The simple reality that's been established time and time again through testing and published articles is that conventional signs of high pressure, such as extraction, primers, etc. are, at best, unreliable indicators of true pressure. Many, many times you will encounter a load that grossly exceeds 52,000 c.u.p. but does not show any conventional signs associated with high pressure. Such loads may be "safe" in the sense that they don't blow the rifle up; most actions are built with a huge margin of safety. But the point that's been made is that it is not prudent to push your luck, especially with older actions.
The 4895 load described above almost certainly reaches or exceeds 52,000 c.u.p. (The Hodgdon Reloading Guide indicates that 45.5 grains of H4895 with a 150-grain bullet in a .308 Win. generates 51,000 c.u.p. The IMR Guide lists a maximum .308 load with the same bullet as 47.3 grains of IMR 4895 with a pressure of 57,700 psi) The only way to know for sure would be to use pressure-testing equipment on that barrel and load. However, we can infer that it exceeds that pressure because of the .308 data. The .303 has virtually the same case capacity and an almost identical bore diameter. Therefore, as a rule of thumb, it is prudent to use .308 loading data, generated through extensive pressure testing with a ceiling of 52,000 c.u.p., as the absolute maximum for .303 loads in the P-14. This is not to say that .308 loads will necessarily be safe; only that in no event should you go higher than published .308 loads, regardless of what the conventional pressure signs may suggest.
The issue about the brass is worth repeating. Because .303s are not normally loaded to the same pressures as modern cartridges like the .308, some manufacturers don't build the cases as strongly as they do the more modern rounds. Sometimes this takes the form of thinner case walls. Therefore, some loads that are fine in a .308 may appear to be too much for a .303 in a P-14. Once again, it pays to work up from below.
The upshot of all of this is that the load being described, in reality, will probably not give any serious safety issues. It will, however, be seriously pushing the redline. Personally, I don't see the need to push the redline. As I've said above, if you want SERIOUS velocity, all you need to do is try a powder like N550, and you should be able to reach near-.30-06 levels out of an ordinary .303 case, within SAAMI .308 pressure levels.
