Chamber Neck Diameter

[nicholasjohn]

I've found loaded factory ammo to have some taper in the case necks. I measured some Norma case necks recently ( 280 Remington ) and they were slightly bigger around right ahead of the shoulder than they were at the case mouth. This stands to reason, because the walls of the case were tapered before the necks were even formed. If the neck walls are slightly thicker near the base of the neck than they are near the mouth, the outside diameter of the loaded cartridge should show a little bigger diameter. I don't remember how much bigger around it was, but I was definitely not the same diameter. Has anybody else noticed this ???

 

[kiwi49]

Chambers have the neck part of the chamber tapered for extraction advantages. The cartridge case will be straight when sized and while loaded for uniform grip/tension on the bullet. Only when fired will the cartridge take on this .001 thousandths taper. the fact that it only occurs when the case has been fired has no effect on accuracy or anything because the neck will be .003 or .004 larger than the bullet after firing.

Just like un-fired brass, turned brass will not have a taper until it is fired in the chamber. Even straight sided cases (Like the 45/70 and others) have a slight taper on the outside for extraction. If the neck was perfectly straight, potentially it could be mechanically bound up if debris was left in the chamber from previous firings but the .001 thousandths taper all but eliminates this phenomena.

The reamer has this same amount of taper in it in order to cut the chamber the same as the Spec. As Shortgrass said "No Problem"

J E CUSTOM

A VERY clear explanation - thank you

 

[J E Custom]

I don't know anyone who ordered reamers with the neck taper in them on modern cartridge designs.

Well, I guess you do now
I order it like the parent case was designed. If it had it that's the way I order reamers, If it didn't have the tapered neck I don't have it ground that way, unless it is a wildcat with very little body taper and then I do. I couldn't tell anyone, that I can tell the difference ether way, but knowing it's purpose I use it when I feel it could help.

J E CUSTOM

 

[J E Custom]

After this discussion, I though I would research this subject to see if I was doing something unnecessary and this was the results of what I found.

Some of the newest cartridges don't have any neck taper in the chamber

Most of the older cartridges do have it and it is normally .001 thousandths taper.

Many Wildcats and European cartridges have .0015 total neck taper

Interestingly the Weatherbys generally have .002 thousandths taper.

And the military cartridges all seem to have between .0035 and .004 thousandths neck taper. The reasoning makes sense that a stuck case neck or separation during battle could be catastrophic.

So once again. the reason varies, but the designer had theirs and knows more about what is necessary than we do and what is needed for the task. With the range of neck tapers used and the quality of ammo and components available the straight neck "Should" Be Ok. But I will continue to use the minimum (.001) until there is proof that straight is better/more accurate.

I did find it interesting that different applications use different neck tapers. Learn something new every day.

J E CUSTOM

 

[tobnpr]

^^^
Good discussion.
Always wondered about the neck taper myself...assumed it would be for extraction.
Seems to me logical, and "couldn't hurt" so to speak- and I dont doubt cartridge designers simply thought the same way.

Probably makes no difference in reality (as mentioned, grinding tolerances amount to half of it), which would be why new designs don't incorporate it. Would be my guess...

 

[J E Custom]

^^^
Good discussion.
Always wondered about the neck taper myself...assumed it would be for extraction.
Seems to me logical, and "couldn't hurt" so to speak- and I dont doubt cartridge designers simply thought the same way.

Probably makes no difference in reality (as mentioned, grinding tolerances amount to half of it), which would be why new designs don't incorporate it. Would be my guess...

My guess is that I can prevent stuck cases and that would/could be important. If for some reason the neck is cut straight and ended up with some taper or fouling the wrong way, it could mechanically bind up the case. and prevent extraction.

With today's precision, I could see the chances of this happening would not be great, but on dangerous game and in battle only one time would be one time to much.

J E CUSTOM