Acceptable tolerance for transforming cartrige cases

M

mapsjanhere

New member
This came up in a thread in the Smithy, but I think it's more relevant here. People have been making .410 brass out of 303 British and 9.3x74R for a long time, despite both cases being about 1/100" narrower than spec for the 410 at the base. Now, you're converting rifle brass into a low pressure shotgun cartridge, so that's probably not an issue. But if you'd be making rifle brass out of rifle cases, what's the acceptable tolerance in picking a donor case?
 
But if you'd be making rifle brass out of rifle cases, what's the acceptable tolerance in picking a donor case
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If I am involved I want a case that is too long. I form 7.7 Japanese cases from 30/06 cases. There is a small difference in case head diameter. Everything else has a parent case.

Then there is the 280 Remington case, my favorite, when it comes to fitting the chamber I can not miss.

F. Guffey
 
If you look at SAAMI drawings, the diameter tolerance for rifle cartridges is almost always minus 0.008", and the diameter tolerance for rifle chambers is almost always plus 0.002". The minimum chamber usually has at least half a thousandth of clearance over a maximum cartridge case, but at the breech is often nearly a whole thousandth larger than a maximum case. This adds up to tell me that nobody in the ammunition manufacturing industry thinks there is a safety problem with a case fit into a chamber that is sloppy by as much as 0.011".

In reality, you can often get away with more slop than that, but how much more depends on the individual cases and the pressures you are actually reaching, so I'm not going to put a number on it.
 
I made 6.5 Japanese brass from .303 British brass years ago. It took some lathe work for the extractor groove and rim. The important thing is: Split the brass the long way and see if the head and web are about the same between the two cases. As already mentioned, the configuration of the brass can be different because the pressure varies between different cartridges. Check thickness all along the whole split piece. Thicker brass up near the neck can get you in trouble also. If you take the time to check everything, you should have no trouble.
 
Sometimes there will be quite a bit of difference and you will want to "fire form" the cases to the chamber.
 
Sometimes there will be quite a bit of difference and you will want to "fire form" the cases to the chamber.
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what's the acceptable tolerance in picking a donor case?
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Acceptable donor case? or: Selecting a chamber first then match with a parent case. Then there is 'fire forming'. I form first then fire. I do not have that problem shared by other reloaders, my firing pins crush the primer before the shoulder of the case collides with the shoulder of the chamber.

There are a number of threads on the Internet that start with "How could this happen". When selecting a chamber, select an action, get to know/understand the design/function.

F. Guffey
 
I think the OP wants to know what brass is safe to form into other cases, not how to soup up his primer detonation. A lot of guys use an old Mark III receiver and just thread and drill a piece of 6" barstock for rimmed cases. Just rent a reamer and use a jam nut with the bar like a Savage barrel and bolt to the bench. Heck, I used to do that with 8" bars and loaded ammo to check reamers I made. I used an Arisaka receiver for the rimless cases to fireform.
 
That's likely cool with most pistol pressures, but if you try to make high pressure rifle cases, you likely want the temper of the brass bar stock a bit on the hard side because you won't have the same work hardening that forming the cases from discs or slugs provides, and that affects its strength as a pressure seal. In many cases it wouldn't be a real issue unless you went into the proof pressure range, but why err on the weak side when tempered stock is available.
 
I use .223 Remington to form .221 Fireball cases. It works just fine, though the cases are thicker, and tend to crack at the case mouth quicker. They work.


I also have used .38 Special brass to make .35 WCF though the harder part of that was lathing the rims down. While time consuming it works just fine.
 
What are you talking about? Who said anything about using brass bars in place of a barrel? I used steel bar ends from a shop I worked at. You bolt the receiver to a bench and you can interchange the bar ends if you thread it like a Savage rifle set-up. It pretty much sets up like standard forming dies.
 
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