Run into tree, boy, without round royal firearm (6-6)

'firearm' is the definition.
(I know that breech loader is a type of firearm)

'run into tree boy without round royal' is the wordplay.
I cannot quite understand how this works, but
'run' could be 'r' (cricket abbreviation) and 'r' is present in the answer.
'tree' could be 'beech' (beech is a kind of tree) and 'beech' is present in the remaining letters.
'boy' could be 'der' and 'der' is found in the answer.
'round' could be 'o' (o is a circular letter) and 'o' is present in the answer.
The remaining letters 'la' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?