What has the "younger son" to do with "cow-punching on the pampas"? in "The Just Men of Cordova"

Question

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:

1. 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?

2. How can someone "run away" and "join the army" in the same time?

3. I found by searching that "swing round" take "to" not "on", so does "swing round on" have another meaning?

Answer 1

In order of the highlighted portions:

It is the younger son in particular because the oldest son will inherit the title and the estate. It's probably entailed on him. The younger sons need to make a living somehow. I note that many of the situations described after it are more likely in novels of adventure and drama than real life.

Leaving the family home abruptly is (with a bit of exaggeration) called "running away from home." It might indicate a younger son who quarreled with his family and enlisted in order to escape them.

Some of them in fact moved to South America, where they might work as a cowboy. Possibly work his way up to being a rancher of some kind and wealth.

"Swung round" here means "turned around." "on her" means he faced her when he was done turning.

"a bag of bullion under each arm to save the family from ruin" -- the younger son might show up with a lot of money, having gotten rich in South America or Australia or some place and unexpectedly save the family in their poverty. (MUCH more likely in a novel than real life.)