I just bought sight unseen a Remington RB carbine. It was advertised as.50 calibre and ‘good for its age’. On first examination it looked pretty grim but while this gun has I think, lost its historical/collector value, it does have potential as a shooter. I want to shoot it but here’s the rub. In finding or creating a cartridge which will shoot – there are two main issues. First, it has a rim-fire block, and second, while it is notionally 0.50 calibre, (i) the bore slug gives it a groove diameter of 0.534 and a diameter of 0.517 for the lands and the chamber cast shows (Ii) the original cartridge is straight walled and possibly as long as 1.5 inches from its base to the commencement of the throat at the chamber, and (iiI) at 0.660, the rim diameter is exactly the same as that for the 50/70 Government cartridge. See attachment.

I can identify no rimfire cartridge (Spencer, Joslyn etc.) whose specs may have been a match for this chamber and upon which I could base the conversion of a centre-fire case to suit, although the 56/56 Spencer comes closest. Because of the suitability of the common rim diameter to the 50/70, I then thought the 50/45 or 50/55 centre fire cases which the 50/70 is based on might have specs which match instead. But they do not – for the simple reason their bullet diameters are about 0.515 – projectiles which are way too small for the bore on this gun.

The best I have been able to come up with for the moment is to paper patch (using .008’ paper) the 0.500’ 400 grain bullet I use in the 12.17 roller, to bring it up to 0.532 diameter. This seems to fit pretty well, although whether the patched bullet would work in a somewhat rough bore is a question for the future. I used this patched bullet to slug the bore and it seemingly was a very good fit. Somewhat hard to drive down the bore, but it picked up the rifling completely and afterwards measured the same groove and land diameters of the casting. So this is one possibility. However, if I was to base the proposed cartridge on a shortened 50/70 case and this bullet (patched or plain), the case would probably have to be substantially shorter than 1.48 inches if it was to have a wide enough throat to accept the bullet. That in turn would present the question of how much powder such a shortened case could carry under a bullet of that length and diameter. Another possibility is to do a little reaming to have the chamber extended to take a longer case. These are considerations I have to work on some more. But any assistance in getting me along the road to getting this thing shooting (the areas of uncertainty I have are captured in the questions below) would be greatly appreciated.

(i) What is the original rimfire cartridge this gun was chambered for – and does anybody have any specs for it in terms of physical dimension s, bullet weight and shape and powder charge,
(ii) Has anybody successfully prepared a cartridge for the 0.50 calibre rimfire,
(iii) Other than the 50/70, does anybody know of a donor case suitable for conversion,
(iv) Is there a mold (mould?) of suitable bullet shape, weight and diameter out there I might be able to get my hands on,
(v) I will need to convert the block from rim to centre-fire. Do-able of course but is there any intrinsic value of this rimfire gun which suggests it would be better to obtain a centre fire block and using it instead,
(vi) Has anybody any experience with the block castings available from The Rifle Shoppe – how much fettling is involved in getting them to fit and work,
(vii) Does anybody out there know where I can get a used #1 centre-fire block for a #1 Roller?
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