The hammer on a flintlock is the steel, what people these days call a frizzen. The hammer cap is a leather cover which covers the face of the hammer as a safety precaution. If the lock were to slip off the half-cock safety position, the gun will not fire because the flint will strike the leather & not the face of the hammer.
The hammer cap or hammer stall is secured to the trigger guard.
Here you can see the hammer cap in place on the hammer of my fusil.
Hammer Stall Documentation.
Lieutenant Colonel Wyatt immediately gave the order, 'Off hammer caps'
Thorp, 1808.
...On Service, leather Hammer-stalls are undoubtedly an advantage to a Battalion, when loaded, and resting on their Arms, as accidents may be prevented by having them fixed upon the hammers of the Firelocks..."
Cuthbertson. (pg 93, XIII).
"It is left to their [officers] discretion in time of real Action to disencumber such men as they may think proper entirely of them [arms], taking care that they be lodged in their Ammunition Carriages and to prevent any possible Accident happening therefrom, thumb stalls have been ordered to be provided which the men are constantly to keep on the hammer of their pieces except when posted centrys." Great Britain, Royal Artillery Regiment Library, Woolwich, Brigade Orderly Book, James Pattison Papers. 1778.
The hammer stalls shall always be made of red Russia leather.
Regulations for the Prussian cavalry, tr. [by sir W. Fawcett].
http://books.google.com.au/books?id=yUY ... th&f=false
The men arms to be clean and in perfect repair, to be completed
with stoppers, pickers and brush, and hammer caps, which ought to be
made of
black leather.
CAMP AT WATSONS FERRY
20TH SEPTEMBER 1761.
REGTL.ORDERS
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On service, leather Hammer-Stalls are undoubtedly an advantage
to the
battalion, when loaded and resting on their arms, as accidents
may be prevented, by having them fixed upon the hammers of the
firelocks.
(Cuthbertson, Bennett. System, for the Complete Interior Management
and Oecomomy for a Battalion of Infantry , Bristol, 1765. page 93)
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