pressures with standard factory loads.

In the same article Dave Manson, then still with Clymer, and not on his own, described how he designed a custom reamer from a seated sample round with a specific bullet.

This should all be right up the "Rifle Looney's" alley, and grist for their mills.

Anyhow, if I ever put together a 458 for "big stuff" I'd use the 458JGSMAG reamer. If I make up 458 for cast bullets for myself I know where there is a slightly used standard SAAMI reamer. I think I would buy it, and have someone remove the total throat, and the throat with a separate throater with a 0.001 over bullet diameter and a 3 degree included angle to a sample round.

Throating does make a great difference in some cases. So it seems.

If you look at SAAMi specs it seems that the older the cartridge the bigger the specs. I.E. look at the difference between the 7X57, the 280, and the 7mm-08. The old 7mm has a neck like a funnel. If I was doing much custom work I'd have a remer made with a neck and throat sized like the 7mm-08, or the neck made sized per the 7mm-08, and throat with my separate seater to a sample round.

I would respectfully suggest shooting and handloading for a .458 long before taking action on the written word.

I do not know how many .458's I have reviewed, loaded for, or owned and I never ever saw one that was anything but accurate.

The written word is often frivalous, and provides incorrect information becuase of limited or immediate experience. A few years back there was a writer who openly stated that .458's had poor barrel life of only around 600 rounds. Because he thinks he saw it in two rifles - key point being the same brand and model for him it was axiomatic that it would therefore ensure in all .458 rifles. He did not consider that he had lousy barrels from a single manufacturer.

It is extremely common for .458's to shoot half inch clover leaf groups at 50 yards with open sights. It is common for them to shoot different bullets weights to a differing vertical point because of variences in bearing surface, velocity and pressures.

It is common for these rifles to shoot flat enough with Barnes, Woodleigh and Swift pointed bullets to take 400 yard shots on deer sized game. They are not, read that NOT, just short range stopping rifles.

I know this to be true because I have done all these things with a variety of .458 rifles with barrels as short as 22 inches and as long as 25 inches.

I own a .458 Model 70 that had previously been worked over by a very good gunsmith. The action is the slickest I ever used. The feeding as perfect as I ever saw on a staggered round magazine. The trigger breaks like a glass rod and the finish has been electroless nickel plated to protect it in severe weather conditions. It has a bedded plastic stock and a juicy piece of walnut in a Supergrade stock.

Topped off with open sights that put 3 X 550grain Woodleighs 1 inch high, inside 1 inch at 50 yards and a 2-7 Leupold that will send a 350gn TSX anywhere you want with absolute authority over what it hits, I know what a .458 can do. All this and with standard reamers in factory rifles.

The .458 pleases me.

Put enough throat and freebore in a 458 Win. barrel and you've got a smoothbore. Alter the feed ramps and bolt face and you can shoot 410 guage shotgun shells!

Some cast bullet voodoo: The case mouths need belled and expanded to fit larger diameter cast bullets. The bullets need to fit the throat snug (but easily chamber), just like filling revolver throats, otherwise a softer alloy needs used to slug up to prevent the bullet tipping enough to strip (it causes leading and inaccuracy).

Short bearing surface bullets in large throats magnify the tipping and off-center entry into the rifling = bad ju-ju.
Making contact with the lands, and at least the tapered throat, will prevent the tipping/stripping/mashing of cast bullets, each of which effects the bullet balance, hence accuracy.

I must have lucked out and gotten a 458 cut with a new (max tolerence) reamer because most bullets under 450gn are completely out of the case neck before hitting the rifling. Think you are correct that they tip in the throat causing accuracy problems.

Have you tried longer (monolithic) 350 grain bullets like the TSX? They seem a lot longer than, say, Hornady round nosed 350s.

The problem I had with 350s was that POI was 7" different from 450s at 100 yards when loaded to full velocity, about 2750 fps.

And your jacketed bullets are going to also be much smaller in diameter, magnifying the problem.

Its not so much the length jump as it is the diameter of that tapered cone...Revolvers live on the long bullet jump and can print 1 inch groups at 100 yards. Throw an undersized bullet in a giant throat and hitting a 5 gallon bucket won't happen, because they can engage the rifling off center, imbalancing the bullet.

Mule Deer's use of a Juenke machine illustrates how imbalanced lead-cores do not shoot as well. A tipped, off-center engagement with the rifling effectively does this too, even to mono bullets.

Weatherby rifles aren't supposed to shoot with their freebore....but most of their ball-seat throats are not much larger in diameter than the bullets, so they still point straight, are guided straight and still shoot very well.

The JGS reamer sounds interesting. Took a look and PTG as well as JGS carry the 458WM-JGS reamer, so there must be a desire for the tighter throat. Wonder how pressure runs with factory ammo?

Have tried 350gn TSX in my 458 and they are too short even though a monometal, but my rifle loves the 450gn TSX.

Have tried a few 450-500gn cast bullets with no luck. Many seem to have reduced diameter ahead of the grease grooves, so they"re out of the case neck before being stabilised in the throat.