I load and shoot the 9x25Dillon from my G-20SF and G-29 and also a S&W 1006, fun cartridge but they don't get up to speed in the shorter barrels. 6" and longer are need to squeeze the velocity higher. Most of the loads are posted in the upper portion of the forum here. http://www.glocktalk.com/forums/showthread.php?t=228796
My work and others too. I've done pull-downs of the Underwood and Double Tap ammos and documented them for reference.
The barrel blanks can be had already bored for 0.308", the outside profile is what will be needed on the milling machine.
It has been talked about making a .308 necking of the 10mm some years back, even by me. I seriously doubt you could get enough powder inside for a 150 gr to achieve your target velocity of 2500 fps using the 10mm parent casing. Maybe with a 10mm Magnum case, but now you're in a whole nother gun platform.
CDS did a 0.224" called the 224BOZ, its velocities were not much better than that of the 9x25Dillon. Light bullet (75gr-50gr) at 2000-2200 fps...
The original test-bed for the .224 BOZ was a Colt Delta Elite. Using lightweight 50gn 5.56mm bullets velocities ranging from 1,750fps to 2,200fps were obtained. Eventually the Glock 20 was chosen as the foundation for the .224 BOZ PPW (Personal Protection Weapon) due to its "positive lock between barrel and frame." (It must be noted though that in this caliber a 5.5" barrel is considered "compact" and a 6.5" barrel is "standard.")
The slides were skeletonized to lighten their mass/weight so they would function reliably.
The .308 would possibly need the same attention for reliability.
Not trying to bust your bubble just providing information to shed light on what you are up against.
Things would be easier if LWD offered a barrel for the G-20 using the 7.62×25mm Tokarev cartridge but with the current government regulations against imports they won't be as cheap as they once were in bulk.
Best regards and good luck with your project. If you haven't stopped by the 10mmFirearms forum please accept my invite. http://10mm-firearms.com/index.php