the thread on the differences between the 458 Win & Lott got me to wondering about a rebarrelled 458 win that I have. the barrelmaker has passed away so is not available. Is there any way short of a chamber cast that would extend into the rifling for me to see what specs were used with the chambering?

Ray B,

A chamber cast ... or ...

Read this and you will be able to figure it out, close enough for reloading purposes:

From the ".375 H&H reloading help" thread, where I got the idea from member eagle27:

quote:

Originally posted by RIP:

quote:

Originally posted by eagle27:
Problem with using a dummy round to determine the chamber/throat length is that the ejector, if it is in the bolt face, or the extractor slipping over the case rim into the extractor groove both create feel as you attempt to close the bolt on the dummy round.

The better way is to close the bolt on an empty chamber and put a cleaning rod down the barrel until it touches the bolt face then mark the rod at the muzzle (ballpoint pen up against the muzzle). Remove the bolt and then insert a bullet into the chamber and push it to stop at the rifling using an empty case, finger, pen etc. Again let the rod touch the bullet nose and mark rod at the muzzle. The distance measured between the marks on the cleaning rod is the maximum overall length, COAL, of a loaded cartridge with the bullet lightly touching the rifling.

Deduct Barnes' 'off the lands' measure from the COAL and load your rounds to the "Barnes" COAL. Of course as mentioned by Trademark, your magazine length may dictate your COAL.

eagle27,

Thanks for goading me into trying that.
I use a cold-rolled 3/8" steel rod in the .458-caliber rifles.
I hold a fine-point Sharpie marker against the rod, precisely at the muzzle crown, and spin the rod to mark it.
Then I cover the mark with transparent tape to preserve it, prevent smudging.



When the rod is marked twice, the data is one number: The distance between the two marks.

If the bullet-caliber slug is flat on the end going into the rifling,
then that one number is the "Throat Jump" of a bullet going from maximum length brass to the bullet-caliber diameter of the throat.

If there is any nose projection of slug/bullet ogive beyond the full-caliber diameter of throat,
then that projection must be measured and subtracted from the distance between the two marks on the rod.

I measure the spacing of the two rod marks on the muzzleward side of each mark, as precisely as possible with calipers.
Not with a ruler vaguely somewhere in the middle of each mark.

Take care and you can get some good results using this method,
repeatable and reproducible for me within +/- 0.005",
plenty good enough for my purposes.

Thanks for the idea.

Here is another method more simple-minded that might be OK for small bores and short throats:



http://bulletin.accurateshoote...ngth-to-lands-gauge/

Be sure the action is cocked so there is no firing pin protrusion when the rod is inserted against breech bolt face.

With a CZ .458 Lott having CIP-2002 throat, it will be impossible to find a bullet long enough to use in the "split-case-neck method."
But a precision-turned brass rod of .458"-diameter with a square/flat base, and about 3" long could be used as the "bullet" in the split-neck-case.

Rip ...

I don't understand the statement about feeling the extractor slipping over the rim messing up the measurement.

Put the rear of the dummy cartridge in the mag box opening and slide it under the extractor as you feed it into the chamber.

I pretty much use the same method for determining max OAL; kinda crude but it works well. I do, however, remove the extractor and ejector for a better feel. After all, that leade angle is only about 1.5 degrees. Also, remove the firing pin assembly for a better feel.

Jim,
I understand why the extractor/ejector/firing pin assembly might be removed for the ultimate sensitivity and resolving power in measurement.

But not at all necessary for the crude measurements needed to get a guesstimate on whether the .458 Win.Mag. is anywhere close to SAAMI minimum.
The biggest source of error can be eliminated by making sure the action is cocked to eliminate any firing pin protrusion at the breech bolt face.

I made a mess of the above "Slug & Rod" method, have tried editing to clarify.

I have since found a much better presentation of the method:

I have another book review.
Excerpt below is for book review purposes of this book:



Book Review: Good book.

I wanted to learn to paper-patch for a BPCR.
I ordered this book and found the genius, artist, author
Randolph S. Wright
has presented what I call the "Slug & Rod" method in a more refined manner:





The measurement he arrives at is the COaL for the specific bullet being used to be touching the lands when chambered.
This same measurement yields what I call the "Throat Jump" from case mouth for any bullet,
if you subtract the ogive length of the bullet nose and also the brass length of the case.

The SAAMI "Throat Jump" for the .458 Win.Mag. is a minimum of 0.6725".
I have arrived at this number by measuring from SAAMI minimum chamber specs plus SAAMI maximum brass specs.

Any throat shorter than that should have the rifle marked as ".458 Winchester Magnum Special" for safety reasons.
Any .458 Win.Mag. throat longer than that may just be called "slop" or "exceeds safe minimum/compliant with SAAMI minimum."

Hornady's (formerly Stoney Point) Overall Length gauge contraption is of course just a variation on this old technique.
Some may like that contraption better.
But they will have trouble finding the ends of some .458 Lott and .458 Win.Mag. throats out there,
by that method,
unless you have a special slug made up to go with the modified case needed for each cartridge chambering being measured.

With any old slug and any old rod longer than the rifle barrel, you can measure any throat with the "Slug & Rod" method.

Rip ...

Sorry to be pissing in your corn flakes here RIP but you are not answering or measuring the question at hand !

The Question was how to measure leade / and rifling angle ???

I presume what is meant is leade angle ( rifling angle can be calculated accurately if twist and true barrel bore and groove diameters are known )

You guys are measuring COL and throat dimension albeit not accurately !

The reason being that the crude measurement using a dowel differs when tangent or secant ogived bullets are used. This means two different measurements for something with a single set dimension.

Just so that we are clear on the matter; the throat is that part of the barrel which is the sum of the freebore and the leade.

Freebore starts at the neck step and stops where at the datum point where rifling commences.

The leade being that measurement where the grooves start to the point where the land diameter reaches full bore. The angle described between these two points referred to a leade angle ( not to be confused with rifling angle)

quote:

Originally posted by Ray B:
the thread on the differences between the 458 Win & Lott got me to wondering about a rebarrelled 458 win that I have. the barrelmaker has passed away so is not available. Is there any way short of a chamber cast that would extend into the rifling for me to see what specs were used with the chambering?

Alf,

Merry Christmas and happy new year!

Thanks for posting the nice presentation of the essential terminology.
A distinction between "parallel-sided-free-bore"
versus "leade-only-free-bore" could be added.
Both contribute to an "effective-length-free-bore" as a finer distinction.
Also one ought to point out that the usual plus-tolerance of chamber length from breech bolt face beyond maximum brass length usually amounts to about +0.015"
(though it is +0.020" in the .458 Win.Mag.),
which should also be added to the "effective-length-free-bore."
Also, the "chamfer-down" at the chamber-neck-2 is usually at a 45-degree angle, and adds a little more length to the "effective-length -free-bore."

The angle of the dangle leade is not the question.
The COaL with a secant or tangent ogive bullet is not the question.

A basic idea about what throating the dead gunsmith used is the question.

Practical is what is needed,
a basic ascertainment that it is not some sort of ".458 Winchester Magnum Special,"
to know if SAAMI factory ammo can be used in it with SAAMI level results in pressure and velocity.



From the above drawing to SAAMI minimum chamber specs and SAAMI maximum brass length,
we see that the "Throat Jump" of a .458-caliber bullet from case mouth to the start of .458" diameter in the leade is:

3.1725" - 2.500" = 0.6725"

I performed the "Slug & Rod" method on my Super Grade .458 Win.Mag. M70 from South Carolina.
I got a "Throat Jump" of 0.675".
That is the tightest-throated .458 Win.Mag. I own.
From this I surmise it was probably made with the SAAMI-spec chamber reamer.
And I surmise that it probably exceeds the SAAMI minimum by only a tiny amount.
Hey, it is "IN SPEC."
Or it is so close that my error (whatever that amount is) means it is "right on" for SAAMI, best I can tell.

Restated:

Throat Jump (X) =
Cartridge Overall Length with bullet touching lands (Q)
- (subtract)
maximum brass length of cartridge (B)
- (subtract)
length (Z) of the nose (ogive) that is less than groove diameter (.458-caliber for a .458-grooved barrel) regardless of tangent or secant ogive type.

Thus:

X = Q - B - Z
or
X = Q - (B + Z)

Just a slug pushed into the rifling of throat and held there with a pencil, plus a cleaning rod and a Sharpie or another pencil:
See Randolph S. Wright instructions.
Maybe Alf will get the correct idea too?

Rip ...

RIP:

Just wondering out loud

Why the insistence on the "SAAMI" spec when the CIP spec is simply what the "Americans" submitted to CIP as the spec ? CIP does not make or set the dimensional Spec the owner of the cartridge sets the dimensional spec.

We further see that Federal, Hornady, Barnes and Remington all submitted their ammo for homologation with CIP ie meeting the accepted CIP dimensional specs ?

Alf,

SAAMI and CIP chamber specs for the .458 Win.Mag. are identical, and always have been, since 1956 American origin.
The 2002 CIP drawings that I have agree with the 2015 SAAMI drawings for the .458 Win.Mag.
Not so for the .458 Lott.

Pressure specs are different however.

2015 SAAMI MAPs:
.458 Win.Mag.: 60,000 PSI
.458 Lott: 62,500 PSI

2002 CIP MAPs:
.458 Win.Mag. and .458 Lott are identical: 4300 bar = 62,350 PSI

If you have a more recent version of the CIP .458 Lott chamber specs,
does it now agree with the 2015 SAAMI .458 Lott?
That is shown in my pencil-on-graph-paper sketch above.

2015 SAAMI Throat Jump for .458 Win.Mag.: 0.6725"
2015 SAAMI Throat Jump fpr .458 Lott: 0.2143"

2002 CIP Throat Jump for .458 Win.Mag.: 0.6725"
2002 CIP Throat Jump for .458 Lott: 0.6725"

I will be googling to see if I can find this:

What is the latest CIP spec for the .458 Lott?
By the 2002 CIP spec it had the same throat as the .458 Win.Mag.

Rip ...

And besides, the USA is not a "Member State" of CIP.
Homologatation versus homophobia: Makes no difference to me.
I reckon the big US ammo makers are forced to homologate their ammo through CIP if they want to sell it in the 14 Member States of CIP.
I would still trust the originating SAAMI spec for the .458 Win.Mag. over the CIP spec: They are the same except for pressures.
Anyway, my .458 Win.Mag. ammo is loaded to RIP spec.

2006 update of CIP switched to the shorter-throated version of the .458 Lott,
and also changed the twist rate to 1:10" from previous 1:14" standard.

http://www.cip-bobp.org/home

http://www.cip-bobp.org/homolo...&cartridge_type_id=3

http://kwk.us/pressures.html

The pressure specs are different because the measuring system is different ! This is not absolute pressure ! Its the pressure read based on the instrument used and SAAMI and CIP utilize different systems ! Conformal vs a channel or"instream" gauging. The latter requires drilling of the case so that the gauge sits in the powder chamber.

Anyway all is good just wondering out loud

"The CIP listings are more extensive. SAAMI standards are voluntary, and a small custom gun maker need not use an official proof load before selling a gun chambered for some obscure cartridge. European laws probably require every gun be proofed to some official standard.

There is a slight difference in definitions between the CIP and SAAMI, and this may explain why the CIP numbers are generally a bit higher; see the section on Statistics, below."

http://kwk.us/pressures.html


Rip ...

The minute changes in throating, free-bore diameters and leade angles cause relatively huge changes in effective length of that throat.
That is why the relatively crude measurement by "Slug & Rod" is so useful in immediately discerning the difference between a SAAMI or CIP minimum throat from
a shorter "special throat."
Also it can easily tell a CIP-2002 .458 Lott throat from a CIP-2006 .458 Lott throat.
Huge difference easily detected by "Slug & Rod."
Very obvious.

Rip ...