[NWPilgrim]

What I mean is that the pressure and velocity increase at the same rate as you increase the powder, within the load table range.

Over a large range the relationship of velocity to powder may be a curve, with small increase of velocity for large increases of powder, then at some point it becomes more linear same % increase in velocity as for powder. Finally at some loading the pressure/velocity is going to spike much more with even small increase of powder. And perhaps with even more powder you reach a point of diminishing returns where velocity barely increases with even large increases in powder.

But reloading manuals stop the maximum load to be less than the "spiking" point in the pressure curve. That is why you seem some powders table maximum set at a point where the pressure is still a few thousand PSI below SAAMI maximum. For that powder/cartridge even a tiny increase in powder will start giving dramatically more pressure beyond the SAAMI max.

So within the min-max range of loading manual tables the pressure and velocity should increase at the same rate you increase the powder.

Linear relationship means a constant rate of change. One for one, or two for one, or three for one, etc. Draswn as a graph of velocity versus powder charge it is a straight sloped line, thus "linear." A 1% increase of powder gives a one percent increase in velocity.

A non-linear relationship is a something where the rate of change in one factor gets greater or lesser with each increment in the other. Thus, at one level powder charge you may have a 1% increase of powder giving 1% increase in velocity. At some other powder charge level you may have a 1% increase of powder giving 3% increase in velocity, or .5% increase. When you graph velocity versus powder charge in this case you get some sort of curved line.

Often though, even with a non-linear relationship, within a narrow range the curve is flat enough that you can approximate it as a straight line, that is, linear. This is what Richard Lee discovered in his testing, at least within the typical range of powder charges listed in handloading data tables. When you go below, or certainly above the listed data then the relationship of powder and velocity may become unpredictable, or at least more dramatic.

Just because H4895 has a longer "flat" aspect to its powder-velocity curve does not mean it is suitable for all cartridges. It is a perfect powder for M1 Garand gas systems, and gives lower than "normal" velocities for the .30-06 (2,700 fps for 150 gr bullet). Hunting ammo for the .30-06 will use slower powders like IMR4350 or H414 and will achieve higher velocities for the same pressure max.

The concept is simple, but not to be confused with pressure and velocity changes that ocurr as the bullet moves down the barrel. There you are comparing velocity to position in the barrel for a single powder charge. I am referring to the muzzle velocity (one specific position of the bullet from the case) versus various powder charges.

Sorry if I am not writing clearly. I am writing paragraphs to describe a single sentence in my head.