Gavin: Antique percussion pistols, or any firearm made prior to 1898, is not cosidered to be a firearm by BATF so you should be O.K. with your pistol. I am not familiar with New Jersey Gun Laws. Someone else here may be, but I don't see that you have a problem with a muzzle loader.
As to you Army & Navy English gun, you might want to post on
www.doublegunshop.com for more information. I have several Army & Navy CSL doubleguns an all are very well made medium quality sporting guns.
Best Regards, George