This kind of 'list of actions' refers back to the subject last mentioned, not a new subject yet to arrive.
It was a fine gun which he forged, stocked, and completed himself.
He forged it, then stocked it, then completed it.
If you think that 'completed' might be considered ambiguous [I don't at all, btw] then try replacing it with a less ambiguous word & see if that works…
It was a fine gun which he forged, stocked, and killed himself.
…you see it no longer makes any sense.
It was a fine gun which he forged, stocked, and shot himself.
Now, this could be ambiguous, but as we've already established you look back in the sentence, not forwards, then you would understand it to mean he used it for shooting [ducks, deer, other people], not for shooting himself.
You could do away with 'himself' entirely without changing the meaning of the sentence.
It was a fine gun which he forged, stocked, and completed.
You can see how this natural tendency works if you intentionally break it by introducing a subordinate clause which pushes our 'last subject mentioned' out of order.
Bath wanted, for old lady, with tin bottom.
There - we broke it. It's not the old lady who has a tin bottom, it's the bath, yet breaking this natural order makes the sentence confusingly ambiguous.
We would naturally want this sentence to be
Bath with tin bottom wanted, for old lady.
Then the tin bottom refers directly to the subject before it - the bath.