I'm thinking .260 or 6.5 CM, maybe .243 is about right.

What's your opinion or favorite round for this duty?

6.5 Creedmoor

You've got the BC edge over the 243, makes the same velocity as the 260, but with a lower ES.

243 win 6 mm rem, 240 WBY mag, 25-06, 257 WBY mag,

Any of these will do great.. I have done pretty well with my little 6mm BR too

Ed

If it is just for varmints I would go with a 22-250. My buddy has a savage that is pretty bad medicine to 800, that is the farthest we had room to shoot it.

Walter

Kinda like asking what color of truck to buy.

Swift

When you get out beyond 400 yards, the wind has a big effect, so I would have to lean to something with good BC. That takes away the 22 and 25 calibers (unless you went 90 grain in 22). If you look at the wind drift of the 22's and compare it to a 6mm at 400 and out, it's a no brainer. 6.5mm is always good, but a fast 6mm with a 105 grain Berger would be ideal. It will be flatter shooting than the 6.5mm, and has less drop and less wind drift. I'm a huge fan of the 6.5mm, but the ONLY time I would recommend a 6mm over a 6.5mm is for varminting. The close second would be a good 6.5mm.

6mm Creedmore... just one more to muddy the waters. Many good choices.

A fast twist 22 caliber would work but the barrel life might be a tad short for PD's. If you want to have any chance of seeing impacts it might be the best option say something running 80gr Amax at 3300-3400fps(22Creed/22-243/22-250AI) As mentioned a 1/8 twist 6mm is probably all around best option as it will handle wind a little better but will need a muzzle break to have any chance of seeing impacts.

I have the 6mm Creedmoor and it will handle 600 yards with the 105gr Berger with ease.

243 with DTAC bullets.

When I lived in South Dakota, most folks shot 22-250 for prairie dogs.

I'd go with a .50 BMG, or maybe a 20MM Vulcan.

22 creedmoor shooting 75 amax at 3350-3420 out of 22 inch barrel

I have shot prairie dogs with a 1:14" twist and 55 gr SGKs leaving at 3780 fps. Outside 500 yards it was tough to get the windage dialed in due to lack of BC. I now run a 1:8" twist and 75 gr A-maxes 3200 fps MV, mucho better-o. But that's about all that rifle is used for.

Even when I go coyote hunting I tend to take my 6.5 Creedmoor. I like having the extra bullet weight and higher BC to reach beyond 500 yards. Yes the 22-250 will do it, but the Creedmoor does it better. But! I don't take the 6.5 Creedmoor prairie doggin due to the volume of shooting, I want to preserve barrel life. Because ofbthis reason I run a less expensive button rifled Shilen on the 22-250 and Savage action. When the barrel gets cooked, I make a call and order a new prechambered barrel. Swap em out myself and no wait on the gunsmith. Saved time and money on a rifle that is going to cook barrels about every three years.

Just the way I address the various tools for the various jobs.

.338 Lapua Mag. hate to wound those fuzzy critters, makes them dangerous. Also good for Van Horn jackrabbits.