The chambers of .22 rimfire rifles made in the early days, until at least 1920, were not really "chambers" as we think of them. They were just an unrifled part of the barrel and may have even had a slight taper. I have 3 Winders and a #3 Ballard in .22 Short, one Winder since lined to Long Rifle. All would readily accept ordinary .22 Long Rifle cartridges. The LR cartridges could be fired safely, but at 50 yards accuracy was a sometimes thing. They would hit somewhere in the tiny targets at gallery ranges (50 or 75 feet). It is not necessary to "rechamber" those old rifles and doing so will only harm them.
With the onset of prone and position match shooting circa 1900, tighter chambers were designed. Today there must be a hundred or more individual chamber designs. The various chambers designed for target rifles were made small and it takes some degree of effort to chamber a Long Rifle cartridge in some of them.
GDU's Martini could be one intended for the 22 Long cartridge, or it could be one with a tight target chamber intended for the Long Rifle. In any event, the easiest way to find out is to try it with some sort of standard velocity, lead bullet Long Rifle ammunition shot from rest at a paper target at 50 or 100 yards/meters.
If it proves to be inaccurate with LRs, try it with .22 Longs. I have had reasonable results at gallery targets with some ".22 CB Long Match" cartridges made in OZ and marketed in the US under the Winchester brand. Those have 29-grain bullets intended for Shorts loaded into Long cases. I suspect that the powder charge is that for the Short. I do not think they are just propelled by the primer.
