Comparisons between the US Navy and the Royal Navy in the 1812 War are very difficult. The US Navy was tiny (in comparison), spent much of its time being blockaded and seldom fought an action with a vessel of the same size. Very few Americans will admit this though, but very few Americans have actually studied the war other than through rose-tinted glasses focussed on the victories gained over the largest Navy in the world at the time.

On the subject of gunnery practice it is difficult to indulge in live practice when you are blockaded in harbour, and it is not a good idea to fire away powder and shot when you are blockading an enemy who may come out at any time.

Dahlgren and Parrott guns were much later, even Colombiads were a bit later. There was basically no difference between British and American guns at this time. I'll be back with more detail when I can find the right book (Arming the Fleet). One main difference (in layman's terms) was that American ships often had thicker sides than ships they were facing - hence 'Old Ironsides'.

There was also a lot of luck involved in a sea-fight in those days, and poor information. An example of this was United States and Endymion (a more equal combat than most). Endymion stopped firing because she though United States had surrendered, and United States though that she had beaten Endymion because the latter had stopped firing.