There were a lot of gunmaking Mortimers- family sort of thing, A few of them moved up into Edinburgh and set up shop, and at least one made it to the US, where he went into gunmaking with Mr. Kirkwood.
Some internet gleanings:
There is a book out called "The Mortimer Gunmakers" by H. Lee Munson.
Partial records of the firm are currently held by John Dickson & Son. http://www.john-dickson.com/the_company/the_company/thomas_mortimer.aspx
John Dickson & Son Ltd
21 Frederick Street
0131 225 4218
0131 225 3658
When Thomas Jackson Mortimer died in 1833 the business at 34 St James's Street, London was taken over by widow, Elizabeth, and their son, Thomas Elsworth Mortimer. It continued to trade until 1835 (the premises were taken over by Thomas K Baker) but in 1834 T E Mortimer had opened a business under his own name at 78 Princes Street, Edinburgh. In 1836 the Edinburgh firm were appointed Gunmaker to His Majesty.
In 1840 the Edinburgh shop moved to 97 George Street, in 1854 it moved to 86 George Street. In 1851 the firm was awarded a Prize Medal at the Great Exhibition in London.
Thomas Elsworth Mortimer died in 1860 aged 53, and the firm was taken over by his widow, Emily, and their son Thomas Alfred Clark Mortimer. Some records state that it was in 1860 that the firm was re-named Mortimer & Son, this is certainly possible but it has not been confirmed, see below.
In 1871 the firm was appointed Gunmaker Extraordinary to the Prince of Wales, but no labels confirming or referring to this have been seen.
It is recorded that it was in 1879 that the firm became known as Mortimer & Son, this is also possible but again, it has not been confirmed.
The Mortimer guns appear to have been better quality guns.