[Unclenick]

Poline,

I've never seen a jacketed RNFP bullet. Are you sure you aren't confusing plated bullets with jacketed bullets? The XTP is jacketed. Plated bullets are softer and need to be fired like cast bullets, or at least not over mid-range jacketed loads.

Lee may not have the data you want, but Hornady has it for the 158 grain XTP's. This is one of the reasons you always look at several sources of load data. Another reason is possibly seeing significant pressure disagreements, as you'll see.

Hornady data:
158 grain XTP seated to 1.450" COL, Winchester WSP primer, Hornady/Frontier brass.

Bullseye: 3.4 grians to start, 4.2 grains maximum without going into +P range
Unique: 3.9 grains to start, 4.8 grains maximum without going into +P range.
HS-6: 5.1 grains to start, 6.2 grains maximum without going into +P range.
VV 3N37 5.0 grains to start, 5.8 grains maximum before reaching +P range.

Hornady says 6.0 grains of VV 3N37 is the maximum load and that it is already +P in their testing. Vihtavuori, on the other side of the pond, operates under CIP pressure limits rather than SAAMI. The CIP has no +P classification as SAAMI does. They use 150 MPa (21,756 psi) for all .38 Special loads. That's above current SAAMI +P. So if you get data from VV's online load data, keep in mind you are looking at exceeding current SAAMI +P, and will be operating in the vague, non-SAAMI region that manufacturers call +P+.

The +P standard has a complicated and confusing history. It was 22,400 CUP originally, then in 1993 it was, for one year, reduced to 20,000 CUP or 18,500 PSI, depending on which kind of measuring gear you had. The PSI number was then raised to 20,000 psi in 1994, because manufacturers complained they couldn't get adequate ballistic performance for +P at 18,500 PSI. Meanwhile, the CIP number suggests that no modern firearm should fail to withstand the higher pressure, anyway. Only some very old timers and early Airweights with their less-than-rugged early aluminum alloy frames need to be restricted to the SAAMI standard load limit (17,000 CUP and also 17,000 PSI).