I have been reloading .223 for a few friends who shoot AR-15's. They gave me a bag of used 5.56 brass to load for them; do I need specific dies or can I use the .223 dies? Are there any do or don'ts when loading the 5.56?
5.56 and .223 are identical. The only thing you may have to change is the FL size die, but only if the reloads are intended to be used in automatic rifles. If so, then you may have to use a small base fl size die.
But, like I said "you may have to" not you will have to. If you already have the dies, load some up and test them. The only real problem you may run into is that they may not fully chamber. If that is the case then switch to a small base size die and you should be good to go!
Some make a hoop and cry that the 5.56 and .223 are "different." Well, semi, sorta. The cases are the same so dies are the same. GI 5.56 is loaded a bit hotter and, sometimes, with different bullets that won't shoot well in standard .223 rifles but we reloaders can ignore that.
Some auto loaders benefit from small base dies but most rifles don't seem to need them.
Couple of things to address here. 1) The difference in the 5.56 and the .223 rem centers mostly around the throating of the rifles themselves, and the max pressures used by SAAMI and the NATO military ammo; a piece of fired brass can be reloaded without regard to which it originally was to begin with. As far as small base dies go, they're not generally required for ARs, but I'd recommend something like the Redding, which sizes down to minimum specs with their standard FL dies. Unless you actually see a problem, don't automatically assume that an AR needs a SB die. Semi autos kill brass quickly enough as it is, no sense in compounding the problem by excessive sizing if you don't have to. 2) Military 5.56 brass, at least that made by LC (our only US Army Ammo plant), has virtually the same capacity as commercial .223 brass, and in some cases even more. This caution got carried over from the military 30-06 and 7.62 NATO (308 Win) brass, which definatelydid have less capacity, and didneedcharges to be reduced compared to what their civilian counterparts would hold. Most LC brass of rcent years has gone 92 to 94 grains, or thereabouts. Compare it to Win, Rem or Fed and they're pretty much in line. You do not need to reduce loads for the .223/5.56mm military cases. 3) It sounds as though you're new to loading for the ARs? Tread slowly, for here thar be dragons. The ARs aren't nearly so bad as the M1 or M14 Service Rifles, but they do have their own share of quirks. Like it or not, they constitute "advanced handloading," as does any semi-auto. Powders must be selected carefully in regards to both bullet weights and port pressures. Primers become quite critical, and correct primer seating needs to be monitored very closely. Gages, from something as simple as Wilson's to as complex as the RCBS Precision Mic or the Redding Instant Indicator are virtually required for properly loading these guns. I'd strongly recommend that you hit Glen Zediker's books on reloading for the ARs, as well as those by John Feamster and Derrick Martin. All will give you some new insights into feeding the mouseguns properly. 4) Loading for friends . . . lotsa folks do it, I'm just not one of them. I won't load ammo for anyone, and there's very few people I'll let use my ammo. By the same token, the people whose handloads I'll shoot in my own guns can easily be counted on my fingers, with several to spare. Yes, I do still have all 10, and I intend to keep it that way. I'm more than willing to help anyone get into reloading, and will teach them anything I can. But I won't load ammo for them, and I don't want them loading it for me. I've seen too many guns blown up, slam fires, out of battery firings, handloader induced jams, etc., to be comfortable with this idea. In today's society, that's just too big a risk for me to casually assume; especially in a Servie Rifle. Your call here, that's just my take on it, for what it's worth. 5) You might take a peek over on usrifleteams.com for some info pertaining to reloading for Service Rifles, as there's several very good threads on this topic over there. After all, the AR's dominate the Service Rifel world today, and for good reason. Great rifles, all the way around.
Anyway, just a little food for thought, and I hope it helps.