Having worked with girl scouts and boy scouts stock length
is the most important gun fit issue for comfortable and and successful shooting. I used an 870 28 gauge for the girl scouts and a 870 20 gauge for the boys scouts and some times the 28 gauge for the smaller frames boys.
The 28 breaks birds like a 20 gauge and so much more comfortable to shoot for 11 year olds. Ammo cost may be an issue for you with the 28 gauge...I reload the 28.
Doc
