same thing, some will tell you different but go ahead and shoot them. The military brass is usually a few grains heavier than commercial. LC99 means Lake City arsenal 1999

I have some rounds with LC 99 and the circle with a cross for the headstamp. It's my understanding that these are 5.56 NATO rounds. Can these be shot in a rifle chambered for .223 Rem. If so, what are the differences? Are they more or less accurate than .223. If they can't be used, what is/are the reason(s). Thanks in advance.

Kevin
There is a difference in .223 vs 5.56mm chamber specs. The 5.56 has slightly different throat dimensions. This has been debated several times, but the general consensus was that it is ok if you have a standard SAMMI .223 chamber and don't use the cheap surplus ammo from overseas (very general statement, some of the overseas ammo IMI,PMC etc is good, some is not).
The .223 rifles I own are AR15's with match chambers (still very generous in size compared to some bolt rifle chambers). I have fired good quality 5.56 in them with no problems except the Winchester Q3131 1999 which was a bad lot of ammo. Popped 3 primers in a row.
I wouldn't worry about good quality 5.56mm ammo.
good luck
Covey16

What do you mean when you say "5.56 'white box' stuff?"

I can only tell you that out of three guns I have had chambered for .223 all shot the military stuff fine. Two were mini-14's and one was a contender. I too have heard it is not a good idea to use militery in a .223 but then my understanding was the 5.56 was just throated a little longer.
How does the C.O.L. of the 5.56mm compare to the recommended C.O.L. for .223 Rem?

mfrankart
White box generally means Winchester q3131a 5.56mm rounds loaded to military specs (fairly hot) duplicates m193 specs. It comes in a white box and is fairly cheap, good ammo for the price.
Lake city comes in a white box also, but Federal now operates the Lake City 5.56 reloading equipment and is not as good IMO as when Winchester ran it.
Covey16

Covey,
There is more difference between the various .223 Rem chamber reamers in use than there is between the SAAMI specs for 5.56 nato and .223 rem. I have used the Wylde chamber, the Compass Lake chamber and of course there is the long Colt chamber and the tight Kreiger. None of these are going to run you into any trouble with any factory loaded ammo with the exception of the Kreiger and Federal Gold Medal Match .223, famous for blown primers. The other exception would be the 80 gr MatchKing which is not factory loaded by anyone (at least for public sale).

Kevin
Mine are a Lothar Walther with a Wylde chamber and a Douglas Air Gauged with a SAMMI minimum chamber and a Wylde throat and leade. I don't shoot many non handloads, but have used Q3131a,IMI and Lake City.I have heard bad things about Sellier and Bellot Nato marked ammo, but have never used it. No problems except the above mentioned lot of 1999 Winchester.
You are right , there are a 1001 .223 reamers out there.
My point was that AR chambers, especially run of the mill rifles tend to be much looser than a well built bolt rifle and that my experience was with AR's only.
Your mileage may vary.
Covey16

All my 223 experience is with AR's. I would guess that a lot of the surplus or cheapo stuff out there is loaded for max velocity only. Most of the guys I see burning that stuff up need to be reminded that there is a front sight on that rifle.