There is no such thing as a Flintlock rifle. There is a rifle and a flintlock. A rifle is a weapon with rifling in the barrel which are curved notches wich cause the bullet to spin as it travels down the barrel. A flintlock is a weapon with no rifling and shoots a spherical ball. It is propelled down the barrel by an explosion caused by a flint stiking a metal plate. The sparks then ignite some gunpowder shooting the shot from the barrel. A flintlock was not a brilliant weapon, they weren't that powerful, they had a poor range and they sometimes misfired.
flintlock rifles
Mainly flintlock smoothbore muskets, with a few flintlock rifles. There were also flintlock pistols and "fowling pieces"- shotguns.
Flintlock pistoles and rifles where mainly used when pirates roamed during 1600-1700
Muzzle loading flintlock rifles, pistols and shotguns.
When describing hunting rifles of the 17th, 18th and 19th century, some writers use the term flintlock, instead of using the fuller description of flintlock mechanism.
Muzzle loading flintlock rifles, pistols and shotguns.
powder horn
Mainly flintlock muskets. There were a few muzzleloading rifles, but many were smoothbore muskets, some shotguns.
As far as I know, not really much, unless they had interactions with Westerners by then which they will have access to flintlock rifles.
From the very beginning. The American War for Independence against the British was waged with Brown Bess and Charleville flintlock muskets, as well as any number of homegrown Jaeger rifles, Kentucky and Pennsylvania Long rifles, flintlock pistols and more. Actually this was all before we were actually the "United States of America." Before we were a nation, we had guns, and have had them ever since.
You would have to explain your criteria a little better. There are several ways in which rifles can be categorised. They can be categorised by action, e.g., muzzleloader (which can be further broken down into matchlock, flintlock, percussion cap, etc), lever action, bolt action, self loading (semi or full auto), breech block, etc.
Varies. European Jager rifles were shorter, and fired larger bullets. The Pennsylvania rifle (miscalled the Kentucky rifle) were of smaller caliber, but longer- about 5 feet.