Q. What is the best round for varmint hunting?
There are lots of options and opinions, but popular choices are the Hornady V-Max and the Sierra BlitzKing, both "ballistic tip" designs. The combination of extreme accuracy and quick expansion makes them an excellent choice. Others include the Nosler Ballistic Tip and the Speer TNT. Generally a very light (40 - 50 grain) round with high velocity (3300+ fps) make the best varmint rounds.
For larger, tougher varmints, such as coyotes, the 60gr V-Max is an excellent choice.
Fact: FMJ rounds like M855 and M193 as well as heavier JSP and JHP penetrate too much to be as effective in varmint hunting. There isn't enough tissue in Thumper for them to yaw. Lighter, weaker bullets are a much better choice.