MistWolf, Wise words.
I think the perfect barrel size for the .308Win is 20". I think in a Bull barrel is the perfect compromise between speed and accuracy.
Also I think the 30.06 needs the two extra inches minimum and can even use extra two more specially using lighter grain military loads (more powder M2 ball cartridge)
.308 win BBLs with 26" barrels are hard to fill with hot gases no matter what powder, even the slowest ones.
30.06 loads in a 26" can provide maximum speeds that is what some manufacturers use so they can advertise a larger speed/energy and write the BBL in little letters or not even mention it but the down side with the largest barrel is that you will have A) Extra weight B) less accuracy (Again all other things being equal) The longer the barrel the lower the tune of that barrel. The lower the tune the less accuracy nodes at your disposal. Can they become accurate, yes? but they need more work with the loads or other methods of harmonic tuning. Also the weight is a factor. Not a huge concern for the bench rest shooter but a big issue for the hunter, a military sniper or the Swat/police operator.
I do hunt with a bull barrel but I can understand folks like something lighter
to carry all day. My primary objective is extreme accuracy even if I have to carry the extra weight. I also understand this is not needed for most hunting situations. When I say extreme accuracy I mean systems that are capable of delivering 1/2-1/4MOA consistently so this allows me to take neck shots if I want at longer ranges. Not that I need this most of the time but in a few occasions has been very handy. As I enjoy the challenge of hunt so much I do not care too much the easy shots but I take whatever the lord has to offer.
.308Win / 7.62x51 NATO.
16" in .308Win only special purpose (socom) and some assault rifles and patrol systems.
18" in .308win some Swat snipers and other special operators. Also some assault weapons.
20" in .308Win is the Sniper/Swat standard, specially urban scenarios. Many hunting rifles and assault rifles.
22" in .308win Many hunting rifles. Some Bench rest rifles.
from there even some 24" and 26" inches...Also Bench rest shooters and high power (long range) shooting to squeeze the last FPS possible with special powder but in reality IMHO totally unnecessary and contra productive but some can make it work for them.
.30.06
18" in .30.06 Never saw one. Would be a big boom!.
20" in .30.06 I have seen some custom rifles for bush hunting but this is a waist of powder that burns outside. Milder loads can be use that save barrel and case life. .308Win would be better here.
22" in .30.06 many hunting rifles and other type of systems around the world. Manageable blast and recoil though.
24" in .30.06 The M1 Garand standard BBL. perfect for the military loads but can do the job even with less length.
26" in .30.06 Can squeeze a little bit more with the right powders but keep in mind the purpose. This is what some manufactures use to put the max speed and
energy in the boxes they sell. Again, weight not a concern for the bench but I don't like that long in these casings.
Going back to my statement about why to start with the bullet is so important is because in the end everything balances itself out.
If you like a military load then the Garand lenght is ideal or the 20" in the .308 win. but if you want more reach then the first thing is that you have to look for better bullets. Even if you consider using the BArnes or other modern all solid copper bullets this is a consideration as they are longer due to the lack of lead. In other words, to meet the grain they are longer, they have to...
Just a quick example of what I mean....
150gr vs 175grSMK vs 168grTSX
168grTSX and 'big momma' 180gr TSX.
Those bullets are normally much longer that means they seat deeper, that means you have less case capacity, more pressure so powder load has to come down. Then the length of the barrel is ok to come down too as there is no so much powder to burn but it is ok to loose speed because low drag bullets fly much better that you can reach longer anyway. The concept of starting slow and ending fast like Bill Alexander preached to sell the grendel.
Now you have more parabola so you need to know your optics and ballistics charts but that is a different story.
I hope this makes sense.
Cheers.
E.