[Pampers]

They're telling you what to use in an M14 for NRA Highpower competition using the most popular 168 gr. bullet.

Check out the Accurate Arms manual which has a whole section on reloading the M1/M14?M1A for Highpower (Across the Course), Long Range (1,000 yds.), and Palma (155 gr. Palma Bullet). Notice the burning rates of the powders recommended. Now check out other manuals for loads using powders of similar burning rates. As a general rule, no powder SLOWER than 4064 should be used for risk of overdriving the gas system. What your worrying about is gas port pressure. Slower powders give higher port pressures and can overdrive the system.

You are not limited to one bullet weight, but you are probably limited to bullets of 190 gr or less due to overall length considerations. AND, heavier bullets are harder to drive fast due to port pressure considerations.