43.5 grains in Federal cases using Federal 210M primers has been Federal's load for their Gold Medal .308 Match with the 168 grains SMK for a lot of years. They get special IMR 4064 for their government sniper ammo featuring the 175 grain SMK (flash suppressant added), but I suspect they also get it specially blended to tighter burn rate tolerance than normal cannister grade for both that and their GM ammo. That's because they've had that exact same load for decades, with about a 0.4 grains spread in the samples I've pulled down.
Federal's claimed velocity is 2650 fps (24" SAAMI standard velocity and pressure test barrel). This round has been the standard for commercial .308 International and 600 yard HP match ammunition for a long, long time. This is not a maximum pressure load. With Hodgdon's own data, interpolated by logarithmic relationships to velocity and pressure to get the small exponential pressure rise factor included, is at about 50,250 psi, so there is room to increase powder if needed. Hodgdon's maximum is 45.9 grains, which is compressed. Indeed, 43.5 grains is very slightly compressed in the Federal cases.
The Hodgdon data also uses the Federal 210M primer and the 168 grain Sierra MatchKing with reference 4064 (a lot right at the target burn rate they have Valleyfield make the powder to), but they use a Winchester case, which is typically roomier than a Federal case and they get about 19 fps lower velocity at the 43.5 grain interpolation, which matches my experience with the different case capacities pretty closely. Manipulating QuickLOAD to match, it looks like the Federal load is at about 52,000 psi in their case and that 43.8 grains would match the Federal load's velocity in a Winchester case. But that still gives you 10,000 psi of headroom (2.1 grains of additional charge in the Winchester case, and about 1.9 grains in the Federal case).
Incidentally, using the Federal load with the 43.5 grain load, different barrel lengths work out as follows:
16" 2411 fps
18" 2484 fps
20" 2547 fps
22" 2601 fps
24" 2650 fps
26" 2693 fps
28" 2732 fps
30" 2767 fps
32" 2799 fps
You can see how the gain from each inch drops off as the barrel lengthens and residual pressure gets lower.
For other QuickLOAD users, I adjusted the default 4064 characteristics to an energy level of 3948 kJ/kg, and the burn rate factor to 0.5739. With the 168 grain SMK and a 24" barrel, the Winchester case with 57.8 grains of water overflow capacity and the Federal with 56.89 grains water overflow capacity, the maximum load pressure and velocity match the Hodgdon data very closely and the velocity matches the Federal data. We don't have the Federal pressure data to compare, but based on my experience with the program and measuring and comparing pressures it predicts to those measured with the Pressure Trace instrument, we are probably within 2000 psi for the numbers all around and within SAAMI limits at all the loads mentioned.
