The Federal Excise Tax on guns and ammo is codified in 26 U.S.C. 4181 and 4182. The percentage is set in stone (it's 10% or 11%).
http://caselaw.lp.findlaw.com/casec...apters/d/parts/iii/sections/section_4181.html
http://caselaw.lp.findlaw.com/casec...apters/d/parts/iii/sections/section_4182.html
Section 4181
There is hereby imposed upon the sale by the manufacturer,
producer, or importer of the following articles a tax equivalent to
the specified percent of the price for which so sold:
Articles taxable at 10 percent -
Pistols.
Revolvers.
Articles taxable at 11 percent -
Firearms (other than pistols and revolvers).
Shells, and cartridges.
Section 4182
(a) Machine guns and short barrelled firearms
The tax imposed by section 4181 shall not apply to any firearm on
which the tax provided by section 5811 has been paid.
(b) Sales to defense department
No firearms, pistols, revolvers, shells, and cartridges purchased
with funds appropriated for the military department shall be
subject to any tax imposed on the sale or transfer of such
articles.
(c) Records
Notwithstanding the provisions of sections 922(b)(5) and 923(g)
of title 18, United States Code, no person holding a Federal
license under chapter 44 of title 18, United States Code, shall be
required to record the name, address, or other information about
the purchaser of shotgun ammunition, ammunition suitable for use
only in rifles generally available in commerce, or component parts
for the aforesaid types of ammunition.
Congress made it, only Congress can change it. The law gives the president precisely 0% discretion on this.