I used to buy either m855/ss109 or mk262. The M198 or whatever the 55gr stuff is is good out of 20" "muskets" but loses out the farther out you get due to the wind pushing it around and loses out the shorter the barrel is.

Plus, many of the optics like ACOGs are made with m855 in mind when it comes to the reticles and drop compensation.

M855 isn't great but it isn't as bad as a lot of people say it is. All the barrels I've had will do 1.5 moa or less with m855 and some have even done sub moa sometimes.

Mk262 is the way to go though if you can afford it or at least zero with it and have a "combat stash" and use the rest for range practice stuff and plinking. It isn't as expensive as it used to be and match grade ammo doesn't go up as much as the other stuff does during panic buying either.