Thanks, Tom. From what I had been able to find only one 3 in. Rifle burst during the Civil War. Now, it looks like maybe at least two. Your account has one bursting in 1863. The following has one bursting in 1864 with the claim that it was the only one to do so.

from: http://www.militarytrader.com/article/Ordnance_Museum_US_Model_1861_3_inch_Ordnance_Rifle/

"In testing conducted by the Ordnance Department in 1856, John Griffen challenged the Ordnance Department representatives to burst the gun. After more than 500 rounds with increasing charges and loads, they finally succeeded only by firing it with a charge of 7 lbs of powder and a load of 13 shot which completely filled up the bore. In fact, only one Ordnance Rifle is known to have burst during the entire American Civil War while firing double canister during the Battle of the Wilderness. It burst at the muzzle, the safest place for a gun to burst. "

Wow, a 100% increase in the number of possible KA-BOOMS. (maybe) OK, so more digging and research to do.