There is no such thing as a "standard issue muslin shirt." The Army issued shirts of domet flannel and wool flannel...why some sutlers decided to make shirts out of modern (i.e. cheaper and lower quality) cotton muslin and call them "standard issue" is beyond me.
It's something to consider getting in the future, and can become an interpretive talking point itself, i.e. why might wool be used for a shirt? Better availability of the raw material in the North, the better durability, the versatility of wool to actually keep you cooler in the summer (wicking away sweat) and warmer at night (you don't want cotton against your skin when it's cold), the fact that woolen base garments would have been very common to people from civilian life, the anecdote about shirt material being used to assist with identifying a deceased's affiliation after Gettysburg, etc.
Marc A. Hermann.
The Daybreak B'hoys.
Liberty Rifles - Hardtack Society.
Oliver Tilden Camp No. 26, SUVCW - Co. I, 83rd NYV, SVR.
Descendant of Pvt. E. Hermann, 45th PA Militia - Capt. Wm. K. and Lt. Geo. W. Hopkins, 7th PA Reserves - Pvt. Jos. A. Weckerly, 72nd PA Infantry - Pvt. Thos. Will, 21st PA Cavalry.