Most every thread Schultz & Larsen uses is metric. I have no idea what metric thread was used on the 54. Best way is to pull the barrel and measure it! That goes for just about any receiver. The reason case heads separate is because the case doesn't fit the chamber properly. I would not count on being able to use cases fired in your current barrel in a new barrel with a custom cut chamber. Factory chambers are always more 'generous' in their size. Why go back to the 7 x 61S&H? Brass has got to be a real 'pain' to come up with, and probably more expensive when you do find it. The 7mm Rem is a good 'sister' to the S&H and a heck more available, too. You will have lost all 'originality' of the rifle when you have it re-barreled. Just an added note here,, if you'd choose to go back with the 7 x 61S&H I would expect you'd have to supply the reamer, as reamers are costly and for a gunsmith to buy a reamer and use it once makes no economic sense to the gunsmith. The 7 x 61 is not a popular chambering in the US. Gunsmiths buy chambering reamers they will use more than once or twice. Expect the cost of the reamer and gauges to be added to the cost of the re-barrel job, And,,,, barrel blanks don't come already 'blued', so expect bluing to be added to the cost, also.