I don't see any reason to not use jacketed bullets. History says the Metford failing was was the hot erosive gases of cordite, not the rifling or steel of the barrel.

I hadn't given much thought to barrel length in these gun so I went out to the shop and measured mine.

Rawbone .303 28"

Bonehill .303 24"

Barnsley 577/450 28"

Greener 577/500 Express 28"

I. Hollis. Westley Richards Musket No. 2 26"