.274" barrel neck vs measured loaded neck (~.269") — what's going on?

[cjjltx]

I know this is a basic question, but I'm a bit confused. When I started working with 6mm caliber, I noticed that the vast majority of pre-fit barrels list a neck diameter of .274", while bullets such as the Berger Hybrid 105 gr have a diameter of .243". Case neck wall thickness for both Lapua and Alpha is around .013" each side, so together that adds about .026". The simple math is .243 + .026 = .269", which matches the loaded rounds I've measured. When I deliberately increase the case neck ID, the bullet simply drops into the case.


So here’s the problem: if the barrel neck diameter is .274", and we know the ideal loaded neck diameter should be barrel neck −0.002–0.003 (i.e. about .271–.272"), how is that achieved? I can’t find bullets with diameters of .245–.246", and I also can’t find barrels with neck diameters of .271–.272". How are these numbers reconciled?

 

[243winxb]

The loaded rounds outside diameter (OD) & chamber should not measure the same. The loaded rounds neck needs .002"+ clearance to allow release of the bullet, without rasing pressures.


Buy a fl *bushing die* & size only 1/2 of the neck. The unsized part will expand to the rifles chamber after 3 or more firings.

Your chamber neck diameter would seem smaller then SAAMI standards. A good thing.

My 243 win brass, fired many times.

 

[dgeesaman]

I know this is a basic question, but I'm a bit confused. When I started working with 6mm caliber, I noticed that the vast majority of pre-fit barrels list a neck diameter of .274", while bullets such as the Berger Hybrid 105 gr have a diameter of .243". Case neck wall thickness for both Lapua and Alpha is around .013" each side, so together that adds about .026". The simple math is .243 + .026 = .269", which matches the loaded rounds I've measured. When I deliberately increase the case neck ID, the bullet simply drops into the case.


So here’s the problem: if the barrel neck diameter is .274", and we know the ideal loaded neck diameter should be barrel neck −0.002–0.003 (i.e. about .271–.272"), how is that achieved? I can’t find bullets with diameters of .245–.246", and I also can’t find barrels with neck diameters of .271–.272". How are these numbers reconciled?

You’re mistaken believing that neck clearance ‘should’ be .002-.003. That is the lower limit, and a dangerous limit to flirt with unless your brass necks are absolutely dead consistent in thickness (aka turned).

More is very much acceptable. Shooting a .269 in a .274 is fine and about right for an unturned neck.

There were days when benchrest shooters thought there was advantage to be explored by making the neck clearance small. That didn’t appear to pan out as winning shooters shoot no turn necks regularly now.

David

 

[243winxb]

When I started working with 6mm caliber,

Need to know what 6mm? Each 6mm cartridge neck, can be slightly different.
Doesnt seem to be a 243 Win?

In 243 win, i tried using 308 win brass, sized in a 243 win fl die, then neck turning, to better fit chamber. Lots of work for no improvement over the bushing die.

 

[HTSmith]

More is very much acceptable. Shooting a .269 in a .274 is fine and about right for an unturned neck.

Yep. You don't have a problem

 

[47WillysGuy]

So here’s the problem: if the barrel neck diameter is .274", and we know the ideal loaded neck diameter should be barrel neck −0.002–0.003 (i.e. about .271–.272"), how is that achieved? I can’t find bullets with diameters of .245–.246", and I also can’t find barrels with neck diameters of .271–.272". How are these numbers reconciled?

You are quoting diametrical clearance, this is to tight.

You want 0.002" - 0.003" on a side which would be 0.004" - 0.006" diametrical.

 

[243winxb]

.002"+ clearance

You want 0.002" - 0.003" on a side which would be 0.004" - 0.006" diametrical.

.002" each side. Total .004" Correct. I may not have made that clear.

 

[bks22345]

I know this is a basic question, but I'm a bit confused. When I started working with 6mm caliber, I noticed that the vast majority of pre-fit barrels list a neck diameter of .274", while bullets such as the Berger Hybrid 105 gr have a diameter of .243". Case neck wall thickness for both Lapua and Alpha is around .013" each side, so together that adds about .026". The simple math is .243 + .026 = .269", which matches the loaded rounds I've measured. When I deliberately increase the case neck ID, the bullet simply drops into the case.


So here’s the problem: if the barrel neck diameter is .274", and we know the ideal loaded neck diameter should be barrel neck −0.002–0.003 (i.e. about .271–.272"), how is that achieved? I can’t find bullets with diameters of .245–.246", and I also can’t find barrels with neck diameters of .271–.272". How are these numbers reconciled?

If you want to change your neck clearance, you change your brass wall thickness or your throat diameter. Not the bullet diameter. IF you want to run .002 or .003 in a .274 neck, you are going to need brass with a .014 wall thickness. If you cannot find that, you will need a custom reamer. But as others have mentioned, most people have moved away from tight necks. .005” is the norm as far as I know.

 

[bks22345]

.002" each side. Total .004" Correct. I may not have made that clear.

So .004 you need .0135 brass.

I don’t know about the smaller stuff but I’ve got Lapua 284 Win brass that’s .015-.016 and Lapua 300 WSM brass that’s .016.

 

[joshb]

So here’s the problem: if the barrel neck diameter is .274", and we know the ideal loaded neck diameter should be barrel neck −0.002–0.003

You’ll have to get a custom chamber cut.
The “tight neck clearance” chamber is an expert level practice for removing variables that may allow the bullet to enter the barrel “crooked”. In laymen’s terms, anyway. Excess clearance is more forgiving for us amateurs that might not turn perfect necks or keep our chambers perfectly clean of carbon rings and such. No turn chambers save us from a lot of precise work.
I like more clearance for my casual shooting guns and most of my custom hunting guns have no turn chambers. I don’t compete so life Is more relaxed.
I just had a 30br barrel made for a hunting rifle. The chamber was cut to a .339 neck dimension. My expanded unturned Lapua 6br cases measure .332 loaded.
Set your calipers at 7 thousandths and look at it. It’s pretty small!