Home Forums Reloading Big Game Rifles A few questions about necking up brass

I am building a 375 WSM. Since it is a true wildcat, I have decided to start the build with brass based off the only set of dies I was able to acquire from CH4D.
I began with virgin Nosler 325 WSM brass, which is very thick compared to other Nosler brass I have used.
I am using 21st Century expanding mandrels and their expanding die.
I lubed the inside of the case necks, and initially necked them up to 35 caliber. Then, lubed the inside case necks again and used a 3740 mandrel anticipating 1thou spring back to leave me with 2-3thou neck tension.
The shoulders had obviously moved during the necking up process. So, I annealed the cases, lubed the neck and body and FL sized them with the intention to turn the necks to uniform the brass before I sent the brass off for the chamber reamer.
The problem I have is that the inside neck diameter after FL sizing the cases is 0.362. Which is obviously far too small and will make neck tension too great. Prior to FL sizing the necks are a consistent 0.373.
Outside neck diameter is .393 on FL sized cases and .406 on mandrel only cases.
According to the Nosler and Hornady manuals I have .405 is the outside neck diameter for .375 cartridges.
Neck wall thickness on FL sized case is .017 and .015/0.16 on mandrel only cases.


So, to the guidance a I am requesting;
How do I remedy the brass?
Is it a die problem or should I just find a way to ream the inside of the brass' necks? at the moment I do not own an inside neck reamer. So, if this is the solution a recommendation on a good one would be appreciated. I am using the 21st Century Neck Turning Lathe. If there is an inside neck accessory for it, I am unfamiliar with it.
Can I simply use a .374 expanding mandrel on the sized brass again to get the inside diameter I want, then neck turn or would this move the shoulders again?

All measurements were performed with a Mitutoyo caliper and Mitutoyo ball anvil micrometer.

If you are questioning why I am doing this vs. ordering a reamer that is available, the answer is that I was afraid with it being a wildcat cartridge that components wouldn't match up. I figured if I could start with the brass formed by the tooling I already owned, I could have a tight chamber reamer made to only allow the brass to grow 2-3thou in any direction. This would allow for considerably extended brass life and also make the brass I formed at the bench be ready for the chamber without fire-forming. I could be wrong, but I'm having fun.

Thank you in advance for any information.

Edit: I forgot to include that FL sizing was done with the expander ball/decapping pin removed.

This is my first experience neck brass up. So, i apologize if my questions seem rudimentary or nonsensical. If any of the method I am using is flawed, please let me know. Thank you!

I suspect the best and only option is to turn the necks.

I sized a bunch of brass from 6mmBR to 30BR and part of the thicker brass shoulder gets formed into the neck (The dreaded doughnut)


I used a Sinclair neck turner with the appropriate diameter Turning mandrel to turn the brass to a uniform neck thickness from mouth to shoulder..

But First! I also needed a Sinclair Expander die and mandrel to push the thicker material to the outside of the neck so it can be trimmed off.
The Expander die also opens the neck .001" over the diameter of the Turning mandrel so the brass will spin when Turning.

I had to be sure to lube the inside of the neck before using the Expander Die as freshly annealed cases may crush and collapse when forcing the tight brass over the Expander die.

That's what I did for my 30 cal but the 375 neck is big and may need a slightly different approach.

Check K+M Neck Turners for the big case options, Good Luck!

PS,
A Power Adapter is also great for holding the case while spinning in a drill for a smooth cut and consistent thickness.
Your hands will be grateful. I used a Lee case trimmer locking stud.

I would just a mandrel that'll expand the necks to your desired OD. I am pretty sure a few companies have custom expanders so you can order about whatever you want.

A set of gauge pins is pretty handy as well. I learned a bunch going through making Mashburn cases and the pins were really handy.

Also, if you can get a FL bushing die it may be pretty useful as well in keeping your necks under control and working the brass as little as possible.

Sounds like a cool wildcat.

The undersized brass makes sense if you sized it without the expander ball.

It sounds like you need to expand, outside turn the necks, resize with expander ball installed, and then expand again to desired inside diameter.

Yes sir, that was my conclusion. I thought I had the correct tool head for my neck turning tool, but apparently the larger caliber turning arbors require a larger tool head than I have. Once the correct head gets here I will start working on turning them.

I appreciate your response. Thank you.

I have been looking at the Forster gauges, but they don't have anything over 30 cal available. Thank you for your response. I am hoping the end product will be as cool as it is in my head.

Check out Amazon for a set from .251 too .500. Pretty reasonable cost. Great for this sorta work.

"The problem I have is that the inside neck diameter after FL sizing the cases is 0.362"

You can not measure this with the blade of the dial caliper, improper measuring tool.

"According to the Nosler and Hornady manuals I have .405 is the outside neck diameter for .375 cartridges."

Best to work off of a reamer print if the gunsmith gave you one, if not, call and ask for one. If he does not have a reamer print, then call the Company of the Reamer he used.

If the shoulders moved during the neck up process, then you were expanding too much in one step. Always put a very heavy chamfer on the inside of the neck that is deep as an aid for the mandrel. Go slow during the neck up, and you may go a little, retract, then a little more. I like Lee Sizing wax in the tooth paste tube that runs about $4 instead of imperial sizing wax. I lube the mandrel with a Q Tip and lube in the inside of the neck.

Real thick necks always end up causing issues. Learn to work off the numbers prior to working brass:

Chamber dimension is: xyz

Target loaded round dimension is: xyz. Here you have established the amount of Grip you want on your loaded round and the clearance in the neck/chamber



Neck dimension after final expander ran through the neck is: xyz which yields a bullet grip = to .xyz

Sized brass neck dimension with no expander ball is: xyz

Sized brass neck dimension with expander ball is: xyz, this may leade to follow up expansion with mandrels to unifor neck grip on the bullet.

Proper tool to measure inside of neck dia is:

https://www.ebay.com/itm/331913296440

The above tool is accurate, and far less expensive than buying gauge pins

Redding sells long tapered expanders, but you do not have the option on sizes. 21st century is the way to go, but maybe expensive to get set up....life time investment.

Darn thick brass causes varying ignition characteristics as the grip can change from round to round. Also, expect variations in Rockwell hardness from individual pieces of brass to begin with. Neck turning will give you great results, K&M neck turners make life easy.

You need to know the numbers you are working off, and expect GREAT results!

At this point, I do not have a reamer print. I am trying to have a reamer made to match the brass I am able to produce with my tooling. Since it is a wild cat, I was doubtful that the available tooling would match well with any kind of reamer. Which is why I am doing the best I can to get the brass as uniform as possible before I send in sample cases to a reamer maker.

I used Imperial in mine. I tried graphite neck lube first, and it was not the right product for the job. I have some Lee wax and will give it a try on the next batch of 100 brass. I lube the mandrel and brass the same way you do.

Thank you for he advice and the suggestion of the tool. It is a new one to me and one I will definitely look in to.

I disagree. The caliper blades will be adequate for this task if one knows how to use them properly and they are in good calibration. The calibration part is important as the blades are easily damaged in use, much more so than the rest of the caliper and many people don't notice the problem. If you have a ring gage handy you can use it to check the caliper blades. That said an inside mic would be nice. You might also just buy a few gage pins around your target size, which would be much cheaper than a full set. The class Z are fairly reasonable.

Gauge pins are not expensive. Quite cheap actually. Buy the sizes you need. For necking up, I fireform. It produces immeasurably better results and work hardens the brass far less.

Under normal circumstances, I would completely agree, but that isn't an option I have available.

The first thing you need to know is the neck diameter that reamer is cutting to. The .405 dimension listed for other .375's includes about .004 of neck clearance. Your cases are much thicker than that, and if the neck is cut at .405, your rounds aren't even going to chamber unless you neck turn first.