For a lot of years, my go-to powder in the .303, as well as a bunch of other cartridges, has been Alliant Reloder 15 [and before that IMR4320]. Between 41.5 grs. & 42.0 grs., as the individual rifle preferred, although way more than half preferred the 42.0 gr. load.
In 17 different rifles, albeit not a P-14 [Lee Speeds, Parker-Hale, Churchill, Globe and Cogswell & Harrison sporterized #1 Mk. III's, but 2 Ruger #1A's are also included, as well as a custom Siamese Mauser with a Parker-Hale barrel], with barrels from 18.5" to 27", shooting in +23.0 to +28.0 C temperatures, with Hornady 174 gr. RNSP-IL, 180 gr. Remington SPCL's, Western 180 gr. RNSP, Winchester 180 gr. ST & PPT, Sierra 180 gr. SPT Pro-Hunter, Speer 180 gr. RN, Speer 175 gr. SP 0.314" diameter bullets designed for the 7.65x53 Belgian Mauser, Lapua 174 gr. SPT, Norma 180 gr. SPT & Boolitcaster 174 gr. Bonded SPT, the 41.5 gr. results were from 2,350 fps to 2,453 [interestingly enough, the shortest barrel did not yield the lowest results, but that barrel had very tight lands & grooves]. The 42.0 gr. loads yielded results from 2,367 to 2,523.