First off, I'd not recommend trying to get more bang from a particular caliber, but rather just go to a more powerful cartridge.
"We need more power, Scotty!"
That said, Lyman 47 shows highest velocities with the 125g Sierra SP (#2120) with the maximum charge of 30.0g Reloader 7 (compressed load/2.200" COL): 2528 fps.
Another load with a Sierra 110g HP (#2110), same COL, shows a maximum charge of 26.7g of AA1680 at 2533 fps.
Now, Speer 13 shows their 125g 303 Spitz-SP (#2213) going fastest with 28.0g of AA1680 at 2544 fps, (compressed load/2.150" COL). With a maximum charge of Reloader 7 (27.0g) at 2429 fps.
Note: Lyman used a universal receiver in their tests; Speer, a Ruger M77 Mkll.
Last edited by CTSixshot; 12-20-2010 at 12:07..