The UK's Finest Independent Forum for Shooters and Gun Owners. Est 2010.

Less force required for the case to fail at the cut, than the force needed to propel the shot and wad through the closed crimp.
Once in the air the weight of shot keeps the unit together until it impacts

Not ideal for over tight chokes though

So why not fully cut around the case?

1. It'll fall to bits
2. Needs to allow the powder time to burn fully