I have converted a couple, both bar action. Essentially you have to change or modify the tumbler and almost certainly replace the main spring as the 'passive' arm needs to be considerably longer.
Some conversions are quite simple, only needing a new spring and an extension on the forward side of the tumbler to pick up the 'passive' arm of the spring.
Others can be far more complicated, especially back action locks which by their slim nature, sometimes won't allow the necessary movement on the spring and/or foul the sear spring.
Back at the height of the hammergun era, the conversion was commonplace and in fact many guns assumed to be rebounding as original were beautifully converted and show no sign of it. This can often be deduced by the date of the gun compared with the date of Stanton's patent.
Thanks Toby. This is a back action Belgian guild gun with beautifully polished locks and a long curved mainspring. "If it ain't broke, don't fix it" probably applies here, but I appreciate knowing what's involved. Here's a few pics of the gun after I cleaned it up, just for grins. I can take a photo of the locks also if you're curious:
Before:
After:
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