If you're going to make silencers, your best bet is to bore only enough room for the inner barrel to pass through. You are least likely to encounter legal difficulties down the road if your silencers have none of the traditional empty cavity associated with airsoft silencers.
As for the G36 barrel conversion market, I don't think that's a wise investment. Existing conversion already exists, and they have proper external finish and accessories to match the rest of the gun. If you're going to make some sort of barrel conversion, your angle would have to be unique. A "short" barrel for a full length G36, for example (omitting everything after the bayonet lug), to satisfy a niche market.
Personally, manufacturing should not be your primary concern or even on the start up checklist. Keep your cost low, retail first and determine what your market is with actual business before venturing into custom parts.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."