My 12 year old Grandson is ready for his first rifle. It is my privilege to buy one for him. My question is; (1) How is the proper pull length determined and (2) Is there a model on the market that would fit him now and provide a bit of room for growth later? I'm thinking of .243 or .257 caliber. I hand load, so I can build the load to match the game and his ability. Suggestions?