William Barr was indeed a NYC gunsmith/gunmaker. He is listed in Vol. 5 of the five volume set of books The New York State Firearms Trade by H. J. Swinney, compiled by Tom Rowe.
Barr is listed in NYC directories as early as 1835. His shop was at 106 Beekman Street from 1835 through at least 1850. He was listed in the 1850 Industrial Census: $500 invested, employed one hand @ $30 per month, purchased 40 barrels, mountings, etc., @ $160; made 40 guns totalling $480, did repairs, etc. worth $750.
There were several other gunsmiths named Barr in NYC also, probably related.