Why same calibre rifles chambers are different sizes?

[disko]

Hi,

I'm doing some reading on reload and accuracy to get my info together for the future... I see that only using brass fired in your own rifle is a popular notion for reloading because using brass that was originally shot in a difference rifle won't suit your rifle as well after you reload it because of difference chamber sizes.

If I had a .308 for example, I would have thought that a .308 is a .308? Regardless of brand or model of rifle, brand of ammo etc. It's all ".308 Winchester".

Why does one .308 rifle (or whatever calibre rifle) have a different chamber size to another of the same thing?

[tarnagulla]

I am not an expert on "why", but I do know that what you have read is correct. As I understand it, as a chamber reamer is used, it wears, albeit very slowly. Over time, before it reaches what the particular manufacturer considers the end of its useful life, as it wears it produces chambers ever so slightly smaller (or tighter) each time.

Cases are produced to a standard size, which means they should chamber in such tighter chambers. If fired in an original chamber (i.e, the largest a brand-new reamer will cut) they will expand to fit (the process is called fire-forming). If you then try to fit such cases into a chamber cut by an almost worn-out reamer, they will only fit with difficulty, if at all, without full-length resizing, which cuts case life by up to half.

In practice, as a Military Rifle Discipline shooter, I tend to keep a separate batch of cases for every rifle I use, of the same calibre. So does everyone else in my Club!

[Aster]

As a specific example, I know in some mounted machine guns they use a larger chamber to prevent jamming.

A bolt action rifle's firing cycle is relatively slow, where as a full auto high-power weapons system is going to be generating huge fluxes of heat and energy very rapidly - punishing the action and brass going through it. The larger chamber in this instance makes the chance of brass jamming less likely due to heat/expansion.

As for your average bolt action rifles, any differences in chamber size are just manufacturer preference for whatever reason they have prioritized. Could be a bit larger for smoother feed/ejection, or could be a bit tighter for velocities sake.

In practical terms, I find using other peoples brass is just a pain in the ass. My rifles load all their cartridges smoothly, in the past whenever I had tested out cartridges made by friends with different rifles though, their's can be tough to feed/eject smoothly.

If you're shooting for ultra accuracy, your brass will place a small part in that too. But for hunting / fun target shooting it's more just a matter of smooth feeding ammo.

[Herdsman]

In practical terms, I find using other peoples brass is just a pain in the ass. My rifles load all their cartridges smoothly, in the past whenever I had tested out cartridges made by friends with different rifles though, their's can be tough to feed/eject smoothly.

This right here.

[inspector]

As a specific example, I know in some mounted machine guns they use a larger chamber to prevent jamming.

A bolt action rifle's firing cycle is relatively slow, where as a full auto high-power weapons system is going to be generating huge fluxes of heat and energy very rapidly - punishing the action and brass going through it. The larger chamber in this instance makes the chance of brass jamming less likely due to heat/expansion.

I dunno, you seen the speed that some trick shooters can put out with a bolt action?

Looks like they'd be going at a few hundred rounds a minute

[disko]

Looks like they'd be going at a few hundred rounds a minute

I'm just a bit slower than that at the moment so shouldn't be a problem