When I made my first rifle I thought breaking through to the barrel channel with the main spring was a no-no, I narrowed the spring to prevent this from happening.
I went from a 32# spring to a 16# one, the gun seemed to go off OK. A few years later and several deer getting away from whoosh/bang ignition I decided to speed things up and ordered a new main spring from Chambers.
I had to put a slight (1/6") notch in the barrel to accommodate the spring. The new spring plus opening the touch hole to 5/64 completely did away with the whoosh and left me with only the BANG.
Don't worry about breaking through with your mainspring and keep your lock functioning like it was designed to.
Now, I need to order a new mainspring for my second build, an English fowler, because I cut that spring down a little as well to avoid breaking through.