I am thinking of having a .223 built with a 1:7 twist and using 77 grain sierras for 400, 500, 600 yard shots at prarie dogs. I don't want to go with more powder, such as in a 22-250, because barrel heat becomes a major issue on 105 degree summer days when shooting at dogs repeatedly.

I want to go with the heavy bullets for long range stability and wind-bucking ability.

What do you think?

I think it's a good plan.........not sure though if you need to go quite as fast a twist as 1 in 7, I think Hornaday recommends a minimum of 1 in 9 for the 75 gr A-max , so maybe a 1 in 9 or a 1 in 8 ?

Yeah i like that plan too. Don't forget the various 68/69er's too-- Sierra, Nosler, and Hornady should give some decent performance at those yardages.

500grains,
I don't know if this matters to you, but I'll tell you anyway. I had a Savage 1-9" twist and it would shoot 40gr BlitzKings more accurately than the 69gr. Sierra Match bullets. If you are going to shoot heavier bullets than the 69grainers you would be better off to go with the 1-8" or 1-7".
One more thing, have you thought about a 22BR? Less powder than the 22-250, but the velocities are nearly equal. I quit the 22-250 in favor of the 6mmBR.

Rem. 222

500 Grs:

I had a Ruger Sporter Weight rebarreled to a 1 in 8 twist 223. I went with a PacNor heavy sporter Stainless barrel ( 24 inch) done by Novesky Gun Works here in Grants Pass, OR. Cost was $275.00 delivered.

The rifle has shot one hole groups at 200 yds using 75 grain Hornady HPs and Amaxes.

I changed the stock to a Hogue Rubber stock for $85.00 and put a Dayton Traister trigger in it.

I have found that the Hornady 75s and the 80 grainers give me much better accuracy than the Nosler or Sierra 77 grainers gave me.

I have been using AA 2520 which gave me the best velocity and accuracy, but after recently loading up some with 27.5 grains of H 380, I chronographed velocity at 3250 pretty consistently, with the accuracy being right on.
H 380 is also real " user" friendly to the handloader for the small neck diameter on a 223 case.

This heavier bullet definitely doubles the range on the 223 no questions as I have used it in 600 yd competition.

When I set up a spot for PDogs, I usually have 2 rifles. 80 % of our shots are 250 yds or less, so I use a load of Blue Dot powder,at 2600fps.
It is Minute of PDog right out to 250 yds. The 12.5 grains of it take a long time to heat up the barrel because most of it is burned up so quickly. Chronographing those loads, it shows very minimal standard deviation in bullet speeds.
45 to 55 grain bullets show no velocity difference at all.

For 250+ yards I use the 223, 1 in 8 twist, or a 22/250. That just depends on how the ammo supply is looking. Strictly speaking, my personal preference is the 75 grain 223 load.

Fragile bullets in the 1 in 8 twist, pushed fast will just vaporize though, I am talking the 45 hornet bullets or the Speer TNTs etc.

For scopes I use a 4 x 16 weaver or an 8 x 32 Tasco on the 1 in 8 twist Ruger.

It will add a lot to versatility going with one with a 1 in 8 twist, plus you can use cheaper 223 brass. In a 22/250 also, I have not been able to get much more velocity out of it with the 75 grain bullets than I am getting with the H 380 loads in the 223, and do so using a lot less powder than in the 22/250.