Case mouth reaming size?

[Twmaster]

I've been converting .223 cases to 7.62x25 tokarev cases for my CZ52.

They need to have the brass around the case mouths reamed else the neck is too thick.

I've been drilling them out with a Letter sized N drill bit (.302"). I need to cast boolits in .311 as my bore is .310".

I can get a chucking reamer in .309".

Ideally what size should I ream the mouth of the case open to?

Thanks!

[cbrick]

Forster has inside NK reamers for their brass trimmer and sells (or did, haven't bought any in years) custom ground diameter reamers. That could be a good way for you to go. A call to Forster and for a minimal extra charge they would ship you the diameter reamer that you specified.

Back when I was more into tiny groups on paper, as in when I could still see well enough to do that I had pretty much given up on inside reaming. I had better accuracy results from outside NK turning to thin necks.

Rick

[Char-Gar]

Inside neck reaming will often be off center and produce necks with a great variation of thickness around the circumference.

The need to inside ream went away when the first affordable outside neck turner came on the scene.

It is far, far better to outside turn the necks to reduce thickness. This will give you neck with uniform thickness and better accuracy.

There are several tools that will do this. I use one that fits the Forster case trimmer, but Sinclair and other sell hand held units.

[Twmaster]

Thanks for the input folks. My question is more about sizing not how to get there. I have a machine shop here at home. I can turn my cases on center.

[Char-Gar]

.002 under bullet diamter

[uscra112]

If you are reaming, the reamer diameter can be +/- a couple of thou depending on the O.D. of the neck as you ream it. Be thinking about aiming for a particular neck wall thickness, look at your reaming setup, and choose a reamer accordingly.

As I remember, the old Wilson reaming technique used a die like a sizer, and the reamer was piloted in the top of the die. The die compressed the neck to a known diameter (and also made it round), and the reamer was guided concentric to the case. It can't have been too bad a technique, Wilson was no fool. But it required the special die and a specially made reamer.

Reaming an unsupported case would allow a lot of error, methinks.

[Twmaster]

That would not be that hard to do then. Take the decapping pin out of the die, run the ram all the way up. Then run the reamer into the case mouth from the top of the die.