Ballistics by the inch 22 data
I was debating barrel length on the munchkin rifle I am working on for my young son. As luck would have it the BBTI folks have 22 data now. The difference in velocity between a 16" barrel and an 18" barrel is practically nil for most loads (20 fps or less). I found it interesting that past about 12" the increases on up were not at all dramatic and if I would leave the munchkin rifle barrel at it's original length of 25" it is a losing proposition over an 18 - 20" barrel.
Ballistics by the inch 22 data
Velocity vs. Barrel Length
I dunno, it sorta looks like it's true: Size matters. 
18" 1158 1114 1510 1407 1342 1181 1212
22" 1141 1083 1569 1430 1351 1293 1242
Not by much, mind you, but in 5 out of 7 cases listed.
The 'losers' in the 22-inch Kimber also appear to have been .22 Shorts (27-gr HP and 29-grain RN). The .22 Long Rifle loads did better in the long-barreled bolt gun.
That's just against the sawed-off T/C. In the case of the 'real-world' guns, the 22-inch Kimber won every time over shorter barrels.
The only 'outlier' was the 23-inch Winchester Model 63. It turned in lower velocities than the Kimber, but that could be attributable to many factors (e.g., wear & tear), plus it was only one case. Whether this result is significant seems questionable.
BALLASTics for kIDS
I'd still cut the barrel off at 18" or less for ballast. I'd bet your son will thank you for it, if not now then later.
And what about velocity? Trust me, the cans will never know the difference.
HTH 
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Graph
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