So, to drop down a bullet weight without getting into pressure problems, how much is a bullet weight? If my load for a K-31 uses a 174 grain bullet, is one weight down 168 or 150?
That's a good question and we can find the answer in Lee's Modern Reloading, Second Edition, (chapter 7, page 105), where Richard Lee tells us that:
Starting loads reduce wear on the gun and lengthen case life by a greater amount than the 10% reduction in charge. Never, never use a heavier bullet that that which is specified. Feel free to substitute a slightly lighter bullet. The velocity will remain about the same and the pressure will be lower. These conditions apply to full loads with normal working pressures. Reduced charges will be explained elsewhere.
My standard load for the .308 Winchester is a 168 grain Sierra Matchking and 43.0 grains of Reloder 15. We've found that load to be a good load in several rifles and we feel that it is one that almost duplicates the storied Federal Gold Metal Match load, at handloader prices. Lots of folks like Hodgdon's Varget in that cartridge as well, and if you look at most burn rate tables, you'll see RL15 and H-Varget next to one another. They are very similar powders.
If you look at almost any good reloading manual, we find that Reloder 15 is also a good powder for 150 grain bullets, albeit at differing charges, but that same 43.0 grain load falls into the acceptable range. It's also useful with 125 grain bullets, although the burning rate of the powder might not be optimal. However, it can be used.
Generally, the rule is that if you've got a good recipe for a particular bullet/powder combination, you can reduce the weight of the bullet and not get into pressure problems. In my case, I was looking for a lighter bullet for grandkid use in that caliber. The fact that one charge shoots well with 125, 150, and 165 grain bullets simplifies my record keeping. It's not optimum, but it works. I'll take some time later and chronograph all three loads and we'll be able to see the difference that bullet weight makes on velocity.