As noted above, for this testing I wasn't interested in getting unloaded, primed cases to fire as Drake Oldham was in his testing. My interest was simply in seeing how many times the pistol's muzzle would squarely strike the floor, which is what is needed for the inertial discharge. Because of this, and not wanting to destroy one my own pistols, I made the drops onto pillows on my carpeted floor, and observed where the pistol landed. Six feet is the critical break over height where the Ti firing pin & extra power firing pin spring are no longer 100% safe for drops onto a hard concrete floor, so that was my selected height for all drops.
Methodology:
Kimber Classic Custom (pre-Series II gun, no firing pin block) with Kimber Laminate grips. The pistol was dropped hammer down, on an empty chamber. A Chip McCormick power mag was inserted, loaded with 230gr Speer Gold Dot cartridges. This was done to replicate the weight of an actual carry loading, but no round was chambered for obvious safety reasons.
Ninety (90) drops were done, again all from a height of six feet.
The first group of thirty (30) drops was done from a firing grip. The pistol was dropped ten times with 8 rounds in the magazine, ten times with 4 rounds in the magazine, and ten times with a single round in the magazine.
The second group of thirty drops was done with the pistol held between my thumb and forefinger to minimize free fall interference from my hands as in the firing grip. The barrel was parallel to the floor with grip vertical to floor (same orientation as a firing grip). The drops were done in three groups of ten, with magazine loaded to the same round counts as in the first subsets.
The third group of thirty drops was done as the second, with only the orientation of the pistol changed. For this group the pistol was dropped with the barrel perpendicular to the floor - muzzle face square to floor.
In the first and second group of drops, the muzzle never impacted. In the third group of drops the muzzle impacted three times. The first muzzle impact occurred on the second drop with eight rounds in the magazine. The pistol was at an approx. 45 degree angle, but even though it landed on two standard bed pillows there was enough force to move the slide back far enough to retract the hammer to the safety notch. The second muzzle impact occurred on the first drop with four rounds in the magazine. Although the muzzle landed squarely, there was not enough force to move the slide rearward. The final muzzle impact occurred on the first drop with a single round in the magazine. This was also a muzzle square to surface impact, and it also lacked enough force to move the slide rearward.
Of note, I had to exert significant grip pressure to hold the pistol with bbl. perpendicular to the floor prior to dropping on the third group of tests. If held where it could swing freely the pistol wanted to tilt to its natural balance with the bbl at a slight angle.
If you combine Drake Oldham's 1 discharge out of 6 drops data (when dropped from 6 feet onto concrete, rigged to ensure square muzzle impact,) with my unimpeded free fall impact data you can draw two conclusions. If you take all 3 drop methods my testing comes up with 1 out of 30 (3 out of 90 reduced to a proper fraction) square muzzle impacts. Combine that with 1 out of 6 discharges, and you get a 1 in 180 chance of an inertial discharge. If you choose to only look at the drops with muzzle square to the floor, my data shows a 1 in 10 chance of impact with muzzle square to the floor. Combined with Mr. Oldham's discharge data that's a 1 in 60 chance of an inertial impact.
Tables with impact points to follow.
