That seems very, very odd coming from Armalite. Very.
YHM's website only shows two diameters for 5/8x24 threading, .920" and .720", but you can have it stainless or black. Four possibilities, when you include color. This would be easy to determine for your barrel, with a tape measure - it's either close to 3/4", or it's bigger than that. With that, you can order the smaller, or larger, YHM thread protector.