The standard for the parent case is 1:9, which will stabilize all but the longest target bullets in that caliber. The slightly slower 1:10 twist should allow a bit more velocity to be realized, but you're really splitting hairs on that count. One of those two will certainly be suitable.
I too would use a little faster twist like 1:9 or 1:9.25. At some point when your grandson gets older he might want to use a heavier and most likely longer bullet in this gun. I have a Savage 243 and they twist their 243's with a 1:9.25 rate. I strongly believe this helps with the heavier bullets that are longer with the high BC's. I'm sure he'll love it !!
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