Rifle is a SIG 556.

I was chrono'ing some LC 5.56 (XM193F) I got from Palmetto and some Federal .223, also from Palmetto. Both loads 55 gr FMJ BT. The LC moved out at an average of 3070 fps, the Federal at 2830 fps.

I haven't weighed the empty cases yet but I've always used my chronograph as one of the guides when developing loads. Does anyone think the case capacity is so different that I should keep them separate and develop loads for each? Or is the .223 loaded a little cooler for use in any rifle?

5.56 Nato Lake City is hot stuff

The throat in a SAAMI .223 is .0250 and the throat in a AR15 is .0500. Both the .223 and the M193 5.56 are loaded to 55,000 psi. Your .223 velocity was lower because it was fired in the longer AR15 throat.

quote:

Originally posted by bigrdp51:
The throat in a SAAMI .223 is .0250 and the throat in a AR15 is .0500. Both the .223 and the M193 5.56 are loaded to 55,000 psi. Your .223 velocity was lower because it was fired in the longer AR15 throat.

I never knew there were this many differences. Thank you.

If both rounds are 55,000 psi and they are fired in the same rifle the velocity would be the same. 5.56 is not as hot as .223. The throat and chamber differences are related to allowing build up in the chamber for full auto between cleaning in military rifles and a shorter throat in .223 for shorter lighter bullets. .223 will generally have a slower twist also.

FWIW. Accurate's data sheet for LT-32 separates 5.56 from .223: the 5.56 is loaded with a tad more propellant than is the .223 with the same bullets.

There is a pressure difference of about 7,000 Psi between the two, and Accurate lists both the SAAMI (.223) and something they call the "CIP Commercial and NATO/MIL Specification" for the 5.56.

The reason I asked was because 3070 fps seemed a bit stout out of a 16" barrel. My three rifles all have 16" barrels.

I don't have a lot of experience reloading this round but I was thinking 2900 or so seems reasonable.

And I always gratefully welcome comments from everyone.

Edit: if it's not a waste of time I was going to compare case capacities too.

Comparing 5.56 vs .223 people focus on nominal case dimensions and capacity. After all, 5.56 mil brass is not much different than most commercial .223 (unlike .308).

They overlook the importance of differing chamber/throat dimensions, and differing loaded cartridge power levels.

It's one of those things you get away with until the day you don't. Maybe an exaggerated comparison to illustrate, but does .45 Super in .45 ACP work as an analogy?

Risk stacking comes into play, as does chronic overload stress. Can't remember the source, but someone here documented some pretty radical piezo pressures from 5.56 in a .223 chamber.

Agree those KBs (other thread) weren't likely due to excess headspace. There is such a thing as normal pressure KB due to such things as reverse-annealed or other failure of brass. But very rare.