Herter's (JP Sauer) single action shaving lead.

[Twmaster]

I've got this fairly nice old Herter's .401 Powermag that is shaving and spitting lead out the left side when fired. I've looked with a light and magnifying glass and cannot see any misalignment of the cylinder and forcing cone.

What else might I be missing here?

 

[Jaymo]

Oversized chamber throats, maybe?
Forcing cone not cut enough, maybe?
Loose lockup, maybe?
Late timing, maybe?
Dunno.

 

[LAGS]

You really need to check the barrel to cylinder alignment with the proper gages and not just a light.
They make rods that fit the bore, and you can tell if the cylinder is aligning with the barrel at the Precice timing needed to align the cylinder with the forcing cone.
It has to be checked in several positions, like when at single action cock, Hammer down with the trigger pulled and at double action firing if that applies to your gun.