This is the internet and I do not know much about nuthin: read at your own risk.
Ferguson and Durs figured alot of stuff out to make the Carbine bore Rifle- Not much translates direct to any other cal.
If the TRS action is a 1 inch lead when you turn the screw one revolution the screw will likely fall further then the bottom of the chamber of a rifle in .54 unless the taper of the chamber is very steep. i.e there will be a pocket below the chamber to trap powder which will get ejected when the breech is closed. This pocket could be avoided with an overly large chamber mouth.
Maybe not a big deal but it will be different then Durs would have built. A screw which moves less in one revoultion maybe would be more correct for a .54.
Before purchasing this item you should have the seller send you a photograph of the female screw pitches above the chamber. How many are there? You cannot fix this if someone got carried away grinding the top flat. Probabley just fine but those pitches are what seals the breech. A 54 is going to operate at much greater pressure then a .62/.65 with very deep grooves and no patch so the 3 or 4 you might see on a Ferguson certainly need to all be there for a .54. The only benefit to a .54 in the same breech width as a .65 is the addition of sealing pitches. In a .54 you should be able to get 5 or even more if the lead is matched to the cal.
Details on the breech plug and chamber are also needed to judge a purchase. The threaded plug hole must be large enough to pass a tapered reamer so the chamber can be formed. If the chamber is already formed volume and shape should be confirmed. If anything needs to be done on the breech end there will be a lathe involved.
Pretty neat item. On the encouraging side 8620 lets you build a safe two piece ferguson to those breech dimensions. I have a notebook fulll of numbers crunched 100 different ways. for me 62 was not doable- .58 was marginal but .54 came out safe even for a coward like me. In fact the Rice early dutch barrel in .54 is an ideal candidate for fergusonifying. (hmm that sounds almost copyrightable LOL)
So I guess TRS does make thier Fergusons two pieces screwed together or did someone cut off the action from a one piece barrel/action assembley?
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If someone buys this please bring it down to your friendly NDT shop and have them shoot a digital x-ray!