As a bit of a late afterthought....... I knew someone in Englandthat had an ex film prop Lanchester when they were semi-legal-after-a-fashion......... He cured several stoppage and ammo problems by drilling into the side of the breech block, threading the hole, inserting an allen key grub screw to lock the firing pin block into the breech block. This eliminated the inertia fire mechanism into a fixed firing pin. As another result of the fixed f/p, he had the advantage of API to boot.
As a matter of interest, I have suggested to Badger that in view of the SMG interest on the forum, where SMG matters seem to be scattered around willy-nilly, that there be a dedicated SMG section. The volume of stuff will never set the world on fire but maybe there'll be sufficient to keep an SMG thread ticking over - especially with the US contingent appetite for Sht barrel rifle variations