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I have done a lot of long range shooting in the past and tries the 223/ compared with a 6mm there is a huge difference, Huge, we were getting very few hits at that distance with even the heavier bullets , switching to my 6mm's we had prairie dog kills repeatedly up to 1000 yds especially with the 105 gr vld's from a 6 Ackley or 6/284, conditions were perfect, 5-600 were no problem at all even with the 70 gr tnt's. The 223's simply did not do anywhere as well at all.
My 223 that day was a Shilen DGA action with a 26" SM 1:9 twist at 26" #7 contour.
My friends with the 700V bolt guns got a few hits, just a few compared to the 6s' and just quit wasting ammo. remember I was shooting at rat size targets nothing like the 500 yds we shot to qualify with M16's.
Even the 6BR of mine with 68-70 gr shot well at 500 compared to the 223.
My friends 223 Ackley did ok with 69gr bullets and is a great caliber and easy to load for.
We had 24 to 36x scopes and Leica range finders to verify plus walked 1/2 way to target area and ranged front and rear and added to verify distance. If your really serious about shooting that distance it can surely be done but a larger caliber will give you much more success.
I do have (my favorite rifle) a 22/6mm that I shoot 68-69 gr bullets in that will do it with no problems but not like the 6mm will.
6mm case necked down to 224.