The can of worms. You have opened it.
Go watch
this. Best video I've ever seen on the subject. He's actually talking about firing factory ammunition, but the relevant stuff for reloading is when he does water volume tests.
I'm also new (newer than you) but I've done a fair amount of research on this. I don't think by any means that I have the final answer on it, but here's my two cents.
AFAIK, the short answer is that the spec for .223 and 5.56 is virtually identical in terms of the exterior dimensions of the cases. Much of the confusion in this area comes from the fact that the
chamber of 5.56 differs from .223, not the cartridge dimensions. 5.56 has a longer leade and generally a looser chamber, and has a higher maximum overall pressure rating, which makes it inadvisable to fire military 5.56 in a sporter .223 bolt action rifle. But that's all about factory ammunition, not your reloads.
The 5.56 spec has a difference in the internal case volume when compared to .223, and you'll generally find that 5.56/military cases are thicker walled than commercial cases, which means that you probably shouldn't take your tried and true load data for your commercial brass and just use it in your military brass. Personally, I treat the headstamp of the brass as one of the variables in a load, so this isn't an issue for me.
Of course there's the crimped primer issue. But if you're picking up range brass I actually see that as a benefit-- you can be sure that any brass with a military crimp has been fired only once.
One thing I actually disagree with in the video above is where he opens the Hornady load data book and points out that load data for '.223 Service Rifle' is different from '.223 Remington' and states that you can't therefore fire 5.56 cases out of your bolt action rifle. There's actually another section after that specifically for 5.56 loads. I emailed Hornady about this directly:
I want to start hand loading for my Savage 10 Precision Carbine. It's a bolt action rifle chambered for .223, and has a 20" barrel with a 1 in 9 twist rate.
I understand that heavier (longer) bullets are more accurate using this barrel's quick twist rate, and that it may not be ideal for 55 grain or lighter bullets. Using Greenhill's formula, I see that I'll want to configure loads with at least 60 grain and ideally 68 grain or heavier bullets because of their longer length and better ballistic coefficient.
I recently picked up your Classic loading kit (great product, BTW) and the 9th Edition handbook only shows load data for .223 Remington up to 60 grains, on a Remington 700 with a 1 in 12 twist rate. Data for bullets heavier than 60 grains is only shown in a section titled '.223 Remington Service Rifle Data', and the tests used an AR with a barrel that had a twist rate matching mine (1 in 9).
I know that the SAAMI specifications for 5.56 NATO differ enough from the .223 specs to make it unwise to use 5.56 rounds in a .223 rifle, so the 'service rifle' part of the load data is spooking me a bit. Is it safe to use the data specified in this section for 68 grain and heavier bullets in my bolt action .223 rifle? If not, where can I find safe load data for these heavier bullets?
Their answer:
..you will be fine to use the Service Rifle data in your Bolt action rifle. These pressure levels will be well below the max range of your bolt action rifle
TL;DR: work up your loads when you use different brass
