[Spirit]

Off the bat, there are two ways I can immediately think of that an alteration would increase the value of a gun.

1. Undoing whatever someone did to lower the value to begin with. Such as refinishing it to the color it was supposed to be, finding period correct grips, removing rust without harming the gun, etc.
2. Engraving the gun.

The thing is, both options usually cost more than the end product is worth, but they do increase the value of the gun. Otherwise, usually changing anything at all outside of something not permanent will always bring the value down, as it takes it a step away from being 'original.'

 

[Spirit]

I’m not sure about your 2nd assertion. A while back there was a thread where someone picked up a SAA which had been well-used, with worn edges, etc. it was subsequently engraved (likely in the last 30-40 years) and then refinished. I didn’t think all the glitz did a thing to enhance the value of what was likely a well-used, but not abused, 1st generation Colt.

I suppose it's not an all encompassing suggestion. As with your example, in the event the gun you're thinking about having engraved is already collectible and rare as it is, then an engraving really won't do much for the gun's value. However, for the vast majority of standard firearms, if you have it expertly engraved and re-finished by a reputable engraver, then undoubtedly the value of the gun will be worth more than before it was engraved and should bring a higher premium at auction, regardless of the collection of opinions suggesting if it did anything to enhance the value or not. Typically the way it goes is you reduce your audience, but to those interested, it's worth more. With that being said, just don't expect to get everything back out of it that you put into it.

However, I agree with you though. When debating having one of my own revolvers engraved, I considered my 1927 pre-detective/2" PPS, which has a well worn finish but is in great condition and not abused at all. As much as I would have liked it, I ended up deciding against it though because the gun honestly has more history and value being left as is. Glitzing it up might look nice but wouldn't enhance the value.

 

[Spirit]

The question is how much does altering a gun affect the value. How much does it devalue them and are their any alterations that increase in value?

As a short answer to the question, the only alterations that increase the value would be things done in factory that set it apart from a normal production gun, such as factory engraving, the gun has been fitzed, etc. About the only other I can think of also is if it went to a well known shop that did their own modifications post-factory. An example would be Wolf & Klar, John Jovino, or King's Gun Sights.

Outside of that, pretty much any alterations will always decrease the value, especially if it was done by an unknown individuals' own personal gun. How much it decreases the gun depends on what was done. However, if it's a 100 year old revolver and the modifications were done very early on, it won't hurt the value quite as much, as it's seen as part of the history of the gun, but otherwise, if it doesn't have provenance tying it to a well known institution, individual, or event, modifying a gun is rarely seen as a positive factor to a collector.