When I was at VMI in the '60s, the Cadet Battery used M101A1's (the WWII vintage split trail 105mm). There was one permanently stationed outside barracks on a concrete platform as the evening gun, but when salutes and honors needed to be rendered, a full six gun battery of 105's went into battery at the end of the parade ground. Back before VMI gave up its horses during WWII, the Cadet Batteries(!) were horse-drawn French 75's, configured as horse artillery. The designated salute battery would apparently gallop (canter?) onto the field and go into battery for the salute. I've only seen it at VMI on film, but there was a horse-drawn Field Artillery half-section at Ft. Sill with a French 75mm which would do the same thing from time to time, especially at firepower demonstrations. It was impressive. In most pre-WWII VMI garrison reviews, there would be two Batteries of horse artillery, two cavalry troops and two infantry companies.
Subsequently, the four six-pounders of the Letcher Battery (guns which served throughout the Late Unpleasantness and have been at the Institute since the 1870s) were brought back to firing condition and now they are sometimes used to render salutes, although the timing is different. It takes close to a minute for a very well trained crew to reload a muzzle-loading six-pounder, even under optimal conditions.
Sorry about getting to this late.
My old unit never had ot do the 21 salute either.
We did, however, get to do the 1812 Overture for a few years at Thomas Bull Memorial Park here in NY.