Re-threading

assuming the pitch was identical I can see your thought process but its not
UNF is 18 per inch
15x1 is 1 per mm

chop 2/3" off and start fresh
 
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UNF is 18 tpi, UNEF is 24 tpi
5/8" is 15.875 mm
So if you'd have to remove the original thread which would leave you less than 15mm diameter.
In any case, by the time you've got the barrel in the lathe, you may as well chop off and start again.
 
ok, i have just purchased a new rifle, i have a moderator which is 5 8ths but the new rifle is in 1 x 15, however the barrel is already down to 19" (its a 6.5 x 55), so i guess id be best off buying a a new moderator....
 
The basic minor dimension of your 5/8UNF thread is 14.1mm. You are looking for 14.9mm OD of material for an M15X1 thread. Therefore a standard re-thead can't be done. If the ~15mm of barrel length is that important to you I would make a collar with 1/2UNF internal thread. Set-up the gun true to bore in lathe. Turn down the 5/8UNF and rethread 1/2UNF. Apply thread locker and screw collar to end of barrel, tightening with a strap. Turn down the collar to 14.9mm and cut a 1mm pitch on it. Re-crown and check concentricity between mod and gun. A bit more aggrevation than normal, but no need for a new mod.
 
The basic minor dimension of your 5/8UNF thread is 14.1mm. You are looking for 14.9mm OD of material for an M15X1 thread. Therefore a standard re-thead can't be done. If the ~15mm of barrel length is that important to you I would make a collar with 1/2UNF internal thread. Set-up the gun true to bore in lathe. Turn down the 5/8UNF and rethread 1/2UNF. Apply thread locker and screw collar to end of barrel, tightening with a strap. Turn down the collar to 14.9mm and cut a 1mm pitch on it. Re-crown and check concentricity between mod and gun. A bit more aggrevation than normal, but no need for a new mod.

That's what I'd do, make the thread in the mod a little tighter and use high strength loctite, saves buying a new mod. :)
 
I have the same situation, with a rifle with 14/1 thread and a 1/2" Moderator, there seems to be plenty of room but I'm no gun smith.
 
19" are you using it for deer ?
You might need a new rifle as 20" struggles to be large deer legal :(
 
I have a wildcat t8 threaded 15x1 with a spigot I was thinking of selling what model is your mod
 
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