Just to be clear here, the mounted shell jacket was the intended dress coat for both Light Artillery and Cavalry. Heavy Artillery were issued frock coats for dress. All three were issued sack coats for fatigue or field duty and it was up to the various unit commanders as to preferred field dress.
Hancock, when 2nd corps commander, preferred rank and file to wear a forage cap or kepi, while officers could chose their headgear preference.
There were units who wore their dress coats and jackets in the field, probably more as an esprit de corps. By far, an unlined sack coat is the most comfortable piece of uniform cloth, in warm weather.
The unlined sack coat was so light and non forming that it did not look too martial when worn in the field, more like men wearing light sweat shirts.
An original unlined sack coat, when held up to the light, may be seen through and when folded tightly, can fit in a gallon sized zip lock bag.
The unlined sack coat was the single most issued piece of, non headgear clothing, issued during the war.