Biggest concern is cracking/breaking/splitting the handle material It's also a bear to try to figure out how to secure the overall knife while peening the tang....without wrecking another portion or the knife. There's a lot of things to take into consideration before deciding to peen a tang..... how do I hold the knife when peening? Did I use a handle material that will hold up with breaking/splitting/cracking, etc? How far can I peen without wrecking something? Will the tang material hold up to peening without breaking? Etc, etc, ect.
In reality, it's a far simpler task to thread the tang, and use a "butt nut"....or thread a buttcap.
Thanks Ed. I was thinking of a contrasting "birdseye" (?) washer.
Brass, copper, or nickel silver don't seem as prone to cracking. I haven't tried to peen stainless yet, so not sure how likely it is to crack as it gets thinner - maybe less likely, I don't know - but it seems like it'd be stronger.
For some hidden tang knives I was thinking to make I thought of the peening as the final step. Everything glued up with horizontal pins and then take the peening slow and careful and polish.
I'd have to experiment with the thickness of whatever material for the tang extension.