George,

The long range CSA breech loaders you are thinking of were most likely Whitworths--several 12 pdrs and less were used that way.

With regards to the large caliber naval Armstrong breech loaders, in Ripley's "Artillery and Ammunition of the Civil War" while discussing a specific Armstrong rifling system on page 139, he says that "the system was discarded in 1863 after breech-loading proved a failure in Armstrong's heavy guns forcing return to muzzle-loading for new designs and conversion of existing 70 and 110-pounders to this form."