Congress is controlling these intrastate acts or products using the power of the Necessary and Proper Clause in conjunction with the Commerce Clause. The Necessary and Proper Clause cannot be used standalone -- it must be used in conjunction with another power.
The Necessary and Proper Clause (listed at the end of Congress' other powers) gives Congress the power to write laws that are both necessary and proper "for carrying into execution the foregoing powers". Without this, Congress would not be able to stop states or individuals from undermining and subverting their interstate efforts.
Except it's all defined so subjectively that it's cumulatively meaningless. Bob Stewart couldn't build enough rifles to have a "substantial effect" on the interstate commerce in firearms. You've said it yourself - it's a tactic, not an objective.
That is indeed true.
But if we allow Bob Stewart to build rifles, then we certainly must allow Bob Jones to build rifles, yes? Oh, and what about Bob Smith? Certainly he must be allowed. Bob Johnson? Bob Washington? A thousand Bobs? A million Bobs?
Looks like "substantial effect" to me. How do you propose avoinding that? You going to put a limit on the number of "Bobs"? Constitutionally?