Is it just because the 500 jeff is moving about 200 or 300 fps faster? Would it be feasible to increase the 500 nitro's velocity by that much or close to it using the right propellants in a suitably strong action?
That is one of the reasons yes.
You can also use whatever bullets you like, which is not always possible in a double.
Seeing as you plan on using a falling block action, yes you can load the 500 NE to higher velocity in that action. You will however not be able to get to 500 Jeff top velocities.
500 NE 3" has a case capacity of 138 and the 500 Jeff is at 158(14.4% more).
You can however go to the 500 NE 3 1/4", this will give you 8% more case capacity than the 500 NE 3". Just remember to work up slowly as these cases where not designed for high pressure but you would most certainly be able to load it up quite a bit over the standard velocity.
My in field experiences over the years and having hunted a large number of DG, have shown that the ideal speed with heavy for caliber bullets in a large bore, falls between 2200-2400 fps. This gives the best performance not only in bullet performance but also in penetration.
Yes there is a big difference between the same premium grade bullet travelling @ 2100 fps when compared to 2300 fps.
Yes all the old NE perform well at 2100 fps, because they are used at very short range and mostly used with more conventional type bullets so the bullets have no issues at that velocity.
Anything over 2400 fps is a waste and penetration actually starts reducing when you exceed this.
I load my 500 Jeff with 570 gr Rhino bullets @ 2350 fps and it has been a superb performer.
We did some penetration test with my 500 Jeff(570 gr Rhinos @ 2350 fps vs a 500 NE 3" 570 gr Woodleighs @ 2100 fps) and the 500 Jeff outperformed the 500 NE by quite a big margin.
Yes the 500 NE is and has been good enough for many years but there is room for improvement especially when used in a strong falling block as bullet selection and regulation are no longer a restricting factor.
If you could get your loads up to about 2250 fps with a 570 grain bullet you would have a very effective big bore.
Good luck and a good choice going to the 500 NE.