Update time...
I'm still very happy with this target, but I recently discovered that the spring was, well, sprung:
See how the spring is kind of crushed and doesn't fill the full gap anymore?
So I did some measuring and bought a 10-pack of springs from Amazon:
https://smile.amazon.com/gp/product/B005KVAPRM/And they seem to work fine:
Here are the new springs alongside the old, battered one:
I don't know why spring did that. I guess there's something about getting repeatedly hit with .30-06 and the like. However, I have nine spares now, so we'll see how it goes.
In hindsight, I would have bought a slightly longer spring, maybe 1.5", but I think these will be okay.
By the way, sorry about the weird blur in those first two pics. I used "Portrait" mode on the iPhone to blur the background, and it did some odd things with the target.