Bottom Line Up Front (BLUF): The difference between heavy and light infantry was as a result of what they were armed with and how they were employed (formation). This was an evolution and not a revolution. It does not impact the use of a line formation but how it was dispersed. During Napoleonic times the regiment was usually three ranks deep and during the Civil War it was two. The rifle allowed the formation to become a little more "loose ordered" or dispersed. The additional range allowed the soldier a little more range so a concentrated fire could be delivered more with a little deviation left and right of the firer.

If you look at the Militia Act of 1792 (Second Act, passed May 8, 1792, Section IV), it states that, "out of the militia enrolled as is herein directed, there shall be formed for each battalion, as least one company of grenadiers, light infantry or riflemen; and that each division there shall be, at least, one company of artillery, and one troop of horse: There shall be to each company of artillery, one captain, two lieutenants, four serjeants, four corporals, six gunners, six bombardiers, one drummer, and one fifer." I interpret this as being that the light infantry were riflemen and those that were not were armed with smoothbores and if they are not light then logic would say they were heavy. Now, the transformation takes place during the Napoleonic wars and the incorporation of the more widely available rifle musket.

Paddy Griffith's "French Napoleonic Infantry Tactics 1792-1815" discusses Heavy Infantry and the debate between the use of the column or the line formations. Interesting read and good background.

"Scott's Infantry Tactics," published refers to light infantry or skirmishers. These were the two flank companies, normally armed with rifles, that were deployed forward or to the flanks of the regiment (remember two battalions per regiment, one light infantry company per battalion) to provide some protection and early warning to the regimental main body.

Hardee's "Rifle and Light Infantry Tactics for the Exercise and Manoeuvres of Troops When Acting as Light Infantry or Riflemen," published March 1855, used his experiences, Scott's ideas, the French manual "Ordonnance du Roi sur l'Exercise et less Manoevres des Bataillons de Chasseurs a Pied," and the technology of the rifle musket to develop the new tactical formations. It is clearly intended to take technology (the rifle), European contemporary tactical thought, and lessons learned from the Mexican War into American doctrine.

I hope this helps.

Respectfully,

Gerald D. Hodge, Jr.
M.A. Military History - Civil War Concentration.
War Between the States Historian
Historian: 39th Georgia Volunteer Infantry Regiment