Case Dimension question?

[Cliff Mitchell]

I'm just starting out reloading. I size my 243 brass today the specs say it should be .276 width at the neck im reading .270 is this going to cause problems. I checked a box of ammunition same brand and it checks at .270. I know this might sound like a completely dumb question but better safe than sorry

 

[sedancowboy]

With or without a bullet seated? .243+.014+.014=.271 sounds about right.

 

[BallisticsGuy]

What specs? From where? .276" would be an acceptable dimension for the neck area on a reamer meant to make a chamber that will eat whatever .243 ammo you put in it but not for the sized brass, with or without a bullet in it.

 

[Cliff Mitchell]

What specs? From where? .276" would be an acceptable dimension for the neck area on a reamer meant to make a chamber that will eat whatever .243 ammo you put in it but not for the sized brass, with or without a bullet in it.

Hornaby reloading book and online. After reading your response I went and found some brass that I hadn't resized yet they're sitting at .276 it's looking as though all the information I'm seeing is for the maximum allowable case size.

 

[J E Custom]

SAMMI Maximum neck diameter says .276 some chambers are more or less than this so I would recommend that you measure a fired case and that would tell you within .001 (Allowing for spring back) of what your neck chamber dimension actually is.

Then after loading a round of this other brass, you should have a .003 to .004 smaller loaded neck diameter than your chamber neck diameter.
To prevent over pressure. Less than .003 is never recommended unless the necks are turned to a precise thickness and even then pressures can appear at .002 thousandths. Up to .008 is still Ok but some consider it excessive on all but dangerous game

J E CUSTOM

 

[DUSTY NOGGIN]

that term is , neck clearance
.276 chamber, subtract loaded round .270 = .006 neck clearance

more than .004 gives some room , when chambering a round and releasing the bullet
more than .008 and the brass neck doesnt make a good gas seal

 

[Cliff Mitchell]

SAMMI Maximum neck diameter says .276 some chambers are more or less than this so I would recommend that you measure a fired case and that would tell you within .001 (Allowing for spring back) of what your neck chamber dimension actually is.

Then after loading a round of this other brass, you should have a .003 to .004 smaller loaded neck diameter than your chamber neck diameter.
To prevent over pressure. Less than .003 is never recommended unless the necks are turned to a precise thickness and even then pressures can appear at .002 thousandths. Up to .008 is still Ok but some consider it excessive on all but dangerous game

J E CUSTOM

I'm slowly learning fired brass is about .276. Factory ammo is running at .270 and my sized casings are running .270 to .271. So I guess I'm looking okay and thank you for your help

 

[J E Custom]

I'm slowly learning fired brass is about .276. Factory ammo is running at .270 and my sized casings are running .270 to .271. So I guess I'm looking okay and thank you for your help

You are good to go.

J E CUSTOM

 

[Cliff Mitchell]

You are good to go.

J E CUSTOM

How about neck width after sizing I'm running .270 which seems to be Factory give or take a thousands. I ran a different manufacturers brass and after sizing I'm running .267 -.268 is this still within the safety zone.

 

[crkckr]

For one, the diameter of sized brass will change when you seat a bullet. Plus there is always some variation in the thickness of the brass from different manufacturers, sometimes a really big difference. When people are chasing that last bit of accuracy they will usually stay with one brand of brass just for this reason. If you're not after that last possible .0001" in accuracy it probably won't matter to you. The thicker brass will change your neck tension, which if varied a lot, will have some effect on accuracy AND pressure. If you're loading any maximum loads, stick with one brand of brass. If you change brands, you might need to work up the load again, watching for pressure signs.
Cheers,
crkckr

 

[J E Custom]

How about neck width after sizing I'm running .270 which seems to be Factory give or take a thousands. I ran a different manufacturers brass and after sizing I'm running .267 -.268 is this still within the safety zone.

Yes. The main thing about neck diameter, is not to get to close to the neck chamber dimension. As long as you have 3 to 4 thousandths clearance between the neck chamber and the loaded round, the case can release the bullet without excessive pressure.

The other brass is thinner and the reason it is smaller. Some brass has thicker cases walls and can cause pressure if this 3 to 4 thousandths is infringed on.

J E CUSTOM