The whole aquafortis thing is confusing because the term is used for at least 3 different things by various people.
Nitric acid is aquafortis. It is used to make a solution of ferric nitrate by adding it to water then adding iron which it dissolves in the chemical reaction, giving off heat and gas. The actual aquafortis - nitric acid - will dissolve your barrel.
Next we have iron saturated solutions of mostly exhausted nitric acid and water. This works great for staining maple but needs blushing with heat plus usually neutralization or st least a good wash out depending on how much acid is left. This stuff, if enough acid remains, can rust barrels in a controlled fashion.
Next we have purified ferric nitrate powder or solution which has no remaining nitric acid. Not useful for browning barrels by itself as far as I know. This may be what Jim Kibler and others sell.
Then Whakon Bay used to sell something they called aquafortis which had nitric acid, water, dissolved iron (ferric nitrate) plus hydrochloride acid. This all rounder would stain maple, rust barrels, and so on.