So what? Meth labs blow up every day.
Drug Lords, Drug Cartels supply drugs. They do whatever it takes to produce and distribute them. If they need to spend $1,000,000 for lab equipment, supplies, airplanes, bribes, tunnels, etc., that's just the cost of business.
In the scenario envisioned by the OP, there will be Gun Lords and Gun Cartels supplying guns and ammunition.
They will spend whatever it takes to set up production and distribution facilities. If a few low level people get blown up now and then, there will be plenty to replace them.
If there is a Demand, there will be a Supplier.
There will always be a demand for guns and ammunition.
There will always be guns and ammunition available, for a price, if customers are motivated to find a source.
Just like drugs.
I assumed we were speculating about a future where people didn’t have access to the modern chemical infrastructure. Meth is made by simply cleaving off a hydroxyl group from a readily available drug.