You said one of them was loaded, double shotted, do you remember what it was loaded with? Just 2 solid shots?
I have an odd fascination with double shotting guns, especially if it's a combination of 2 different types of projectiles.
It was loaded with two 12pr solid shot - just a charge of loose powder (not in a cartridge as was another I unloaded) with two shot above - no wood bottom, no wadding etc. I saw lots of solid shot over there - never saw a shell, but they are rare it seems, even in most places where they were once issued in number - unfortunately......
In British service double shotting could include a shell, (or other projectiles), if so it was placed over the shot, not below for obvious reasons. The shell would have a wood bottom (all shells did unless being used as hollow shot or if they were mortar shells) held on by tin straps or rivets - depending on the period in question. Wood bottoms therefore for naval service shells - where double shotting was usual - were fully hollowed out so that the base of the shell made contact, metal to metal, with the top of the shot that lay beneath it. Or else the shot would impact the shell - through the wood bottom - with enough force to potentially break the shell. To hold the wood bottom in place, in the period following the use of tin straps, two small copper rivets were driven into two rivet holes in the shell, sufficiently far enough away from the exact center of the base to where the wood bottom afforded enough thickness to admit & hold a rivet. Whilst for Land service the same shell had just a single larger rivet hole dead center of the base - its wood bottom had enough thickness of wood to enable this, as these were not going to be used for double shotting. Therefore the number & position of rivet hole/s - if present - can identify the nature (Land Service or Sea Service) and, to some extent, the approx manufacture period of a shell. If no rivet holes then the shell predates the introduction of rivets. The fuze hole details are also a key to determining shell type & approximate manufacture dates. This all pertaining to British common shells etc, I know little or nothing about ordnance of other countries.
I hope this makes sense, a shortish description of an involved & fascinating (to me anyway) subject.
Adrian