Hastings makes them, but as he grows it isn't the barrel that matters, it is how the stock fits him. That is what will need to be changed.

An 18 inch barrel is a little short, but the only thing it effects for the most part is how the gun swings on moving targets. A longer barrel will have more mass in the front will help you follow through with the swing better. Shorter barrels tend to be louder as well.

I personally wouldn't waste my money on a barrel extension. If I wanted a longer barrel for a 870 I would just look at getting a different barrel.

Paul