Although not my longest the one I am most proud of was with an AR15 20" 8 twist shooting 50 gr zmax with 8208 @ about 3350 fps scope was an old Nikon Monarch 4x12. With the farmer there a ghog came out @ 480 yds I dialed it in and shot prone with bipod and rolled him with I shot. I don't know which one of us was more surprised.
What was your longest 223 varmint shot and what kind of load did you use to achieve it? Rifle and scope?