I'm personally not extremely familiar with Civil War artillery, but I am not aware of many two-pounder guns. Maybe McNeil meant to report two [six-]pounder guns in his description of Lieutenant Alexander McLaren's battery.
In an August 16, 1862 report of Colonel James McFerran he mentions 2nd Lieutenant William Caldwell of Company L, 1st Cavalry MSM and his two six-pounders. The M1841 six-pounder, although somewhat outdated, was used by some Union and a number of Confederate units during the Civil War. The gun had a tube length of 60" and a tube weight of 884 lbs. Its bore diameter was 3.67 inches. At a 5% elevation, it had a range of 1520 yards. This may have been the type of gun that Captain Johnson's artillery company employed, but again I would look for someone with more knowledge about Civil War artillery to comment on whether this is likely or not. Anyhow, I hope this helps.