Likely you’ve seen this photo before:
The view overlooks Belle Isle with Richmond, Virginia in the background. Naturally what I’m drawn to is the cannon. The size leads me to identify the field piece as a 6-pdr gun.
But several details indicate this is no regulation 6-pdr. The breech face has a ring, or perhaps an oversize base for the neck of the knob. Just beyond the base ring is a vent astragal, or ring. Hard to be sure, but there don’t appear to be rimbases between the trunnions and barrel. There’s a gap between the cap square and the trunnion. The gun seems too small for the carriage. A non-regulation gun on a carriage for regulation guns.
The gun in the photo reminds me of the weapons produced by Columbia Foundry and John Clarke (predecessor of Bellona Foundry) prior to 1820. But the angle of the gun prevents any direct comparison. Nor are any markings visible.
However, there are two iron guns at Fort Branch, North Carolina which resemble that gun at Belle Isle.

Neither this gun or its partner piece have any useful markings to aid with identification. But the form has several features matching that of the Belle Isle gun – wide ring at the base of the knob’s neck, vent astragal, and lack of rimbases.
Archaeologists recovered the guns at Fort Branch with their carriages. Neither exhibit the size problem between the cap square and trunnions.

The other gun was dismounted from its original carriage and placed on a reproduction carriage for display.

In external form, there are several features in common with the 6-pdr bronze militia guns on display at the National Guard Museum, which I discussed in an earlier post. Again, wide ring at the neck of the knob, astragal ring, and no rimbases. But of course bronze instead of iron.
Restricting the conversation to iron guns, another surviving 6-pdr, this one at in Sunbury, Pennsylvania, also resembles the Belle Isle gun. The Sunbury gun, however, has rimbases. I mentioned it in a post last year.

Regardless of any similarities or dissimilarities that I might suggest, the exterior form of all guns mentioned above had fallen out of favor by 1861. Odds are those guns were cast well before the secession crisis.
I can offer nothing conclusive about the Belle Isle gun. Perhaps that gun was from a pre-war militia battery. Or perhaps some old weapon purchased on the market in those hasty early days of the war. All I can say with certainty is that a handful of very similar guns saw service in the Confederacy.