I'd increase the requirement to $300 for newly made select fire weapons. That old Vietnam-era M-16 that grandpa has in garage would still require the $200 tax stamp for transfer. An SBR/SBS stamp would also stay at $200.You plan on bumping the tax stamp up from 200 USD? Would it track inflation?
More importantly putting tactical rubber bands on your semi auto AK wouldn't be a felony anymore as it is now private arms manufacture. You'd still get fined for operating without a license though.