Underhammer Issue…need help

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Option;
If the breech threads still fit snug but the barrel just slightly over-clocks past the register-mark when snugged up & pinned to the receiver, that issue can be resolved by applying a 'light coating' of silver solder to threads on the breech plug.

I've not worked on H&A underhammer guns so if the pin is loose other forum members have covered cures for that issue.
 
Just send it to Doug and the crew at Pecatonica I have 4 UHs they have converted for me with no problems
I just got off the phone with them. Very nice folks. I’ll be sending the gun to them. They are very reasonably priced, too. Now I’ll see if the seller will offer some sort of compensation.
 
Here's a picture of a genuine h&a .50 setup. It's a 5/8" plug. You indicated your's is a .58? Looks like some one tried using a .50 plug instead of making a proper plug. The threads in your .58 breech barely even engaged the rifling in the bore (with only 5 threads) from the look of it. I'm thinking you dodged an issue. Firing that as it was sold might've sent the barrel a' flying at the very least. I included a pic of my .54 plug. Note the relations between the .625 shank and the threads. Your .58 should have a 3/4" plug and the barrel rethreaded as well. On the plus side, if you find another pin on barrel your in business!
 

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Here's a picture of a genuine h&a .50 setup. It's a 5/8" plug. You indicated your's is a .58? Looks like some one tried using a .50 plug instead of making a proper plug. The threads in your .58 breech barely even engaged the rifling in the bore (with only 5 threads) from the look of it. I'm thinking you dodged an issue. Firing that as it was sold might've sent the barrel a' flying at the very least. I included a pic of my .54 plug. Note the relations between the .625 shank and the threads. Your .58 should have a 3/4" plug and the barrel rethreaded as well. On the plus side, if you find another pin on barrel your in business!
I would have dodged the issue by not buying it! Thanks for the pics. Yes, I was never going to shoot it the way it is and this has been confirmed by multiple people. The folks at Pecatonica are going to fix this mess.
 
A picture is worth 1000 words, they say. Wow. What it should be vs. what I have.
 

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