Check my math?

[Sumtoc]

Have always used bushing dies to set neck tension but now loading 223 Rem for F-Class and trying to get better control with mandrels. I'm using unturned Lapua brass which is pretty consistently .013" neck wall thickness. My sizing process goes like this: deprime, .2240" expander mandrel (oversize to remove any dents and "reset" new brass), FL S die set to bump shoulder back .002" and with a .245 neck bushing, expand with .2225". Nervous about the calculations, please check my math.

The math:
Loaded neck diameter - 0.2495 (measured)
.002 tension = 0.248/7 bushing, or
(0.2495-0.002) = 0.2475
2*.013 (neck wall)=.026
0.2475-.026=.2215 ID
.001 spring back = .2225 mandrel for final neck sizing

Thanks guys

 

[std7mag]

What is the neck diameter after you size it, without a bullet in it?

 

[jelenko]

On neck tension, don't you need to substract .002 neck tension for both sides? I.e, you subtract .004 for neck tension. [Just like you're doing for wall thickness.]

 

[Sumtoc]

Sized empty neck measure 0.2475-0.2480 which seems Ok for 3x fired brass?

Jelenko, not sure why but I've always been told to subtract the target neck tension (like .002") from the loaded neck diameter to get the correct bushing size.

 

[jelenko]

Sized empty neck measure 0.2475-0.2480 which seems Ok for 3x fired brass?

Jelenko, not sure why but I've always been told to subtract the target neck tension (like .002") from the loaded neck diameter to get the correct bushing size.

Oops. Not enough coffee.
Neck tension is the total amount of interference on both sides.

 

[jepp2]

Nervous about the calculations, please check my math.

I'm an engineer, but I feel careful measurements with quality instruments will be more useful to you than theoretical calculations. You don't say if you anneal or not. That can influence springback. And remember springback works in the opposite directions for sizing down and expanding up.

I don't see how running a 0.224" mandrel through your fired case necks provides any value. When you size down with your bushing and expand up with your mandrel, it will take care of any imperfections.

If you want to move any irregularities in the case neck from the inside of the case neck (result of the bushing sizing the neck down) to the outside of the neck with the mandrel, the mandrel is going to have to actually expand the case neck in a measurable amount.

Springback changes from work hardening of the case necks will show up in actual measurements, not in calculations.

If your loaded neck diameter is 0.2495" and you want 0.002" neck tension, the case neck before bullet seating should be 0.2475". But when I see measurements that have 5 ten thousandths in them, I suspect those are caliper measurements and not micrometer. Calipers really aren't capable of measuring to that level.

 

[RegionRat]

I agree with jepp2

If you are going to get picky about your prep process, you will need to use the right tools and instruments for measurement.

A micrometer is necessary for what you are doing, but I will add you will want to also have a spread of mandrels and use them as gage pins to know what your resulting ID becomes after all is said and done.

A tight slip fit of a known gage pin (or mandrel) is very bottom line in terms of your actual ID results.

Use a micrometer to verify your mandrel diameter. Things are getting better in terms of mandrels or bushings actually being the diameter they are marked, but not long ago it was common for them to be significantly wrong.

 

[Ned Ludd]

Forget the indirect measurements and calculations. Measure the outside of the neck before/after seating a bullet to determine neck tension/interference fit. To identify the correct bushing, you literally need to test a few. Bushings are often be off by +/- .001" from the stated diameter, so predictive calculations don't really help much. You need to measure the necks of sized brass to know what the actual bushing you are using is doing.

I've used Lapua .223 Rem brass for F-TR loads for many years. When I re-sized brass with a bushing die as the sole [neck] sizing step, a 0.248" bushing was the perfect size to yield ~.002" neck tension. When using a mandrel as the final neck sizing step, you want a bushing that is slightly smaller so that the mandrel will do work and open every neck up slightly. So something like a 0.247" bushing would be a good starting point, but again, you need to measure the necks after sizing with a specific bushing to know what diameter it will actually generate. It may be that a particular 0.247" bushing could yield an outside neck diameter as small as 0.245" to 0.246", in which case you would be needlessly over-sizing the necks.

In any event, once the necks have been sized to a slightly smaller diameter with the bushing die, spring-back of the brass following removal of the appropriately-sized mandrel will actually cause the necks to close up by about .0005". So in my hands, a mandrel that is .0015" under bullet diameter will yield very close to .002" neck tension. In the case of .223 Rem brass, I routinely use a 0.2225" mandrel as the final [neck] sizing step to yield ~.002" neck tension. If a bushing is used that leaves the neck diameter too small relative to the selected mandrel, it may be that one could get more than ~.0005" spring-back, and the necks would be needlessly over-worked.

It may seem like a pain, but the whole bushing diameter variance thing has led me to the obvious conclusion that I generally want to have a few different bushing and mandrels for each cartridge. For .223 Rem as an example, I might want to have 0.245" to 0.248" bushing in .001" increments, and 0.2220" to .2235" in .0005" increments (21st Century makes these). That way, you will usually have the correct bushing and mandrel, regardless of the whether the bushings actually yield values equal to their stated measurement. BTW - the 21st Century mandrel diameters are spot on.

 

[Sumtoc]

Thanks guys. This is all great feedback.

Jepp2, I anneal after every firing. And you're right, the first expander mandrel is optional/not required for fireformed brass but it is definitely needed for new brass. I'll reserve that step for new cases.

Ned, it appears I am using a the same final mandrel as you suggest but I need to be a bit gentler with the bushing, I'll swap the .245 for a .247

 

[Ned Ludd]

Thanks guys. This is all great feedback.

Jepp2, I anneal after every firing. And you're right, the first expander mandrel is optional/not required for fireformed brass but it is definitely needed for new brass. I'll reserve that step for new cases.

Ned, it appears I am using a the same final mandrel as you suggest but I need to be a bit gentler with the bushing, I'll swap the .245 for a .247

You never know. For example, I've had different 0.248" bushings yield different diameters for sized necks using the exact same Lot# of brass. Using a 0.2225" mandrel with Lapua brass ought to yield a final neck diameter of 0.2475" or 0.2480". Thus, a 0.247" mandrel that sizes true would be perfect. A 0.248" bushing that sizes slightly small, or a 0.246 that sizes large could also do the job. I'd just test a few bushings you have to see what comes closest, or order 0.247"/0.246" bushings if you don't already have them. The 0.245" bushing might be working the necks a bit more than is necessary, even if the final product is acceptable. It's something you can experiment with and tweak over time as needed.

 

[BoydAllen]

I would do this. Size with a bushing of your choice. Expand with the one you usually use prior to neck sizing. measure the neck after, seat a bullet and measure again. Adjust your mandrel size to hit your goal. Have a good neck mic. that I use to set up my turning tools and check necks as I am turning, but for my final test for loaded neck OD and neck tension I measure sized and loaded necks.