From chapter 9 of The Just Men of Cordova (1917) by Edgar Wallace:
“A gentleman and a policeman?” asked the earl incredulously. Horace nodded. “A new profession for the younger son, eh?” remarked Lord Verlond sardonically. “No more running away and joining the army; no more serving before the mast; no more cow-punching on the pampas—”
A look of pain came into Lady Mary’s eyes. The old lord swung round on her.
“Sorry” he growled. “I wasn’t thinking of that young fool. No more dashing away to the ends of the earth for the younger son; no dying picturesquely in the Cape Mounted Rifles, or turning up at an appropriate hour with a bag of bullion under each arm to save the family from ruin. Join the police force, that’s the game. You ought to write a novel about that: a man who can write letters to the sporting papers can write anything.”
Actually there are three obscure points for me in this part:
What has the younger son to do with "cow-punching" or "holding a bag of bullion under each arm for the sake of family", and why "the younger son" in particular?
How can someone "run away" and "join the army" in the same time?
I found by searching that "swing round" take "to" not "on", so does "swing round on" have another meaning?