I've not tried copper nails to kill trees/stumps. I've read a lot of home type remedies for killing trees and many of them probably work. Dumping salt on a stump seems like it would be lethal to me.
I googled the copper nail thing. Some people claimed it worked, but most seemed to indicate it was a "wives tale".
The Amazon link above seems to be just some guy buying copper nails and reselling them as a supposed tree killer. However in his description he says to pound the copper nails every 1/2" around the tree.
If one thinks about it, that's really close proximity and disrupting a lot of xylem and cambium tissue. Given that copper is somewhat phytotoxic to local tissues, that may work.
However, I don't think a few copper bullets or a few nails would kill a tree. I've shot dozens of copper jacketed bullets into trees before with seemingly no effect.
As you know trees do occasionally die as a matter of course, so it might be difficult to draw a linkage of copper/fatality from the bullets you retrieved.
There could be some variation of susc. of copper damage in different species of trees. It's well known some trees are more susc. to copper damage (at least in the foliage) than others.
I also think it's possible cutting a tree down could possibly make the tree more susc. to copper nails (i.e. kill the stump).
Cutting a tree down is pretty stressful to some species. For example, large peach and cedar will rarely grow back if sawn to the ground. The stress of cutting the tree down, might increase the susc. of copper, particularly if it was nailed in the phloem. Bushhogging a tree is even more stressful. I've bushhogged a lot of trees. It shatters the stump, and most trees won't grow back.
I've also used pure roundup or tordon to kill trees. Supposedly sometimes this can kill trees nearby if the roots have grafted together, so one should probably be careful with this method, especially w/ tordon.
As an aside, I've also used a small amount of tordon to kill woody type seedlings around some fruit trees at the farm and not seen any damage. As you know Tordon does have "carryover" but it takes a lot in the ground to kill trees.
Here's what a prof. of plant sciences had to say about the subject of copper nails:
"TO UNDERSTAND the potential impact of embedding a poece of copper in
a tree trunk on the long term health of the tree, some basic plant
physiology and chemistry needs to be considered. In order to kill a
tree, a toxin must interfere with cell division in the regions from
which a tree grows - root and shoot tips and the cambium, a ring of
dividing cells inthe stem and roots. In addition, inhibition of a vital
process such as photosynthesis will have a similar deleterious effect.
Implanting a piece of copper in a tree trunk will only affect such vital
processes if the copper is transported from the implant to the roots
and shoots. There are two routes that copper could take. The first is in
the xylem, the woody tissue that forms the bulk of the tree trunk
(wood) but also forms the main transport route for water from the soil,
via the roots to the leaves. Movement here is controlled by the rate of
water loss from the leaf and this process is regulated by stomata on the
under surface of the leaf. Materials move passively with the flow of
water, although those with a positive charge will fix to negative
charged sites in the walls of the xylem tissues. The phloem tissue
(bark) is highly specialised and is responsible for transport of
products of photosynthesis from leaves to shoots and roots. It can
rapidly seal off any injured tissues. Copper from an implant would need
to dissolve before it could move to roots or shoots and affect plant
vitality. The pH of the phloem and xylem sap is slightly acidic (pH 5 -
6) so some copper would slowly dissolve. Copper binds preferentially to
the xylem tissue and shows limited mobility as a cation. It readily
forms stable organic complexes with small molecules such as amino acids
and appears to move through the xylem in this form. These complexes are
very stable and may not dissociate at the end of the transport pathway.
If so, these will not easily pass across biological membranes and
inhibit metabolic activity. Copper movement from leaves, via the phloem
is very slow so the redistribution via this tissue from an implant would
also be slow. The slow rate of copper release from a metallic implant
would be unlikely to cause significant problems for a healthy tree. As
the main route to living tissue would be via the xylem, the patterns of
water movement within a tree would also be important in the subsequent
transport of copper. These vary with tree species - in some water
ascends straight up whilst in others, water movement occurs in a spiral
of verying pitch. Several implants would be required to make certain
that all parts of the tree crown were reached by copper. In conclusion, I
would consider it unlikely that a single copper implant would prove
fatal to a healthy tree; an old or already debilitated tree may prove to
be more susceptible.
(Professor) Nicholas W Lepp, Professor of Plant Sciences, Liverpool John Moores University, Byrom Street, Liverpool (firstname.lastname@example.org)"
While I think it's perhaps possible copper nails driven every 1/2" around a tree might prove lethal, I suspect anything much less would have little impact on most trees.
As you know, there are lots of "leaps" and exaggerations made in horticulture. I thought the one below rather humorous. The person claimed copper nails kill trees because the copper turns into "arsenic". Now that would be quite a feat of alchemy!
BTW, love your "find" close to the pond. Interesting what people leave behind. At the farm, I found an old piece of railroad rail someone had made into an anvil. Now I use it as an anvil.