Lets say your getting a custom bolt action built for tactical purposes. You've chosen the parts, the general size, and the weight of the rifle. Now comes the caliber. You've set yourself at a 20" barrel and are unwilling to change the length .

What caliber do you chose and why?
If the barrel is set at 20" what calibers do you avoid cause it would just be a waste?

.308 Winchester.  My favorite rifle caliber. Near worldwide availability.  Anything more probably needs more barrel length.  Anything less is, well, less.

My next choices would be: 7.62x54R, and 5.56x45.  I like military calibers for availability.

Karl

I have been back and forth on this whole deal for a long time. I have conveniently set the caliber range from 308 win to 338 lapua mag.

Now comes the part of everything in between.

6.5-248, 260 rem, 300 win mag

the choices are there. Its just that I've set VERY lofty goals towards this project

Weight constraints, Length, Cost! No I'm not joking....... and don't call me shirley.

I believe that I can find a happy medium. Not too heavy, able to spank an unfriendly at a long distance, I can run with it for several miles........ yes run!

I think a better question to ask is what do you consider a kick ass cartridge/platform and why.

If you are not too concerned about ammo availability, and you don't want the .308, then the 6.5mm and 7mm are good choices. I'm not talking magnums either. I'm talking about the 6.5X55 and the 7X57. The 6.5X55 and 7X57 Mauser rounds are two of my all time favorite cartridges. Loaded with SMK's or Lapua bullets to their full potential, they have better long range performance than the .308, with mild recoil. Of course, you could get similar, if not the same, performance from their modern day counterparts, the .260 Rem. and the 7mm-08, which may actually be better out of the short 20" barrel you want.