257 Roberts Neck size

[GRUMPA]

I'm playing with these at the moment, and just wanted to double check the size of the neck with a bullet in it. The Spec calls out for a .290 diameter neck and I just wanted to know from a few folks what the diameter is on what they have. All this does is tell me what diameter reamer to get. No way this conversion is going to work without reaming the necks, brass is really thick on this conversion.

Thanks for the help....

[JonB_in_Glencoe]

OK, I checked what I had.

First batch, the brass was loaded x2
"W-W super+P" brass, FL sized and with a .2585 boolit seated
the necks measure from .288" to .289" about 5 samples measured.

Second Batch, loaded many times.
"W-W super+P" brass, Neck sized only and with a .257 J-word seated
the necks measure from .286" to .288" about 10 samples measured.
>>>Also, I found a "R+P" mixed in with this second batch, it measured .285" with the .257 J-word seated.

I also checked some fired cases, "unsized", they measure .291" to .292"

I hope that helps you out.
Jon

[GRUMPA]

Thanks John, there sure is a big difference from spec to actual size....

[EDG]

Grumpa
You can make or buy a case neck turner with a pilot that will turn the necks in your lathe.
I doubt that it is any faster but it is adjustable and will let you cut the necks to any thickness.

The cartridge specification has a tolerance of +.000 -.008

The neck of the loaded case is .290 Max .282 Min

The neck of the chamber in the rifle is .291 MIN and .293 Max . The tolerance for the chamber is +.002 -.000

Note with the fatest case neck and the smallest chamber neck there is still .001 clearance.

[GRUMPA]

This conversion in particular I was going to put in the case forming section. I have a feeling it'll be lengthy since I plan on posting a bunch of pictures with it. So far I only have the die set, the gauge is on order so who knows when I'll get notified, more than likely when I'm in surgery this week since that's the way my luck seems to be.

To be honest I've never turned a neck down in my life, only reaming. When I ream them I hook up my 5C collet chuck and use a steady rest on the neck. Seems run out is extremely minimal and it keeps the axis of the case pretty much dead on. The first thing I do with the reamer is shorten it, and I indicate it in to make it within .0001 for height and taper and just go at it.

I've had folks write me back that have neck turned, and after using the brass they get from me remark on the improvement they get. When they write back and I find out they neck turn, I explain how I do it. After that I never hear back from them so I take it they're taking my advice.

When I start a new conversion I try to get as much info as I possibly can and dissect everything after that. But I try and keep in mind the boolit users and make allowances in neck diameter with that in mind.

I'll have to look a bit closer at the SAAMI drawing I have, I must have missed the tolerances but right now I have other things on my mind...

[Clark]

I had Forster hone out my 257 Roberts Ackley full length sizer die neck to 0.284".

My rifle chamber is .288", the two rifles I made with that reamer.

8mm brass can be necked down to 257 and not turn necks if it is RP or WIN.

But Lapua or Norma thick necks cannot go half that far without being turned down.

I sometimes have to turn the necks down to 0.0135" thick.