Found this little tip out while looking for a deer rifle for my 7 year old son. If you or your little one are looking for the right size gun, here is an easy and sure way to do it. Once you've insured the gun isn't loaded, place the butt of the gun inside the bend of your arm with your index finger point toward the trigger. If the gun is to long you wont be able to get the first bend of your finger on the trigger or you might not be able to reach the trigger guard at all. If it's to short you can probably fit half or even your whole finger over the trigger. You want to find a gun or stock that will allow you to fit just the first bend of your finger over the trigger. Once you find this you've found the right gun for you. Hopefully this will save you a little time and money down the road.
Good Luck!
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"Its common since to take a method and try it. If it fails admit it frankly and try another.
But above all try something."
Franklin D. Roosevelt