Good Morning,
as I wanted to change my barrel of a 645 from 45HP to 45ACP,
a gunsmith I know has found the (lower acp) barrel. It is ok but I don't know,nor does he as he's quite unprofessional, if it is for a 4506. The problem is that I cannot fix the barrel spring and guide in the notch: it flips always away. With the HP barrel I don't have the problem, as you see the "notch" in the acp is slightly smaller.
I hope you understood as my english is not very good.
Regards
Matteo

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They appear to be slightly different in angle and cut. What exactly is a HP barrel ?

Exactly what I was wondering.

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Waiting for the break of day

Why not go to a professional gunsmith, for starters?

Sorry for the late reply the 45 HP is a caliber used in italy until the 90 as it was forbidden the 45ACP. i bought the barrel anyway and I will try at the range on sunday. Will keep you posted. Still don't know if 4506 and 645 share the same barrel

I had the same about the HP so I took the, apparently, unusual step of actually "Googling" it! Here is a link to an article American Rifleman | The .45 HP Load by Wiley Clapp. It would appear OP has one of these hermaphrodytic contraptions. If there were only 200 or so is it any wonder no one has ever heard of it?

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Gunsmithing since 1961