300 Blackout neck turning diameter

[brstevns]

When neck turning 300BO made from 223 brass, what diameter do you turn the neck to. Plan on using 309-311 diameter cast bullets.

[kenton]

It looks like the SAAMI spec is .334 max neck diameter for a loaded case.

https://saami.org/wp-content/uploads...2021-06-02.pdf

https://saami.org/technical-informat...ami-standards/

[BK7saum]

I do not neck turn 300 blk cases. I only use those brands known to have thin, correct thickness necks when formed and verify neck diameter on loaded cartridges before first firing. I don't have those numbers in front of me, but approximately 0.003" below the OD of a fired case neck.

[brstevns]

Is it best to turn before or after sizing?

[popper]

I size and then turn, looking for a ten thous neck thickness. Normally just takes off the high spots. PPU is normally thick but wall thickness is pretty uniform. I use the RCBS tool and put the case head in a large elec drill chuck. Cut slowly in one pass. The hand holder tool will wear out your fingers. I use a 0.332 NOE sizer insert to check loaded rounds for neck fit.

[BK7saum]

It would be hard or almost impossible to turn before sizing. You would be trying to turn a tapered case.

I really don't know how you would accomplish that task. Maybe size in a 338 whisper? die (338 off of 221/223), turn with a 338 mandrel, then size down to 300 BLK. That is essentially what I do making 7mm-08 amd 260 rem brass. I use 308 LC brass, turn to correct thickness, then size down in the appropriate die(s). It removes the potential for a donut at the shoulder. I could perform a cleanup cut after sizing, but do not feel that is necessary for my purposes. They are not match rifles.

[brstevns]

It would be hard or almost impossible to turn before sizing. You would be trying to turn a tapered case.

I really don't know how you would accomplish that task. Maybe size in a 338 whisper? die (338 off of 221/223), turn with a 338 mandrel, then size down to 300 BLK. That is essentially what I do making 7mm-08 amd 260 rem brass. I use 308 LC brass, turn to correct thickness, then size down in the appropriate die(s). It removes the potential for a donut at the shoulder. I could perform a cleanup cut after sizing, but do not feel that is necessary for my purposes. They are not match rifles.

I was wondering about that myself, I will turn after sizing.

[country gent]

On my tight neck 243 that has a .267 neck I turn to .266 loaded round dia. What I recommend is to cast your chamber and see what the neck dia is If it is .334 I would turn to .002 under ( .332 loaded round dia) for a AR or semi auto. If a bolt action .333 or .001 under. If your going to neck turn you may as well gain all you can. Fitting the neck may gain some accuracy and case life.

[gloob]

I don't see how you benefit from casting your chamber, unless you have a method of making your necks thicker.

If your cartridges fit and fall back out the chamber, already, just turn to w/e thickness where the lowspots come out of the majority of your cases.

If they don't fit, turn them until they do. Then another thous or two smaller for autoloaders.

[country gent]

If he has thick necks from forming then he needs to know what his chambers neck dimension is to know what he need his loaded round diameter to be. The .334 listed above has tolerances also. a range that is acceptable. Knowing what his firearms neck is may save him some work and allow for a fitted neck.

[crashguy]

I do not neck turn 300 blk cases. I only use those brands known to have thin, correct thickness necks when formed and verify neck diameter on loaded cartridges before first firing. I don't have those numbers in front of me, but approximately 0.003" below the OD of a fired case neck.

here is a link that lists the good and bad brass for conversions

https://www.300blktalk.com/forum/viewtopic.php?t=88599