I'm speaking especially about the Americans that have the legal ability to buy these weapons.

I get that but there's about 258.3 adults in the US 18 or older, 196.8 adults 21 or older (some jurisdictions require a buyer to be 21).

If we apply the most conservative estimate of 30% of adults living paycheck to paycheck that leaves 180.81 million 18 or over adults, or 137.76 21 or over adults.

Averaging the 2 numbers gives us 159.285 million which is right in the ballpark of your estimate but we'd still have to account for the 32% that already own all the guns (there will be some overlap) and anybody precluded from owning a firearm (criminal record, active TRO/OFP etc.) and anybody who has to get a government issued permit first.

That's why, while I agree with your point, I think your estimate is high at 150 million.