Just bought another dozen ammo cans locally.  They are marked  5.56 and have a
o solid dot which must stand for ball then a circle with a + I would imagine that would be nato cross, Then a stencil of a picket fence type thingy which must mean it must have been  in clips, then the # 10 which I would imagine mean 10 rounds per clip.  Then a four leaf shamrock.  That has me beat.  Any ideas of what the shamrock is the symbol for?  It might have cost me a little more for the cans locally but it was sure nice to examine them first and be able to pick my own.  Thanks.  

How about the "NATO Ammunition Interchangeability Symbol"?

From Cruffler, referring to the 7.62:

"Based on the existence of these two distinct markings, it is evident that there are at least two types of NATO 7.62mm military ammunition.  The first, bearing only the cross-in-circle marking, will function in all 7.62mm rifles and machineguns fielded by NATO armies, but may or may not provide the expected ballistic performance.  The second, bearing both markings, will not only function in all arms, but will provide the ballistic performance prescribed in the STANAG (a 147 +/- 3 grain bullet with a muzzle velocity of 2,750 +/- 50 feet per second)."

Quoted: How about the "NATO Ammunition Interchangeability Symbol"? From Cruffler, referring to the 7.62: "Based on the existence of these two distinct markings, it is evident that there are at least two types of NATO 7.62mm military ammunition.  The first, bearing only the cross-in-circle marking, will function in all 7.62mm rifles and machineguns fielded by NATO armies, but may or may not provide the expected ballistic performance.  The second, bearing both markings, will not only function in all arms, but will provide the ballistic performance prescribed in the STANAG (a 147 +/- 3 grain bullet with a muzzle velocity of 2,750 +/- 50 feet per second)." www.cruffler.com/NATO-Interchangeable.jpg

I am a little confused.  The can is marked 5.56MM 830 rounds not 7.62.  Please bear with me on this one brillo.  

Maybe it is 55gr stuff?

The first, bearing only the cross-in-circle marking, will function in all 7.62mm rifles and machineguns fielded by NATO armies, but may or may not provide the expected ballistic performance. The second, bearing both markings, will not only function in all arms, but will provide the ballistic performance prescribed in the STANAG

M193 is not NATO standard, M855 is, but M193 will still function in all NATO 5.56 rifles.

5.56 can be loaded to STANAG stds too.  In fact all sorts of things have STANAG stds like mags and such.

The point of Cruffler's post was to show that ammo loaded to NATO and STANAG standards doens't necessarily mean the same thing.

+1

The cross in the circle denotes that the ammunition is interchanable (i.e chamber anf fire safely in NATO firearms) with other ammunition of the same calibre, also bearing that cross in the circle mark. But will be ballistiacally different

The clover leaf means that additionally the ammunition will expect to have equal ballistic performance, as other ammunition with the clover leaf.

See the markings on these boxes :-

5.56mm Boxes

Now I get it.  Thank you all.