If people can still buy overpriced guns and mags, they can still afford to buy ammo. You are most likely curbing a very small percentage of people who cannot afford them.*
You assume that their endgame is simply making ammo more expensive. They don't want more expensive ammo, they want ammo that is priced so high that the market cant support it.
Over priced guns are one thing but people are still pricing at a point that the market continues to support sales of those products. In other words, price hasn't exceeded demand. If you price those same guns at 500% of the current going rate, those sales are going drop. The market can only handle so much until demand will drop. Price will exceed demand. In a free market, then price will fall to meet demand.
The ammo taxers want to increase the price via tax until the demand stops. Since it isnt a free market because the price is driven by tax, demand will stay low. No ammo, no guns.
"He who laughs last, laughs dead." Homer Simpson