In my collection, I have a similar Westley Richards .375 2.5" but with 26" barrels. Also, in my collection, I have four other .375 2.5" British doubles, not Westley Richards; one being a hammer gun regulated for 300 grain bullet--all others I have a regulated for 270 grain bullet. I have handloaded and shot them for years. All of mine have 26" to 28" barrels. I would be highly suspicious of 18" barrels, thinking that they may have been cut short by some previous owner, unless shown otherwise by Westley Richards the company. I am not saying that they didn't make them that short originally, only that it is highly unlikely!