Jim Watson
Member
And Tokyo Arsenal is not a member, so any new 6.5 Arisaka is reverse engineered from 70 year old enemy gear.
Using a rimmed cartridge headspace gauge
- Thread starter MrMagumba
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What most people call "headspace" has another factor, sometimes called case support. A case must be supported in some way against the firing pin blow and against being pushed too far into the chamber. Support by the case shoulder is not really a very good way of doing that; it was adopted only because a rim interferes with feeding in a repeating or auto weapon. Contrary to being precise, it is very imprecise, with a lot of places to go wrong in rifle chambering and cartridge manuacture.
In some cases, the shoulder is not at all good, being either too small or at too shallow an angle to be effective. There are plenty of examples; H&H developed the belt not to strengthen the cartridge base, but because the small and shallow shoulder of the .375 rimless was simply too small. The same is true with cartridges like the .35 Whelan and even the .35 Remington, which can have misfires if the chamber and the ammo are not quite perfect.
A semi-rim, like a belt, is a compromise between the good support of a rim and the smooth feeding of a rimless (cannelured) case.
Jim
In some cases, the shoulder is not at all good, being either too small or at too shallow an angle to be effective. There are plenty of examples; H&H developed the belt not to strengthen the cartridge base, but because the small and shallow shoulder of the .375 rimless was simply too small. The same is true with cartridges like the .35 Whelan and even the .35 Remington, which can have misfires if the chamber and the ammo are not quite perfect.
A semi-rim, like a belt, is a compromise between the good support of a rim and the smooth feeding of a rimless (cannelured) case.
Jim
Very interesting analysis. I have had misfires in the 35 Whelen and I believe it was due to the slight shoulder. I sized my cases to maximum cartridge headspace. When I close the bolt, there is no movement as the case is an exact fit to the chamber. I don't like doing this as I normally want a bit of clearance.
Thought I replied to some of this but must have got lost. Interesting stuff here. Thanks Jim K for the reamer pics and explanation of how they are used. The smith should be able to leave the cut just short enough to keep the case set back far enough to be in the bolt face recess. Jim Watson- good idea on cutting the chamber for my ammo. I'll plan on providing resized brass and factory ammo to make sure it all works. Boom Boom- thanks for the reamer source, and the light recoil is a major requirement for this project, so I'm sticking with the 6.5.
With permission, a bit more. The original reason for the shoulder was the result of two advances happening at once, around 1885-90. The first was the development of smokeless powder. Smokeless powder was more powerful than black powder and allowed a significant increase in bullet velocity. But when designers tried to speed up lead bullets, they found the bullets just stripped in the rifling; bores leaded badly and accuracy was poor. The French solved the problem by using solid bronze bullets but that was expensive, production was slow, and the bullets were lighter than ideally they should be. The Swiss (I think) came up with the idea of a lead bullet with a jacket of copper, brass or mild steel. That solved the problems of leading, but required harder and more expensive barrel steel, not to mention stronger steel to stand up to the greater pressure of smokeless powder.
But even with smokeless powder, bullet velocity was limited and efficiency poor because of bullet weight. So they wanted to reduce the bullet weight to increase velocity. And that meant a smaller diameter bullet, about 7-8mm seemed pretty good but some people thought 6 or 6.5 mm, even as small as 5mm, would be better, and lighten the soldier's load.
But if the cartridge was straight, as most black powder rounds were, with a slimmer case they couldn't get in enough powder. So, they took an old idea, the stepped down cartridge, which had already been used in some Winchester cartridges for another reason - to increase the powder charge while keeping the cartridge short enough to work through the old Henry action.
So we had the necked cartridge with rounds like the .303 British and the .30-40 Krag. But it still had a rim, and while the rim had advantages, it did get in the way in rapid volley fire, then a military necessity. The Germans came up with the answer in the form of a cannelure, or "channel" around the base of the cartridge that would allow the fired case to be extracted without that bothersome rim. But then how to support the case? The answer was obvious; use the shoulder, and that is what they did.
So, contrary to what some seem to think, the modern necked, rimless cartridge was not handed down from some gunners' heaven. It was the product of a lot of research and work and testing. What is the next step? What will those working in ordnance research today come up with? Probably not anything as radical as the modern cartridge was in the 1890's, or metallic cartridges were in the 1850's, but there is something out there waiting to interest and surprise us tomorrow.
Jim
But even with smokeless powder, bullet velocity was limited and efficiency poor because of bullet weight. So they wanted to reduce the bullet weight to increase velocity. And that meant a smaller diameter bullet, about 7-8mm seemed pretty good but some people thought 6 or 6.5 mm, even as small as 5mm, would be better, and lighten the soldier's load.
But if the cartridge was straight, as most black powder rounds were, with a slimmer case they couldn't get in enough powder. So, they took an old idea, the stepped down cartridge, which had already been used in some Winchester cartridges for another reason - to increase the powder charge while keeping the cartridge short enough to work through the old Henry action.
So we had the necked cartridge with rounds like the .303 British and the .30-40 Krag. But it still had a rim, and while the rim had advantages, it did get in the way in rapid volley fire, then a military necessity. The Germans came up with the answer in the form of a cannelure, or "channel" around the base of the cartridge that would allow the fired case to be extracted without that bothersome rim. But then how to support the case? The answer was obvious; use the shoulder, and that is what they did.
So, contrary to what some seem to think, the modern necked, rimless cartridge was not handed down from some gunners' heaven. It was the product of a lot of research and work and testing. What is the next step? What will those working in ordnance research today come up with? Probably not anything as radical as the modern cartridge was in the 1890's, or metallic cartridges were in the 1850's, but there is something out there waiting to interest and surprise us tomorrow.
Jim
Clark
Member
Yes, Jim K seems to be able to think.
My father went to Rock Island Arsenal to pitch this patent in the early 70s
https://www.google.com/patents/US3901123
It would keep a machine gun operator on top of a vehicle inside a turret.
But they were obsessed a the time with Stoner's caseless cartridge idea.
A little later my father's corp hired Stoner's little company at Niagara Falls to develop one of my father's cam operated large machine gun designs, so they were one the same team then. Stoner was concurrently working on a gas large machine gun.
None of the 4 above mentioned ideas made money.
What did make money for my father was the M55, M107, and M110 as the result of responding to a request for quote [RFQ] from Detroit arsenal. The arsenal will tell you what gun, what diesel engine, and what transmission to design into it.
When I started designing on weapon systems a generation later, it was deja vu all over again.
The military has the money for new weapons and they don't want any suppliers doing any thinking. It is one giant Dilbert cartoon. It is what happens when those in charge are not as smart as those they are bossing around.
The technical term here is "case support" or "cartridge support." That is what stops a cartridge and holds it against the blow of the firing pin. In rimmed or semi-rimmed cartridges, it is the rim. In a rimless rifle cartridge like the 8mm Mauser, it is the shoulder. In some pistol and a few rifle cartridges, it is the case mouth. In a belted cartridge it is the belt. In all cases, the headspace is measured from that stopping point to the face of the breech.
So why? Why not just use the shoulder? In the early days of rimless cartridges, designers didn't fully trust the case shoulder or the case mouth to provide proper support since cartridge manufacture was still something of a "by guess and by God" affair. (Browning had problems with case support of pistol cartridges and his earlier cartridges all have semi-rims.)
In the case of the 6.5x50 Japanese, the cartridge is supported by the rim, not the shoulder, so the shoulder is, to some extent, irrelevant.
So the headspace of the 6.5x50 is the thickness of the rim, just as in a rimmed cartridge. Of course, the chamber is made so its shoulder will provide support to the shoulder of the cartridge, but the case does not "headspace" on its shoulder but on its rim.
Jim
So why? Why not just use the shoulder? In the early days of rimless cartridges, designers didn't fully trust the case shoulder or the case mouth to provide proper support since cartridge manufacture was still something of a "by guess and by God" affair. (Browning had problems with case support of pistol cartridges and his earlier cartridges all have semi-rims.)
In the case of the 6.5x50 Japanese, the cartridge is supported by the rim, not the shoulder, so the shoulder is, to some extent, irrelevant.
So the headspace of the 6.5x50 is the thickness of the rim, just as in a rimmed cartridge. Of course, the chamber is made so its shoulder will provide support to the shoulder of the cartridge, but the case does not "headspace" on its shoulder but on its rim.
Jim
Jim Watson
Member
Yet reloaders of belted cartridges are often told to ignore the belt and resize for shoulder headspace control. Case life is better. Maybe accuracy.
So why not treat a semirim the same way?
I would expect the OP to handload, he isn't a soldier who only wants to get the shot off and the gun to function for the next.
So why not treat a semirim the same way?
I would expect the OP to handload, he isn't a soldier who only wants to get the shot off and the gun to function for the next.
The belted cartridge is simply a variation of the rimmed cartridge. Contrary to the usual idea, the belt does not add strength to the cartridge head (it is too far back to do that), it simply acts as a rim for rounds which have too small a shoulder to provide case support on the shoulder (e.g., .375 H&H Magnum). In many modern "Magnum" cases, the belt really serves no purpose except to act as a rim and look impressive. Nonetheless, it is the belt, not the shoulder, that provides case support.
Jim
Jim
PRD1
Member
O.K. ...
I've read all the responses to your OP, and, unless I've missed something, no one has actually told you how a gunsmith determines the depth of chamber and/or rim recess needed to achieve proper headspace in chambering a barrel - I'll try.
There are a couple of things to do first:
1. determine how long the shank of the barrel must be - this can be done by measuring the shank of the original barrel, or, and this is the better way, by measuring the actual distance from the receiver face to the forwardmost surface of the bolt or breechblock. Allowing sufficient clearance between the butt of the barrel shank and face of the breechblock or bolt (and remembering that torquing-up the barrel will usually result in the barrel shank being 1-2 thousandths closer to the breech face than the pre-installation measurement) will give you the correct length of barrel shank. If installing a barrel with a pre-threaded shank, and/or in the case where you do not have the means of altering the shank length, or the barrel has features which will not permit altering its length (index timing, sight location, extractor cuts, etc.), you can still determine the space between the butt of the barrel and the breech face by making the same measurements and remembering the 'crush' factor.
2. knowing how much clearance there will be between the breech face and the barrel shank when the barrel is properly wound-in, you can calculate the necessary protrusion of the base of the case from the chamber mouth to achieve correct headspace - in chambering the barrel this can be measured directly with a depth micrometer. It is customary to leave the protrusion 2-3 thousandths longer than the required final depth, so that, after installation, the chamber can be carefully hand-reamed to achieve final headspace.
You need to remember that some bolt-action rifles, semi-autos, etc., have more complex requirements for shape of the barrel shank, due to the need for clearances between the protrusions on the bolt itself, the extractor, etc., while others, and many single shots, have flat shank butts ('98 Mausers, Krags, etc.), but that all of them will have a rearmost surface of the barrel shank that will serve to measure from when determining the depth of the chamber. As you will be measuring from the base of the headspace gauge to that surface, it does not matter whether the case is rimmed or not, so long as your measurements are carefully and accurately made, and you keep the relationships of the dimensional measurements you have made on the receiver and the barrel shank in mind.
Work carefully and the results will be correct.
I hope this will be helpful.
PRD1 - mhb - Mike
I've read all the responses to your OP, and, unless I've missed something, no one has actually told you how a gunsmith determines the depth of chamber and/or rim recess needed to achieve proper headspace in chambering a barrel - I'll try.
There are a couple of things to do first:
1. determine how long the shank of the barrel must be - this can be done by measuring the shank of the original barrel, or, and this is the better way, by measuring the actual distance from the receiver face to the forwardmost surface of the bolt or breechblock. Allowing sufficient clearance between the butt of the barrel shank and face of the breechblock or bolt (and remembering that torquing-up the barrel will usually result in the barrel shank being 1-2 thousandths closer to the breech face than the pre-installation measurement) will give you the correct length of barrel shank. If installing a barrel with a pre-threaded shank, and/or in the case where you do not have the means of altering the shank length, or the barrel has features which will not permit altering its length (index timing, sight location, extractor cuts, etc.), you can still determine the space between the butt of the barrel and the breech face by making the same measurements and remembering the 'crush' factor.
2. knowing how much clearance there will be between the breech face and the barrel shank when the barrel is properly wound-in, you can calculate the necessary protrusion of the base of the case from the chamber mouth to achieve correct headspace - in chambering the barrel this can be measured directly with a depth micrometer. It is customary to leave the protrusion 2-3 thousandths longer than the required final depth, so that, after installation, the chamber can be carefully hand-reamed to achieve final headspace.
You need to remember that some bolt-action rifles, semi-autos, etc., have more complex requirements for shape of the barrel shank, due to the need for clearances between the protrusions on the bolt itself, the extractor, etc., while others, and many single shots, have flat shank butts ('98 Mausers, Krags, etc.), but that all of them will have a rearmost surface of the barrel shank that will serve to measure from when determining the depth of the chamber. As you will be measuring from the base of the headspace gauge to that surface, it does not matter whether the case is rimmed or not, so long as your measurements are carefully and accurately made, and you keep the relationships of the dimensional measurements you have made on the receiver and the barrel shank in mind.
Work carefully and the results will be correct.
I hope this will be helpful.
PRD1 - mhb - Mike
Zeke/PA
Member
Let a GOOD gunsmith do it!
Pricey ? Yeah , but it's done right!
Pricey ? Yeah , but it's done right!
TRX
Member
> With 303 there is a huge range.
Commercial .45-70 varies from "can't close the bolt" thick to "OMG headspace" thin.
Commercial .45-70 varies from "can't close the bolt" thick to "OMG headspace" thin.
MrMagumba, it is possible to use a field gage or a no go-gage. There are cases that are held to the rear with a belt and or rim. In the beginning there was little concern with what happened to the case body in front of the belt when the trigger was pulled. The purpose of the case body was to seal and fill the chamber. After that came the obsession with getting cases to last forever.
I have one belted magnum chamber that is as close to head spacing on the belt and the shoulder of the chamber at the same time as possible.
F. Guffey
I have one belted magnum chamber that is as close to head spacing on the belt and the shoulder of the chamber at the same time as possible.
F. Guffey
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What receiver was used for building your Whelen? And then there is that "maximum cartridge head space", my cases do not have head space and I have fired cases in chamber with .127" clearance between the shoulder of the case and shoulder of the chamber without a failure to fire. And the shoulder on my cases did not move.
Case head support. there is case head protrusion and unsupported case head. I measure case head protrusion and I measure unsupported case head.
F. Guffey
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