I agree if it's what the op wants, heck ya why not. I realize the "fact" but my point was the gain in accuracy is insignificant. .5" @ 100 is nothing special, it is just a well built machine doing it's job, regardless of barrel length, action length or any number of other factors. By all means build it.
Well, it's also a 'fact' that higher muzzle pressures (all else equal) hurts accuracy.
And this is why longer barrels work better, and more accurately in large capacity cases, than barrels that are too short.
Whenever you go to any extreme, with anything, you pay a price.
You wouldn't want to go ridiculous long for a little more velocity, adding whip.
And you don't want to go ridiculously short for accuracy, adding fireball.
Both hurt accuracy.
What point blank BR shooters do, is far far away from LR hunting/shooting with a rifle in 300wm.
PB BR shooters can take advantage of ~22" barrels because they're shooting the tiny 6ppc with a very fast powder, resulting in low muzzle pressures despite. They're also using better releasing FB bullets.
I would go 28-30" for a 300wm. With this, you can burn a hi% of the powder INSIDE the barrel instead of hitting the releasing bullet base with a giant blast of spiking pressure OUTSIDE the barrel.
You can use a larger diameter & fluted barrel for stiffness/weight, get very clean bullet release, and better accuracy than you're gonna end up with from 20".
Yes a longer barrel will give you lower muzzle pressure, to a point. If you use powders that are on the fast side for your given combination, you can get along with a lot shorter barrel than if you're using a compressed charge.
I pulled out some pressure graphs from my old 7mmstw, and though the pressure was still dropping as the bullet exited, the pressure was below 20,000psi at 1.5 milliseconds, while it has only dropped to 12,000 psi at 2.5 milliseconds. Neither pressure is doing much to move the bullet. The load was 81 rl25, 140 sierra fb, 215, rem. brass. The pressure peaked at 60,000 psi at .6-.7 milliseconds.
I highly doubt he'll notice any difficulty shaving down to 20" from the normal 24" of a lot of the factory 300win's out there. 100-150 fps would be a lot with equal or better accuracy if he watches his powders. I wouldn't burn ball 50bmg powder or retumbo in this one, though. That would CREATE problems on its own. Stick to the idea that the pressure needs to be off by exit, and use mid-speed powders.
Another thing that hasn't been brought up. Why not go with a 24"-26" barrel and a single shot action if shorter is a premium. That way you can use any powder you want and stay stiff enough to be accurate in a sporter taper. My 338 is a #1, and while it is rather critical as to hold and bagging for accuracy, I'll put it up against most any rifle out there short of a ppc for accuracy.
Even with Imr4350 and 165s in a 20" tube its a giant fireball, build it if you wish, but I call it pi$$ing in the wind!
I don't get a fireball with my 22-250 in a 14" barrel. My father doesn't get a fireball with his 300 wsm with a 14" barrel.
You have got to get a shorter barrel to see it's capabilities before you summarily strike them down. If our 14" barrels are just fine, why will his 20" barrel be a problem??
I've shot I've shot 300 win's with 22", 24", and 26" barrels, and you loose a few fps, but it's more the barrel (whether it's fast or slow) than the barrel length.