243 AI neck diameter question.

[Prairie605]

I noticed the other day while testing some new loads on my 243 ackley that I couldn't push a bullet into the neck of a fired piece of brass. My 30 calibers have always been loose when trying the same thing. I could probably force it in but didn't really try that hard. It was recently re-barreled with a Bartlein and is shooting great.
The fired brass neck measures .274 before sizing. The sized brass measures .270 before loading. And the loaded round measures .272.
I checked 20 pieces of the Lapua brass I shot yesterday and all were exactly the same at .274 after firing.

Should I be concerned about this? It's not a tight neck chamber as far as I know. Just the standard dimensions off of a PTG reamer I believe.
I've read that .003 difference in loaded and fired brass is good. And I've read that you add .001 to the fired brass to account for brass shrinkage after firing, so if that's the case I have the .003 needed for good expansion?

 

[Mikecr]

I'm trying to understand why you think there is a problem. Your clearance and sizing is good.
Is it because bullets pass through an expanded neck?

If this is the case, remember that a bullet is fully released even at 1 trillionth (0.000000000001) over bullet diameter. So with a loaded neck OD of .272, the bullet would fall out with that same neck expanded to .272000000001".
Does it take much pressure to do this? No, very little, way less than needed to obturate bullets.
This release pressure is effective neck tension, which is adjustable.
It also means that ANY neck clearance, any at all, is sufficient to release bullets. I have tested this from .001 to .000001 clearance(best I could tell), and while POI changed, velocity did not.
This was a highly controlled experiment -do not try it otherwise.

 

[Prairie605]

Mike,
My original concern was that I couldn't take a bullet and easily put in the neck of a fired case. My 30 calibers, after fired, you can drop a bullet thru the case neck with room to spare. The 243 is really tight doing the same activity.

Wanted to make sure the chamber wasn't cut too tight.

 

[Mikecr]

I'm sorry for my misunderstanding.
If the fired neck measures .274, and sized/seated measures .272 (which is good) then you have 2thou of something in them necks to stop the bullets. Sounds like donut.

Do you FL size the necks?
How deep does a bullet drop freely in a fired neck until stopping?

 

[Prairie605]

The bullet won't go in to the fired case at all without a lot of force.
Yep, i full lenght size the neck.

 

[J E Custom]

You should have a "Minimum'' of .0033 thousandths difference from a loaded round to a fired round.
I recommend .004 difference to prevent any pressure problems and to allow for bras that may be a little thicker (Not Turned).

SAMME neck dimensions are .277 and free bore dimensions are .2463 (.0033 larger than the bullet. So if your neck dimensions
after firing are less than .2463 you do not have a SAMME chamber and may Have a tight neck chamber.

If you have the correct diameters before and after firing the necks could be long jamming them into the throat preventing them from expanding at the front of the neck. you can take one of the fired cases and chamfer the inside of the neck with your loading chamfering tool to relieve/increase the
case mouth and if the bullet goes in all the way after being chamfering on the case mouth, you need to trim your cases .010 shorter (All should be the same once you determine you actual chamber length.

I have seen this many times (The neck chamber was to short for the case length).

J E CUSTOM

 

[Mikecr]

The bullet won't go in to the fired case at all without a lot of force.
Yep, i full lenght size the neck.

Then your measurements are wrong. Stop FL sizing your necks also. Pick up a bushing die.

 

[Prairie605]

JE customs,
I chamfered a fired case and there was no change. I trimmed the case down .010 and tried again. Really no change.
I can push the bullet in and once it gets past the boattail, i can spin it with minimal pressure. They just dont fall in the fired case like the other rifles i have had.
Its still tight, but doable with a little force.