How to size a neck bushing for use with a mandrel?

[Wheres-Waldo]

6mmBR Peterson Brass
~0.013” neck walls
Annealed after every firing
.242” Mandrel
.265” SAC Bushing in LE Wilson FL Die

My bushing is leaving my neck at about .2655”, which put my neck ID at .2395”.

Is approximately .002” of interference enough for the mandrel, or should I get a .263” bushing?

I’m comparing ES/SD between my standard load and incorporating a mandrel, and so far the numbers are horrendous for the mandrel group.

Wanting to make sure I’m doing things properly.

 

[Maurygold]

Mandrels arent worth the hassle and have very little if any benefit. You can easily shoot under 5-8 sd without one.

That said I would mandrel to 2-3 thou under the bullet diameter. I think the 242 is too big

 

[Wheres-Waldo]

Mandrels arent worth the hassle and have very little if any benefit. You can easily shoot under 5-8 sd without one.

That said I would mandrel to 2-3 thou under the bullet diameter. I think the 242 is too big

I’m coming from about 25 ES and maybe 7 SD without it.

If it CAN reduce me further, it’s hardly a hassle for me personally. I have the set-up, just looking how to properly use it to judge it’s effectiveness.

Especially considering I’m not able to throw .02gr resolution charges yet.

 

[Herb Stoner]

I size the neck to 2-3 thou under my mandrel size. It gives the mandrel something to work with. I anneal so the slight additional working of the brass is a non issue. Mandrel is typically set up to give about 2 thou neck "tension".

I don't disagree that it may or may not yield better results but I do most case prep on a Dillon 750 so its not an extra step - no extra hassle.

 

[Maurygold]

I’m coming from about 25 ES and maybe 7 SD without it.

If it CAN reduce me further, it’s hardly a hassle for me personally. I have the set-up, just looking how to properly use it to judge it’s effectiveness.

Especially considering I’m not able to throw .02gr resolution charges yet.

You won’t be able to shoot the difference in 5 vs 7sd and you may not even get down there

I get playing with new stuff but don’t get too caught up in thinking you need the mandrel.

 

[RegionRat]

TLDR: about 0.002" is a fair starting point for "neck tension", but there is always the devil in the details.

There are pros and cons to every neck prep method. Folks find many ways to prep their brass and some of them are the best shooters in the world, and some of them are terrible examples, while their method may be called the same.

To be of help, yes a mandrel can be a great method when done well.
The main issues I will bring up are in two big buckets and are true for all prep methods.

One bucket of issues deals with the idea of elastic strain versus plastic strain, or yield versus springback in different words.

The same tools (bushings, mandrels, dies, etc.) used on brass in different hardness conditions can land you at a very different final diameter.

So it is important to consider the work-hardness state of the brass when discussing a particular bushing or mandrel diameter, not just the thickness. Starting from a HV of 90 is different than starting with a HV of 120.

Another bucket of issues deals with the tribology, or friction coefficient of the neck-bullet pair. Discussing internal ballistics results without remembering that folks treat and clean their brass differently can often lead to wild debates that are caused by details left out of the discussion. Even the base style of the bullets and the chamfer method can give wild differences in the outcome.

One way to short circuit all the above science, is to measure the seating force as a way to judge the effects of your neck process. While gage pins and micrometers are helpful, they don't reflect the final answers.

We tend to talk about the interference of the neck to the bullet in terms of colloquial "neck tension". Many competitors do not agree on the philosophy of neck tension, so be prepared to hear all sorts of things from "soft seating" with the chambering, to folks who are well into yield when seating their bullets, in effect using their bullet as that mandrel.

Whatever path you choose in the end, you must consider the hardness state of the brass and the tribological effects of your chamfer and cleaning methods.

At some point in the journey of shooting life, you should play with jam versus jump, soft versus hard seating, annealing, cleaning, chamfer versus flare, etc., just to get some firsthand experience and then make up your own mind.

 

[padom]

I’m coming from about 25 ES and maybe 7 SD without it.

If it CAN reduce me further, it’s hardly a hassle for me personally. I have the set-up, just looking how to properly use it to judge it’s effectiveness.

Especially considering I’m not able to throw .02gr resolution charges yet.

Im not going to repeat a topic that's been beat to death and documented extensively on this site...

But there are many benefits of a mandrel. And they aren't a hassle... unless you find running your brass into a mandrel is a hassle.. I dont use dies that squeeze the neck down and mandrel all in the same die..

I do all my precision rifle loading on a 550c.. decapper in station 1, Wilson FL Bushing die in station 2 to squeeze necks down and bump shoulder, and 21st century mandrel in station 3 to set final neck tension. All happens while I just pull the handle.

I use a bushing to squeeze my neck down 0.004 under loaded neck diameter, I then run my brass into a 21st Century mandrelnto open it back up 0.002 for a final 0.002 neck tension..

Now, what bushing and mandrel to use to get your desired neck tension is a little trial and error due to springback...

You may have to go up or down a bushing size or mandrel size to get your desired neck tension. I have 3 or 4 bushings of each caliber I shoot and 2-3 mandrels to play with 0.001-0.004 neck tension..

Low SD is only one of the things we are chasing using mandrels with bushings. Extreme Accuracy and working the brass as little as possible and keeping donuts away are some other reasons to consider.

If your looking for Consistent 5 shot groups in the 0.1-0.2moa this can help

 

[CK1.0]

I think a separate mandrel step is worth the squeeze, and I don't think it's a hassle at all compared to all the other annoying things we have to do when prepping cases to be loaded.

That said, rather than swapping bushings (as a .265-.266" bushing for a 6mm works great for me and sounds about right), I'd try a slightly smaller .241" mandrel (that's what I've used for a long time and have had great results with). I think I'd always prefer a hair more neck tension than I might need versus maybe not having enough...

I mean, in a perfect world with no spring-back, a .242" mandrel will only leave you with 1 thou of friction-fit... so IDK if going 1 thou smaller/tighter will make that much of a difference or be a revelation, but I'd lean towards going tighter/smaller (in fact, I've been meaning to pick up and try an even smaller .240" mandrel at some point to see if that might even be a little better).

 

[Baron23]

I use a mandrel and find it utterly simple, quick, and easy to do. I agree with @CK1.0 that mandreling is the least of my challenges in reloading.

I do not use a bushing die. I just took the expander ball out of the Redding die.

Mandrel is sort of reasonably new to me. Now, I anneal (AMP) every firing and have found that with spring back I needed to use the .264" mandrel in order to get .002 interference fit and it is one reason I'm glad that I bought a mandrel set from 21st Century. I have no idea how you can just take a .002 under caliber mandrel and just assume that will yield .002" interference.

I don't really care what size mandrel gets me there and I confirm I.D. with pin gauges. Does it make my ammo better...eh, probably not. But I like to control my case prep to the degree reasonable and mandrels give me very consistent neck sizing.

Cheers

 

[Wheres-Waldo]

I have no idea how you can just take a .002 under caliber mandrel and just assume that will yield .002" interference.

The thought is that a .242” mandrel with annealed necks will give a little bit of spring back, probably winding up between .2410 - .2415. As long as it’s opening the neck up from a reasonably undersized starting point. Not sure if the spring back factor is a product of just annealing or that in combination with how much the brass is being moved/worked in that operation.

I probably need to grab a few gauge pins myself.

 

[RegionRat]

I probably need to grab a few gauge pins myself.

If you have a spread of mandrels in fine steps, you can just use those if you don't have the gage pins.

Owning some mandrels above and below the one you think you need, is cheap enough and can always serve as a gage pin too. If it ever comes to pass that some brass finds itself on your bench that is different, you will be glad you already had the other sizes. YMMV