8X63 FN Conversion ---Why?
The 8x63 Swedish was developed in 1932 as a Military cartridge.
The book "Handloaders Manual of Cartridge Conversion" lists a load of 57 grains of 4320 with a 170 grain bullet for a velocity of 2820 f.p.s.
A 1967 Hornady reloading manual gives an 8x57 Mauser load of 52.5 grains of 4320 with a 170 grain bullet for a velocity of 2800 f.p.s.
Next: Semi-automatic firearms such as the FN 49 operate from gas pressure. This pressure has to be "balanced" to operate the action, and the FN system can be adjusted. If underpowered, it will not function, if over pressure, it will be hard on the action, or even dangerous. Remember, your nose is just 6 inches behind 50,000 pounds per square inch of pressure.
The cost of such a conversion would be expensive. It would involve cost of a reamer, labor for a gunsmith ( if you can find one who would agree with the conversion --- and deemed it safe ), and probably magazine or feed rail alterations. The action is designed to use a cartridge 1/4 inch shorter and the feed lips, etc, may not be in the right place.
Before you do it, try some of the MODERN loads from Norma or other European manufacturer. The 8x57 can be made to perform almost to the level of the 30-06 but the reloading manuals tend to be on the lower end of the scale because of LIABILITY. There are several 8x57 cartridges, the earlier being .318 bore and after 1900, generally .323 bore. European hunters are well versed in the difference, but here in Canada and the USA, many are not. A .323 bullet fired in an old Mauser with a .318 bore is dangerous, so these loading manuals, having no control of the type of firearm the reloader will use, tend to be conservative in their loading.
Best of all, buy a FN 49 in 30-06 Calibre. It will probably be a lot cheaper, and you will have the same thing as what your are trying to accomplish, loading dies, etc, are readily available and cheaper, and there is still surplus 30-06 available.
Why try to re-invent the wheel?