Q: What looks like a .303 but isn't?
A: An RFI .410 Musket of course!
This is an SMLE manufactured by BSA Co. in 1917, originally in .303, then converted at Rifle Factory Ishapore (RFI) in 1931 to a .410 Musket. These conversions from rifled .303 to smooth bore .410 were done from 1927-1950 in India, ostensibly for use by police, guards, etc. that needed to be armed, uh, but not too well armed. They carried the musket mostly for authority, I suppose.
It's a single shot conversion with the magazine removed and plugged with a wooden block that also serves as the feed ramp for loading single cartridges. Note that the rear sight is also pinned through the slider to make it a fixed "adjustment".
These are not your normal .410, they were made from .303 cartridges that had not been necked yet. simply a straight wall cartridge crimped over the lead ball. You can see from the photos that the 97gr .410 lead ball is driven by just 13.3gr of powder. The ball is well padded by fiber wadding (it's not nitrocellulose, I did an ignition check) and cardboard discs.
I shot all 10 rounds from one package at a 25 yard target and it gave a solid 24moa, i.e., about a 6" grouping. No hang fires, no failures to fire. It is a powder puff load and is really just a hoot to shoot! The rifle was on my want list and a recent pick up, but the ammo I got from Dan's Ammo while he still had a sea-tainer full of the stuff a couple of years ago. I bought two of the little crates of 180 cartridges (18 packs of 10), i.e., a life time supply. It's 1960 Pakistan Ordnance Factory (POF), so fresh 61 year old stuff.