What's the story with this? Performance, reliability???

I take it your talking about 5.56x45/.223 and 7.62x51/.308? Because as far as I know 7.62x39 and 5.45x39 are military only calibers. Meaning there are no "Sporting" (SAAMI?) versions (I don't mean that there aren't any sporting rifles in these calibers).

Gun Zone has a quick write up on the differences.  Ammo Oracle talks about it to.

Basically the military versions are bigger (not really, but it's easy to think of it this way). So it's not safe to shoot military versions of a caliber in a sporting chamber. But visa versa is ok. Chamber configuration is really the difference. Also accuracy might suffer when shooting .223 through a 5.56 chamber (because the longer leade in the military chamber makes the .223 bullet jump further before finding rifling).

That said, many people say they've put thousands of rounds of 5.56 through .223 chambers with no problems. I think what might potentially cause the most problems is having a very tight match cut .223 chamber and shooting NATO 5.56 ammo in it.


Edited to put the right damn cartridge size in.....thanks Schulze!

I'm dealing with 7.62 only. I contacted a company today that does several build services on receivers they sell. I asked if they could press my barrel back in for me after they parked and rivet pressed my trunnions on. They said yes but that they would headspace for a sporting rifle setup as the military spacing is "too long", this is "how they do it". Honestly I think they were after the redrill and pin necessary to change headspace so accordingly they wanted $150 for this. I deferred and said "have a nice day". Granted I know there in business to make money so I wont be taking that service although I will do the rest.

Well, they are right about one thing. Longer *would* be a better descripiton than bigger. This would be the increased leade in the military round.

However, just as you take a 5.56x45 and “shorten” it to make .223, and take a 7.62x51 and "shorten" it to make .308, shortening a 7.62x39 would have to make a different cartridge (5.56x45 and 7.62x51 *are* technically different cartridges than .223 and .308).

If this is so, and the company in question is shortening the chamber from the military configuration, what cartridge is the result?


Edited to put the right cartridge size in.....thanks Schulze!

Modifying the chamber shorter - bad idea!

Reseting the headspace to tighter specs than military - OK but why bother?

Quoted: Reseting the headspace to tighter specs than military - OK but why bother?

Yeah, and I suppose that this is probably all they really meant. Cutting chambers to tight tolerances is SOP on a match rifle.

Spiff, why do you not accept site IMs?

.308 = 7.62x51, not 7.62x54

Quoted: Spiff, why do you not accept site IMs? .308 = 7.62x51, not 7.62x54

Yup, my bad. Sorry for the mis-information everyone. What makes this even worse is that I *own* an M1A!! And I have hundreds of Australian 7.62x51 surplus.

I don't know why I haven't set up site IM's. This is the first time I know of that anyone
has tried to IM me!!