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I have been searching day and night with no luck I have a mark vii and want to get the 50 ae conversion. I can not find it anywhere. All I can fine is the mark xix barrel conversion.
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You essentially need the whole top end (barrel / slide / slide innards) from a MK XIX. I don't know if MRI ever sold a "conversion kit", or just complete .50 guns back when they were doing the MK VII.
I found a stripped slide on Gunbroker several years ago and bought it. I then slowly pieced together all the small parts for the slide to make it work. I did this mostly so I had a whole top end so I could go between .357 and .44 at the range without pulling the bolt out and potentially launching springs and small parts to places unknown. If you haven't looked, buying small parts from MRI is not cheap. I think in the end I ended up spending as much as if I'd just bought a used MK VII. But I digress.
I have also seen DE "parts kits" occasionally on Gunbroker. They often have everything except the frame. Likely from guns that have been confiscated by the police and "destroyed". But the non-receiver parts can then be sold off. Your best bet might be to just keep watching there. I think the last kit I saw went for $600 or so. May or may not be worth it to you.
Having eventually bought a .50 barrel for mine, I find I shoot it once in a while, but the .44 is much more enjoyable to shoot.