6mm ... to .257 Roberts ???

[Trinidad Bill]

Need to be schooled on how to re-form 6mm to .257 Roberts.

I was fortunate to have purchased some 6mm brass from a member with the intent to convert these to .257 Roberts.

I have never done this before, so I need some tutelage from those who know.

[Mk42gunner]

The easy way is to just run the brass through your .257 Roberts sizing die and load as normal. The expander button in the .257 die should expand the 6mm case neck just fine.

Do be sure to clearly mark the boxes once they are loaded with .257 bullets, You don't want to try putting one down a 6mm barrel.

Robert

[Trinidad Bill]

Well that is pretty straightforward. My dies are CH and all steel. Is that going to be an issue? I do have a Lee Necksizer die which would be carbide.

[15meter]

I've made 257 Roberts brass in the past and it's been long enough I don't remember the donor case. I do remember it being an easy conversion.

And you may want to read up on annealing your brass to extend case life. No need to go through all the work of reforming the brass and then start losing it to cracked necks on the first couple of firings.

I would anneal after the forming and before I loaded the first time.

Going up from 6mm to .257 neck thickness shouldn't be a concern.

Sometimes when converting other calibers, I have had difficulties with initial chambering, if you experience the same thing, it's a fair chance that the shoulder isn't quite right.

I have taken a shell holder and ground 10-20 thousandths off the top and full length sized with the ground shell holder. After first firing you can usually go back to a full height shell holder.

CH dies are good dies, shouldn't have any issues there. I use Imperial sizing lube that I put on ever neck ID with a Q-tip. Go slow and easy, in to the die a little bit, back out and rotate the brass 30-40 degrees and back in again. Wouldn't hurt reapplying wax as you are necking up.

Good luck!

[TD1886]

I've made 257 Roberts brass in the past and it's been long enough I don't remember the donor case. I do remember it being an easy conversion.

And you may want to read up on annealing your brass to extend case life. No need to go through all the work of reforming the brass and then start losing it to cracked necks on the first couple of firings.

I would anneal after the forming and before I loaded the first time.

Going up from 6mm to .257 neck thickness shouldn't be a concern.

Sometimes when converting other calibers, I have had difficulties with initial chambering, if you experience the same thing, it's a fair chance that the shoulder isn't quite right.

I have taken a shell holder and ground 10-20 thousandths off the top and full length sized with the ground shell holder. After first firing you can usually go back to a full height shell holder.

CH dies are good dies, shouldn't have any issues there. I use Imperial sizing lube that I put on ever neck ID with a Q-tip. Go slow and easy, in to the die a little bit, back out and rotate the brass 30-40 degrees and back in again. Wouldn't hurt reapplying wax as you are necking up.

Good luck!

Before grinding some off the top of the shell holder cam over your press by adjusting the sizer dies in more. Sometimes that gives you enough to chamber if they were tight upon the first sizing. Often another brand of shell holder may give you more as they all aren't exactly the same height.

[Rapier]

So true, if you have a group of shell holders try them all until you get one unloaded die formed case that allows the rifle's bolt to close. Once the case is fire formed the case will go right back in the chamber. Trim the case after fire forming, unless it is over length when die formed.

[MostlyLeverGuns]

Lube the inside of the case necks BEFORE running over expander, easier on brass, sizing die, press and you. I use Hornady Unique Case Lube on Q-tip inside neck. Annealing after expanding usually helps case life. When working with fired brass of unknown history, if I have problems chambering, I run the cases into an RCBS Small base 30-06 die without the expanding unit. This seems to reduce oversized cases, cases fired in oversize chambers so they sort of reconditioned, then size and load normally.

[Kestrel4k]

+1 on the Hornady Unique case lube for sure.
That is an interesting tip on the small base '06 dies, thx.

[Blkpwdrbuff]

You shouldn't have any trouble at all.
I open .243's up to .257 for my .250 Humdinger all the time.
Follow all the previous advice and life will be good.
Blkpwdrbuff

[15meter]

Before grinding some off the top of the shell holder cam over your press by adjusting the sizer dies in more. Sometimes that gives you enough to chamber if they were tight upon the first sizing. Often another brand of shell holder may give you more as they all aren't exactly the same height.

My list from last year, post #246:

https://castboolits.gunloads.com/sho...oad-for/page13

I'll still look at grinding a shell holder as a viable tool in case conversions.

About 25 out of my list entailed a case conversion of some flavor. Some because of unavailability of brass, some just to see if I could do it and some just because I'm a curmudgeonly cuss at times.

A few the ground shell holder just made life easier.

Rarely have I seen much more than standard manufacturing variation in shellholder heights.

YMMV.

[MarkP]

Make sure the expander in your die is tapered and will fit into the 6 mm mouth and has a longer lead angle than the RCBS expander shown below;



If it looks like this probably will not work very well and may buckle the shoulder of the case while expanding due to downward force on the neck.



This is a Hornady elliptical expander ball and will work perfectly.

[Trinidad Bill]

Wow...a lot of great information here guys, thanks.

I am concerned about the expander ball on my CH dies. The expander ball leaves strong grooves inside the neck after sizing...I am assuming the helps hold the bullet in the neck. I had never seen that before. I may need to get the Hornady sizing die for the taper.

I did order some Imperial sizing lube, I normally used Lee Sizing Lube. Looks like I need to also try some Hornady Unique lube.

Never have annealed anything, but I will. I have been reading about the annealing process...there seems to be many!

I do have a lathe so I can trim a shell holder if I have trouble with chambering after sizing and annealing. I am assuming this allows the sizing die to reach farther downward towards the case head... in case there was bulging from the pressure exerted during the neck expansion and sizing?

Would the NOE expanders plugs with the Lee Universal Expander help at all in this process?

[Kestrel4k]

You might think about carbide expanders for your dies; I have swapped some of these into my most-used die sets and they do an excellent job of neck expanding after resizing.
Should be an even greater benefit if opening up the necks slightly as you'd be doing.

[Trinidad Bill]

What is a good source for carbide expanders?


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[Kestrel4k]

It's been a while since I changed mine out; my fuzzy recollection that one of our large manufacturers made RCBS-compatible stems for direct replacement, but not compatible with Lyman die sets.
A little carbide button captured on the decapping stem assembly; money well-spent most certainly.
It may have been Hornady ?

[15meter]

What is a good source for carbide expanders?


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Redding makes taperd expanding buttons. I actually went looking for the one I used to expand .243's to .260's to see if it would fit on a CH die stem, can find a CH die, can't find the Redding expander button. Bummer. It's in my shed, "somewheres".

Don't remember if it's carbide, it's been a dozen plus years since I did the 243 to 260, but I remember it being pretty easy.

[TurnipEaterDown]

I find the Hornady elliptical expanders work really well for necking up.
Their neck size dies are/were also designed for a range of calibers / body diameters.

So, I simply use a handy Hornady neck sizer, bought a array of extra buttons of various sizes as replacement parts(they all use the same stem diameter and thread in my collection), and swap them out to what I want to open the neck.
Don't interpret this to mean that decapping happens in die used this way, but I have necked belted magnum cases from 30 to 375 using an old monkeyed up Hornady 22 cal NS die, 338 BM to 416, 300 WSM to 35, 30-40 Krag to 416, etc. You just lube the inside of the neck and run it in far enough to get it over the button.
Sure, the case body isn't guided in this set up, and you could get concentricty issues in first go around for neck enlargement, but this can well be followed up by FL sizing in your "real" die to get you where you need to go and straighten things up a bit. Really easy for me anyway.

[Texas by God]

I ran a Norma 6mm brass through my NOS .257 Roberts CH dies with just Hornady One Shot. It was a slightly rough feel, but not hard.
I’d recommend a regular case lube for this.
I don’t have a Roberts rifle to check it in, but the Norma case looks great side by side to a real .257 Roberts case.
My die also has the fine knurling on the button.
I always thought that was for grip when you replace the decap pin.
With a good lube, I’m sure it’ll work.
Back in the days of cheap bullets and cheap primers, my brothers and I would probably just shoot 6mm ammo in the .257 rifle for giggles.
22-250 to .250Savage worked just fine.

[Trinidad Bill]

I did take the CH FL sizer and moved the decapper/neck sizer button down so it protruded through the press. I then lubed (Lee lube) up a 6mm case neck and up sized the neck with the button slowly rotating and moving up and down. Was not too difficult. I then removed the decapper and sized the case body. I then seated a bullet (fit perfectly) to the desired depth and cycled the case (minus primer and powder) in my 257 Roberts receiver. Cycling/seating the case in receiver worked smoothly.

I will do a couple more, anneal the cases, then load up for a test.