It would appear that Nirvana is just starting his shooting career.
If that is the case, a .22 is just the thing for teaching mechanics. Ammo is cheap, encouraging a lot of shooting. Recoil is absolutely nil, which removes one potential obstacle to learning proper mechanics. With a highly accurate .22, he can eliminate "the gun" as a potential cause when judging his mechanics and result accuracy.
A .22 is a great gun to use to learn to shoot. No matter age at which you begin.
I'll say this again too... 870's are cheap and reliable but if they don't fit you, you won't shoot them well. Remington stocks in general have a lot of drop; the 870 does.
Typically, Length of Pull is 14 inches, Drop (Comb) is 1 1/2 inches and Drop (Heel) is 2 1/2 inches.
Drop at comb is probably the most critical of all the factors involving fit.
It is at the point of the comb that our cheek rests while sighting. If this distance is right, then when we "spot weld" our cheek on it, our eye falls naturally in line with the sights. We can throw the gun to our shoulder for a quick shot and not have to readjust the position before firing.
It is obvious that this measurement is extremely important on a shotgun stock, or any long gun with no rear sight. The drop at comb IS the rear sight.
If the comb is too high, the eye is forced high, also, and we will tend to shoot high. To counteract this tendency, we mash our cheek too hard against the stock, trying to lower the eye. This isn't a comfortable, natural way to aim, and, in addition, will lead to our being smacked hard in the cheek from the recoil of the gun.
If the comb is too low, the eye is too low, and we will tend to shoot low. To avoid this, we find ourselves placing our cheek very lightly on the comb, or even raising our cheek off the stock a bit. Neither is good for accuracy.
The combination of your particular body dimensions
, length of pull and "slope" from Drop at Comb to Drop at Heel determines where your eye will be in relation to line of sight along the top of the barrel.
If a gun fits you, when you naturally raise it to your shoulder, the comb will meet your cheek in a place that puts the eyes right on the line of sight down the barrel. It is amazingly easy to hit with such a gun.
Do yourself a favor: get fitted and research manufacturer stock dimensions. Buy something that is as close as possible to your measurements. It may cost a bit more but it is really invaluable.