Shotgun or rifle?
I'm working on this exact issue right now.
I bought an adult-size Silver Hunter 20ga(great shotgun, love the thing).
When my son was 10, I called Browning and ordered the Micro buttstock, which included an action spring tube(shortened).
He hunted that for 3 years and this year, we are retrofitting the adult spring tube and buttstock.
If you own a Micro shotgun, it has a shortened spring tube and the spring tube is threaded on the end to accept the nut that attaches the buttstock.
Adult buttstock requires adult spring tube.
Let me know if you need more information.
Browning CS has been great to work with.