It doesn't take much gravel to shear a pin in my experience, but then again, the last time I ran a snow blower was about 25 years ago back when I lived in New England. Had gravel driveways in both CT and VT, and in both cases small pieces of gravel could jam an auger or impeller enough to shear a pin. I think it's a bit of a random thing, since I also saw plenty of gravel come out the chute that did not result in a sheared pin. Just depends on how much clearance there is around the rotating parts to see if it can possibly happen.
I would suggest getting the parts diagram for the blower and make sure you have the right pins for both auger and impeller. I do remember them being different on the machines I ran, most likely because there were different diameters and speeds, so obviously the torque that can shear the auger is different than what could shear the impeller. It would be rare I'd think for both parts to use the same exact pins.
