Brian - The physics is inflexible, I'm afraid. Use the same load in a 24, then a 16 and, if my maths is correct, that's the result.
Of course it's possible to use a hotter load to claw back some of the lost velocity, but that's not the point I was making.
Kevin, I am in form for a lively discussion on this one. I am increasingly enamoured of short barrels these days. In these days of almost universal mod use, who would entertain a 24" 308? I would say 20" is the maximum barrel I would consider, not the minimum. Now, I will concede, as the calibre gets smaller, and the powder used gets slower, you do need a bit of extra barrel to get the most out of it.
The physics are not that inflexible. A judicious choice of bullet and powder can maximise velocity from a short barrel. The loss of velocity is not linear for every inch of length lost. Certainly when you go below 19", the velocity might drop off a little faster, but not enough to really matter. I would guess that the vast majority of woodland deer in the UK are shot at under a 100m. Lets use the quoted velocity of 2600FPS with a 150gr bullet. Of the cuff, that is roughly the velocity a convential 308, wringing out every drop of velocity, would carry at 175 - 200m. Do you believe a 308 lacks the energy to kill a deer at 175m?
I read a very favourable review of that RPA recently, but I cannot remember where. Now I would say that 16" is a bit short, but it is an interesting idea. As mentioned earlier, I used a custom 260rem a couple of weeks back. It had a Barnard action, a sodding big Swaro scope, 19.5" LW 1250 weight barrel and an ASE Utra hanging on the end. This little beauty handled like a wand, seriously, like a convential 24" rifle without a mod. I couldn't believe. My 260 is now away to have the appropriate modifications done.