By this I read - your cylinder bore dimensions (smaller/larger) cause the case extraction test to be inaccurate. Should the case extraction test be avoided?
Absolutely not! Smaller cylinder bore dimensions cause excess pressure. A looser forcing cone allows the bullet to transition more easily into the barrel. That is not the only reason that some cylinders show signs of pressure sooner than others. In some revolvers the cylinder walls are thinner, such as light weight 38 spl's, or some replica 45 colts. Those thinner cylinder walls expand slightly under pressure and contract after the pressure is relieved. This behavior is a certain sign that the cylinder wall is being stressed beyond it's reasonable limits. Brass flows at around 65000 psi, that flow will be visible as deformation in the case head, it usually flows into the extractor recess. This is an important number, because it is far outside safety limits. Most reloaders have seen the early stages of this in rifle loads that went over. That said, take a micrometer and measure your cylinder wall thickness at its thinnest point. Hardened 4140 steel has a yield strength of somewhere around 100 to 130 thousand pounds per square inch. Remember that number. Lets say that you measure .150. and lets also assume that you are measuring a 38. Now measure your cylinder diameter, should be around .380 or something like that (probably larger).
Now plug the numbers into this equation. This equation calculates the hoop stress on an internally pressurized, thick walled cylinder. This is a tensile stress.
X=(((a*a)*p)/((b*b)-(a*a)))*(1+((b*b)/(a*a)))
p is pressure
X is the stress value
a is the inside radius (diameter/2)
b is the outside radius (2 times thickness of wall at thinnest point plus inside diameter all divided by 2)
I won't show my work, but here is the hoop stress on that cylinder at 65000 psi. 124000 psi. That exceeds the bursting pressure of some revolvers, and comes dangerously close to that of others, depending on the Rockwell hardness of the cylinder at Rockwell 25 the yield point is 100000 psi at 34 it is 132000 and at 38 it is 155000. your knife blade is about 40 on the scale. That should scare the crap out of anyone pushing the limits of a revolver.
Although , I agree that some listed loads seem ridiculous, unless I can find a higher maximum load listed in some published reputable manual I will not exceed it. Remember, smokeless powder is progressive powder. As pressure increases, burn rate increases, thus causing the pressure to increase faster, at some point these rates spike sharply upwards. In some powders this can be just above max.
A 10 percent reduction in max. is not a hard and fast rule, it is rather a general rule. In most cases a starting load will be listed, if it is use it. If you are loading someone else's pet load you need to find a manual that lists that load. The reason that I say this is that a hand full of powders have proven dangerous if reduced too much, Hogdon does not recommend reducing H110 loads by more than 4 %. Some rifle powders show the same behavior.
