09-28-2022, 04:19 PM
Fess Wrote:I wonder if the Grendel is unusually sensitive to this issue. WAY back when the issue of Quickload predicting overly high pressure for the Grendel was first raised, a plausible explanation was that on this cartridge, the base of the bullet is so close to the primer that the ignition of the primer started the bullet moving. This resulted in a larger effective case capacity before much of the powder had started to burn. If the bullet is pinched in place by the neck hitting the beginning of the freebore, the pressure increase would be even greater (combination of smaller case capacity and higher bullet clamping pressure).
I could see how this is plausible as compared to a more cylindrical case like 223/556. It's all about the diameter of the mouth relative to the diameter of the case. If they are closer in size to each other, then the pressure will relieve itself better and more easily. All the gas in the chamber will head straight out the mouth, relatively speaking. But in the case of the grendel, all the gas in the case now has to funnel through the smaller mouth relative to the case diameter. It's like when you put your thumb on a garden hose mouth. You restrict the mouth area and you create back pressure which lets you spray water at higher velocity. Without your thumb, the back pressure is lower and the water just dribbles out.