The Howard Francis carbine was a design submitted to the British Ordnance Board for consideration in 1943 – one of many weapons proposed to help meet wartime requirements. Specifically, the Howard Francis carbine was a conversion of a bolt action SMLE into a semiautomatic pistol caliber carbine.
Chambered for the 7.63mm Mauser cartridge (the same as used in the C96 Broomhandle Mauser pistol), the conversion was done by plugging the chamber of a .303 Enfield barrel and then re-reaming it to fit the pistol cartridge. If you are wondering why the 7.63mm Mauser was used instead of the more potent but dimensionally identical 7.62×25 Tokarev, consider that the Tokarev cartridge was not in common use in England at the time, while C96 pistols were well known and popular. The 7.63mm bore was close enough to the .303 to allow its use without altering the actual bore of the barrel.
Anyway, the mechanical action of the carbine was quite simple. In place of the regular turning bolt, a heavy blowback bolt sans locking lugs was made. A tubular extension out the back of the action contained a recoil spring (rather like the Soviet DPM) and the extension locked into the rear locking lug recesses of the receiver – a clever use of those existing features. Upon firing, the heavy square breechblock assembly would simply blow backwards against the recoil spring. Its striker would reset, and be ready to fire again with another trigger pull. The prototype examples made were semiautomatic, not fully automatic.
The barrel was cut back to 12.75 inches (324mm) to make the weapon handier, and the final product weighed in at 8 pounds, 1.5 ounces (3.7kg). Not much less than a standard rifle, which would make it feel virtually recoil-free to shoot, I expect.
Approximately the front third of the original .303 magazine was removed, and the remaining portion of the magazine used as simply a filler block in the action. The magazine actually feeding the system was a 12-round proprietary design similar to the Browning High Power magazines in use by British forces (but not identical, because 7.63mm Mauser is too long to fit in a 9mm Parabellum magazine). The example pictured below does not have a magazine, unfortunately.
Reportedly the weapon was tested from 30 out to 200 yards and was pretty accurate (not surprising from a fixed barrel carbine, really, even though the rear sight was a simple open V notch mounted on the moving breechblock). However, it suffered from feeding and extraction problems – also not surprising for a prototype carbine converted to use a new and proprietary magazine. After those test results, no further actions were taken to develop the design. The STEN was well into mass production by this time, and there was no compelling reason to adopt a converted SMLE using a non-standard cartridge.
The Howard Francis carbine pictured below is in the National Firearms Centre collection at Leeds, and may well be the only one still in existence.