Sizing down brass

[ForgeValley]

So like an idiot I thought I was getting out of 7 SAUM and sold all my precious ADG brass. I now miss it and want to shoot it again. I have some 300 SAUM brass, and thinking about sizing it down. I've not done this before, so did a little research and found guys posting processes that were 10 steps long and a total PITA (annealing, neck turning) to guys saying knock it down with two bushings and you are good to go.

So before I jump in, what's your process. PITA or easy peasy?

 

[RegionRat]

Any time you go down this road, the issues make this an "it depends" answer.
Two things drive this, the original thickness of the brass being reduced will cause it to effectively be thicker, and the diameter of your chamber.

The brass doesn't evaporate, so going down in diameter means you get an effective thickness increase. If that is okay you don't worry about it. However, to determine if it is okay you have to go forward and find out if your chamber and sizing process agrees.

Even a no-turn chamber has a say in how thick that brass can be in terms of performance. Sometimes you get away with the short version you mentioned and don't need to turn, other times you are not happy unless you do.

 

[ForgeValley]

Any time you go down this road, the issues make this an "it depends" answer.
Two things drive this, the original thickness of the brass being reduced will cause it to effectively be thicker, and the diameter of your chamber.

The brass doesn't evaporate, so going down in diameter means you get an effective thickness increase. If that is okay you don't worry about it. However, to determine if it is okay you have to go forward and find out if your chamber and sizing process agrees.

Even a no-turn chamber has a say in how thick that brass can be in terms of performance. Sometimes you get away with the short version you mentioned and don't need to turn, other times you are not happy unless you do.

So if I am reading your answer right, it's a matter of necking it down and seeing if for that particular chamber, it works out and gives performance I am happy with? If not, it will be neck turning?

 

[RegionRat]

So if I am reading your answer right, it's a matter of necking it down and seeing if for that particular chamber, it works out and gives performance I am happy with? If not, it will be neck turning?

Yes... and not just that chamber, it applies to your neck prep process in general when compared to virgin brass.

It is a given that you will want to seriously consider annealing cause you are investing time in brass that will get more than the usual cold working to start with, so you take care of this investment by doing that step to lengthen your case life and put the hardness closer to what your previous regular sizing process would want.

Even then, if you don't turn it... from then on your process is still changed since that thickness is now part of the rest of the cycles and to work out it must be accounted for.

You would normally have some homework to determine your neck tension with any new-to-you virgin factory brass, so it is all those normal issues plus the issue of starting with brass that is worked just to get it started. If you don't turn it, be prepared to account for the extra thickness in your prep process. On the other hand, once it is turned, you are typically better than average in terms of the neck prep from then on. YMMV

 

[Schw15]

I shoot a 7mm-300 win mag I have to neck down 300 win mag brass to 7mm. I do all my necking down in one pass with either a bushing die or normal full length die. I lube the necks with imperial wax. I don't neck turn but my reamer works with the necks getting thicker. After necking it down I run it threw a expander ball to make sure my neck tension is same but switching over to a mandrel for that. But I never have any issues and I've necked down 30-06 to 6.5 in the past no huge issues

 

[bax]

So if I am reading your answer right, it's a matter of necking it down and seeing if for that particular chamber, it works out and gives performance I am happy with? If not, it will be neck turning?

SAAMI says the necks on your 30-caliber brass and 7mm SAUM brass should be no more than 18 thou thick.

30 caliber bullet diameter is .308, max diameter for the round is .344, .344-.308=.036/2=.018.
7mm bullet diameter is .284, max neck diameter for the round is .320, .320-.284=.036/2=.018.

In my experience I would guess that the neck thickness is more like 0.015. I'm not measuring real brass. When I measure real brass, I see neck thicknesses around 0.14 to 0.16 - so I am guessing about your brass. You need to check your brass and chamber.

SAAMI says that the minimum chamber neck diameter for the 30 is .345 and the max diameter of a loaded round is .344. The 7mm SAUM the chamber number is .321 and the max round diameter is .320. Using their numbers, there is a one-thou total clearance, half-thou on each side of the neck. Using my .015 number, there is a seven-thou total clearance, 3.5 thou on each side. Again, you need to check your brass and chamber.

When you size down your 30-SAUM, those necks will get thicker. I'm going to guess about 2 thou thicker, now necks are 0.017. (see calc below) Now the neck OD of a loaded round is .284 + 2 * (.015 + .002) = .318. With a chamber diameter of .321, you have a 3 thou clearance, 1.5 thou on each side. I guessed that the necks would get 2 thou thicker, suppose it is 3 thou? Now the calc is .284 + 2 * (.015 + .003) = .320 and you have 1 thou total clearance, half-thou on each side. That is about one-sixth of the thickness of a piece of paper.

You turn necks to get back to the standard clearance.

How I am guessing about extra thickness. Using your 30 caliber brass and the SAAMI drawings, imagine a circle half-way from the inner wall to the outer neck wall of the brass - OD is .320, ID is .308 so a circle with diameter .314. The circumference of that circle is .9865. Now imagine the same circle for 7mm - the circumference for that circle is .9205. The new circle is 7 percent smaller than the old one. The bullet (the I.D.) didn't get any smaller so the neck got bigger.