This is a .300 inch Rubin cartridge as acquired by Britain from Switzerland for trials in 1886/7. The case is straight sided and has a split brass collar inside the neck to hold the bullet. There is no headstamp and it has the typical Rubin very flat, square edged, primer and the radiused extractor cannelure. The bullet and case mouth are coated in a soft, sticky, wax.
This cartridge has not been interfered with so it presumably still contains its 70 grain compressed blackpowder charge. Measured dimensions are:
Cartridge overall length: 78.2mm (3.079”)
Case length: 53.4mm (2.10”)
Bullet diameter: 7.95mm (0.313”)
Neck diameter: 10.25mm (0.403”)
Head diameter: 11.7mm (0.461”)
Rim diameter: 11.6mm (0.457”)
Rim thickness: 1.7mm (.067”)
Note the bullet and neck diameters may be a trifle high as I didn’t want to disturb the wax coating any more than necessary.
Reference: .303 INCH, A History of the .303 Cartridge in the British Service. By P. Labbett & P. J. F. Mead. ISBN-0-9512922-0-X. 1988.