Max neck thickness of necked down brass?

[JackSprat]

I have necked down 308 brass for my 7mm08 every since I got ,and never thought twice about neck thickness,but here recently I got some 308 brass from a member on here,and I necked some down to 243..I ran them thru the 7mm08 dies,and then the 243 dies every thing went fine,and out of the twenty I did I had no problems except for 2 dented shoulders from too much lube..I measured the neck of them after loading,and they measure .274,and from looking at the dimensions of the 243 in the loading manuals they have the neck measuring .276,so I'm assuming that .276 is max diameter..Am I right in assuming these are safe,or am I missing something?.Thanks

 

[Toprudder]

I'm no expert, but I think what is important is the diameter of the neck after you seat a bullet. It must be able to fit the chamber of your gun.

 

[someguy2800]

correct

 

[JackSprat]

I'm no expert, but I think what is important is the diameter of the neck after you seat a bullet. It must be able to fit the chamber of your gun.

The .274 was what I was getting after seating a bullet.

 

[someguy2800]

Do you have any fired cases out of the gun? Measure the diameter of a fired case. If its more than one of your loaded cases by a couple thousands or more then you are good to go.

 

[JackSprat]

Do you have any fired cases out of the gun? Measure the diameter of a fired case. If its more than one of your loaded cases by a couple thousands or more then you are good to go.

Yes,I have plenty of fired brass .Thanks,That makes a lot of sense..

 

[243winxb]

.276 is max diameter.

Of a loaded round, correct. Watch for a donut right at the neck/shoulder junction..

SAAMI has drawings of cartridges and chambers. See link below.

 

[JimKirk]

If you really want to know for sure ... Buy a tubing micrometer and measure the thickness of the downsized neck thickness compared to normal factory made brass ... You'd also need to do a chamber cast and measure it ... That if you want true measurements...

 

[Bart B.]

Mic a bullet diameter and a loaded round diameter. Half the difference is the neck wall thickness. A couple thousandths spread is not an issue.

As long as the loaded round's neck is at least .005" smaller than chamber neck diameter, all's well.

Less clearance doesn't align shoulder headspacing bottleneck case necks any better centered in chamber necks when fired than those having lots of clearance. 243 Win case necks center perfect in 308 Win chambers.

 

[spitballer]

measure the thickness of the downsized neck thickness compared to normal factory made brass ...

I'm inclined to agree with this. My only experience is with .223 but with an undersized neck I understand the importance of neck wall thickness. My clearance is only .002 just like yours but my neck walls must be .012" and under because the chamber neck is only .026" bigger than the bullet itself. Judging from the numbers you put forth with your rifle it would appear that either your caliber uses a thicker neck wall or the standard chamber allows a couple of thousandths more clearance than does the .223. If you're necking down from a larger caliber you might even be able to run a thicker neck wall in lieu of an undersized neck, but this is beyond my scope of knowledge. All I know is what the old timers continue to drill: consistency, consistency, consistency

 

[JackSprat]

Do you have any fired cases out of the gun? Measure the diameter of a fired case. If its more than one of your loaded cases by a couple thousands or more then you are good to go.

I don't have anything more precise than cheap calipers,but my fired 243 cases measure .276,and my loaded shells of necked down 308 measure .274, .I shot a few today,and they shot good without any signs of pressure..Sorry for the poor picture quality,but I am not a very good photographer.