Resulting from some earlier postings I have looked up all the information I can find on the origins of British Fusiliers and why they wore uniforms similar to Grenadiers.

In the late 17th Century British battalions were mostly armed with matchlock muskets. These men wore hats with floppy brims, which by the end of the century had been turned up to form tricorns. Grenadier companies were formed in these battalions and they were armed with the new flintlock muskets (also known as fusils) since a burning match would have been hazardous for the grenades. The Grenadiers needed both hands free to light the fuze on their grenades so slung their fusil over their shoulder to do so. To facilitate this slinging of the musket they wore mitre shaped Grenadier caps, which eventually evolved into bearskin caps.

In 1685 the Royal Regiment of Fusiliers (later 7th Foot) was formed (with some initial intake from Tower of London Guards). This had a role of guarding the Artillery train and so all the men were armed with fusils to prevent burning matches required for matchlocks being a danger to the gunpowder in the train. They were also called upon to assist in manhandling artillery pieces from time to time and so needed to sling their fusils. This is why they were issued with similar mitre caps to Grendiers. The British Army being what it is, the Royal Fusiliers concentrated on their elite Grenadier status and moved away from the less imposing artillery guard role as quickly as possible.

By the end of the 17th Century two other existing Regiments were converted to Fusiliers. These were the Royal Scots Fusiliers (21st Foot) originally formed in 1678 and Royal Welch Fusiliers (23rd Foot) originally formed in 1689. Most sources give no reason for this but I did find one mention of the Royal Scots Fusiliers also being used in the Artillery guard role. I would surmise that all three were used in this role, even although neither of the latter admit to this on their own Regimental websites, merely stating that they became Fusiliers as an elite status. Both of these Regiments were then armed with flintlock fusils and both adopted fusilier caps.

By the Napoleonic wars all British infantry was armed with fintlock muskets. The three Fusilier Regiments therefore had exactly the same weapons and organisation as everyone else. They however considered themselves as "Grenadier" batalions although in truth they had the same recruiting system as all other infantry and therefore no real claim to elite status. However a great deal of being elite is considering yourself to be so, and this undoubtedly was true of all three Regiments. After the Napoleonic wars other Regiments were granted Fusilier status, including 5th Foot, who had long had aspirations to such status and already wore grenadier style white plumes.

Other armies had different nomenclature, so that a French fusilier was a centre company man and a Prussian fusilier was a light infantryman.

I believe what I have outlined above is correct, but would welcome comments

Rod