Interesting, I have never tried it. It is documented in other theaters of the war that Confederate gunners used rail spikes. In the Diary of Cpl. Timothy Regan he noted in his 29 August 1862 entry that his company was fired upon by rail spikes. Col. Benjamin Harrison also reported Confederate cannon hurling rail spikes at the Battle of New Hope Church. See The Life of Gen. Ben Harrison by Gen. Lew Wallace at page 204. And as I previously mentioned, rail spikes were shot at the Union lines at Vicksburg, see Vicksburg, 1863 at page 419.

I would guess that the spikes were bound and wrapped in paper or cloth prior to being loaded. They may have even employed a wood or metal disc at the bottom of the "round" much like the cotton-wrapped grapeshot rounds. I would love to conduct experimental archaeology as to the feasibility and effectiveness of rail spikes as canister. Now if I could only get someone to loan me their 6-pdr cannon.