Link to Western Va county maps showing the evolution of counties from 1738 on:
Virginia's Counties in 1738
Virginia's Counties in 1770
Virginia's Counties in 1776
Thomas Jefferson to Lafayette May 29th, 1781 (Lafayette in the American Revolution Vol IV page 143)
"I am sorry it has not been in your power to send me the County Returns of Militia. I assure you returns weekley are indispensably necessary to enable the Executive to keep Militia in the Field. I did however on receiving Information from Colo. Walker that the Enemy were reinforced call for one Fourth of the Militia of Washington, Montgomery, Botetourt, Rockbridges Augusta Rockingham and Amherst, which (the last excepted) are our best rifle Counties. They will rendezvous at Charlottesville and there expect your orders..."
An article titled Henry County. From Its Formation in 1776 to the End of the Eighteenth Century (The Virginia Magazine of History and Biography Vol. 9, No. 4, Oct 1902) contains some very interesting extracts detailing the prices of arms for the use of that county during the American Revolution. In all, Henry county impressed 10 smooth bored guns/shot guns and 8 Rifle Guns for the militia of the county from 1776 to 1781.