Why aren't case neck dimensions listed in drawings?

[njc110381]

I've stumbled across a bit of a confusing point in the reloading of my 8x60R and wondered if anyone on here could offer advice?

 

I'm shortening and necking down 9.3x74R S&B brass to fit the chamber, which I have got on just fine with. The trouble is that I am squeezing down the neck area of the finished brass from below the shoulder point on the 9.3, so the necks are thick and when a bullet is seated it is sticking in this area to the point that the cartridge will not fully enter the chamber. I've thought of a solution to the problem - machine a mandrel to bullet diameter and center it in the lathe chuck, then mount the cases onto it and turn down the necks. That will save buying a turning tool at £80+ and will be just as accurate. I can grind a cutter that will run into the shoulder without damaging it just like the cutters on the specialist tool.

 

My problem is this. I've measured the thickness of the brass in the necks of my cases and it is .017 - but how do I know what it is supposed to be? Common sense would suggest that the SAAMI drawings would have this dimension listed, but so far I'm unable to find one with this info? I don't have a tool to measure the chamber with.

 

I've measured other cases (the original 9.3 brass, a .375H&H and a .243) and they all have a neck thickness of around .012. That would suggest to me that when the bullet is seated in my formed cases, the neck is .010 oversize? But I'm not sure and figure this is quite important!

 

I suppose I could run a thou off at a time until it fits, but how much play should there be to allow a safe level of expansion when the cartridge is fired? Again I don't know the answer, but once I've machined a case down until it fits I will know the exact measurement of the chamber and will be able to then deduct from that to the required spec.

 

Of course part of me wants to just crack on and machine 5 thou off of the brass and that will make it the same thickness as all the other cases I measured, but that would be a little wreckless as I have no idea what I'm doing!

 

What do you advise gents?

Edited August 4, 2013 by njc110381

[rem708]

You are on the right track. I do this all the time but I do use a proper case trimmer/neck turner however a lathe is just as good. Yes make a mandril to support the case. It will be smaller than a bullet diameter but not a lot. It needs to be a slight friction fit in a sized case. Ensure you lube it.

Trim/turn a few cases until they chamber without force but don't cut too deep into the shoulder at this point. the lathe cutter end should be curved contour of about 1/10" diameter

Now you need to fire form these few cases. Check the fit of a unfired bullet in the fired case. It should be a loose friction fit. If its still tight then take just a little more off with the lathe. This takes a bit of time but you are 'fitting' the cases to the receiver. Once you get the right fit after firing size the case and measure the neck outside diameter. This is what you should always use for all other cases.

I never cut too deep into a cases shoulder until its been fire formed the first time. I strongly recommend just neck sizing from then on. In that way you wont get any metal flow from the shoulder into the neck. If you have to full length resize at some stage then just run your fingers gently over the shoulder onto the neck after the first firing. YOU will detect a deforming *slight bump) if metal has flowed in which case just cut it back to the shoulder again.

 

It is a lot of work but worth the effort in the long run as it makes the case fit your gun perfectly.

[Underdog]

My old speed manual does matey .

For the 8mm Mauser it says neck od of.3493" .

Take a .323"bullet off leaves around .026"/2=13....bet you you not far off at 11-12 thou thickness.

 

U.

[seeker]

I use a kit for neck turning - the expander mandrel is about 2 thou under and the turner mandrel about 3thou under bullet diameter.

Have you access to cerrasafe (or similar) to get a chamber/leade cast?

[njc110381]

I've got a block of cerrosafe - never thought to cast it and measure that! I bought it to check that the bore was .323 and not .318 as the older 8mm guns are.

 

I'll try a case at the spec Underdog has given me. I'm not too bothered about the cases being a perfect fit as there's only so much accuracy you can drag out of an 80 year old drilling anyway. It's for 100 yard deer so if I can get it to group anywhere under 2" at 100 yards I'll be happy. My main concern is the safety aspect as I don't really want to blow the old girl up or worse still lose any of my digits!

 

Thanks for all the replies. I'm much happier going ahead with this now and have a couple of lines of attack should the first plan fail.

[Underdog]

You wunna blow it up

 

U.

[activeviii]

you know a case fits before you seat the bullet. measure the outer. you know once you seat a bullet it doesn't fit, measure the outer. take one from the other and that's what you need to take off. (meerkat)

[njc110381]

The bullet will expand a correct case by around 3 thou. If I take the case down so it's the size of a sized case when it's loaded it will be too loose I think?

 

Anyway, if you're so clever why haven't you done it for me already?

[activeviii]

cus you smell