The shortest standard factory Auto-5 barrel was 60 cm which is 23.622" It would have been marketed in the US as a 24" barrel.
Browning would accept custom orders in odd lengths, but I dont know of any in particular that were short.
Jeff
ps: Dont cut down a good original barrel, or I'll put a cures on your house....
If you must cut one buy a clapped out barrel with a polychoke on it and carve away.