Mondragon’s 5,2mm Cartridge is a "Piston "Design.
The Disc serves to support the Long Bullet in the short neck (prevent distortion when loading from the magazine) and also to act as a Pistopn (Like a Steam Locomotive Piston) to guide the Bullet into the rifling on firing…the Gas pressure exerts evenly on the Disc and the Bullet, pushing both together, until the disc reaches the shoulder, where it stops…the bullet by now is engaging in the rifling, and the pressure fully engages the base of the bullet, pushing it down the bore.
This allows for the Rapid-burning characteristics of early (1890s) Nitro Powders. ( Retarded & Progressive Powders only came out during WW I).
It also allows for proper stabilisation of a very long bullet, given its mass and small diameter.
Mondragon used the “Piston” system in several of his cartridges ( 5,2 and 6,5mm, and others) The 5,2 cartridsge was used in the M1893(?) Straight Pull/ Self-Firing magazine rifle ( three position selector, Safe, Single shot( single trigger pull) and "Self-firing " ( manual repetition…keeps trigger depressed, and Bolt locks and fires automatically. ( Like a “tricked up” Winchester LA).
Mexico bought a small quantity of the Mondragon rifles in the 1890s, and occasionally (Rarely) one will show up, usually “poquito descompuesto” ( as described in the 1967 G&A Article…one of the very earliest of my acquisitions of Gun Magazines…, I have it archived in my Library.). They were used to destruction during the Mexican Revolution, as long as ammo lasted. (Ammo Made by Polte, Rifles by SIG Neuhausen, Switzerland).
The ammo was loaded by taking the cylindrical case, fitting the Piston and Bullet over the Loaded (Primer and Powder) case, seating it at the cannelure, and then necking the shoulder and neck over the Bullet. As the Case diameter is quite large, and the bullet quite small, there would have been several sizing steps.
Truely a “small calibre Magnum” in any era.