I am not an engineer, nor an expert. However, I do consider myself to be "scholarly" when it comes to guns, reloading, etc. So, here is my nickels worth:
I have some Lapua 6BR brass that I load for competition in my 6.5BR XP-100. It has such a small flash hole that I have to buy smaller decaping pins to use in my resizing die. My Remington 7BR brass has small rifle primers and a flash hole that is just a little bit larger and accepts the standard decapping pin. A buddy of mine has some early 7BR brass that is probably necked down .308 brass. It has a large rifle primer and a little larger primer hole yet. Same type brass used in same type gun with different size primer holes.
Stimpy's example with the garden hose is a good illustration here also. Garden hose with no constriction and you get high volume coming out of end of hose. Add a little contriction and you get higher pressure within the hose and the water squirts out further. Now restrict it a little more. More pressure in the hose and the water squirts out even further.
Small primer hole has a flash that is restricted and comes out like a torch flame. It forces it's way into the case to ignite all the powder. A little larger primer hole has less restriction so you have a wider flame pattern that is not quite as long. It doesn't penetrate into the powder as fast as the more restricted flame and the powder at the bottom of the case is burning faster than the powder toward the neck or your case. Open up the primer hole a little more and you get a ball of fire coming out of the flash hole igniting the powder in the bottom of the shell only. The pressure is generated by the acceleration of the powder burn rate. BUT, while this pressure leaving the primer cup, you also have pressure builiding within the cartridge case. Your pressure within the case is going to build and force your bullet out of the case before the pressure is great enough to start pushing back thru the primer hole. Per some of the bench rest experts, pressures are more controlled when the flame lights more powder down the length of the case and thus you get more consistent pressures and initiation of momentum within the cartridge case.
I too have seen the brass with the drilled out primer flash hole for shooting wax bullets and plastic or rubber target bullets. The main reason was to keep the initial flash from burning the back of the bullets. It allows all the pressure from the primer charge to exit the primer pocket, into the empty case, and you get a fast pressure equalization. You have no powder igniting so no pressure is building back into the primer pocket. It is all filling the case and initializing bullet acceleration. If you don't like the speed of your wax bullet with a standard primer, try a magnum primer and see how much more you get.
Just adding another nickels worth of info into the plethora of knowledge base in this forum.
Steve
