My thoughts on most SMG conversions is go with a striker system. The bolt on some of these is marginal in weight, they work fine in open bolt config because they are slamming forward when the round goes off. In addition to the weight of the bolt, the forward inertia also has to be overcome. On most hammer fired systems the bolt is being reduced in weight with a slot for hammer clearance plus you no longer have the added resistance of that forward motion. A couple of the PPs 43 conversions done here in the USA have experienced blown cases with surplus ammo, and their makers recommend not to use surplus ammo because of the problem. Kinda defeats the purpose, I think, cheap milsurp ammo is one of the major reasons for owning one.
OTOH, a striker fired system keeps most of the bolt weight there, plus the weight of the striker is added to the bolt mass when fired, along with a bit of forward momentum from the striker slamming forward and the added spring force for the striker.