My local gun dealer can get me a 30" barrel on backorder for my 11 year old son ...

My local gun dealer can get me a 30" barrel on backorder for my 11 year old son for his trap shooting team or should I just get the 28" barrel and it's available just looking for thoughts on the what the difference he uses a870 with the stock cut off right now for length of pull of 13" with a barrel of 30" that has been bored out specifically for 16 yrd trap line