Ammunition in most common usage at sea during the Napoleonic Wars consisted of:

Round Shot - solid iron ball used to damage the hull of an enemy ship.

Chain-Shot - Solid iron balls linked with chain, used to damage masts, yards and rigging of an enemy ship.

Cannister or Grape Shot - Tin Cannisters filled with musket balls, used primarily against personnel.

Prior to 1820 the Royal Navy did employ vessels called Bomb Ketches in attacks against shore installations, these ships were armed with Mortars or Howitzers firing shells.

Nelson's maxim was for getting in as close as possible in any engagement. Weight of shot became all important, this led to the Royal Navy arming Frigates almost exclusively with carronades. These light guns fired very heavy shot (Victory's main armament consisted of long barrelled guns firing 32 pound shot, the carronades on Victory fired 64 pound shot). Although the weight of shot was greater carronades had much shorter range, the application of this philosophy meant that during the War of 1812, the Royal Navy's Frigates (normally 38 or 28 guns) were at a distinct disadvantage when they enountered the larger and more conventionally armed American Frigates (44 guns).

I have never heard of a 74-gun ship being described as a third rate. Agememnon at 64-guns was a third rate.