Depends on what varmint you are banging. I like 17s, 20s and 223s for PD size of varmints and I also use all of those for coyotes. But is I am serious about killing coyotes, I only use 223s, 22-250s and 220S guns.
If I could only have one varmint gun, it would be a bolt 223.
Exactamundo.....
