That's the only viable way to get Serpents anyway.
I wish I was better at sweeping minion waves with the barrels, anyone have some tips on the best way to time it so you can clear them with the parlayed barrels? I usually can only manage this mid to late game, and at that point there are teammates killing the minions with me :(
Well, there is a couple ways. The easiest way to set off a parrrleyed barrel early on them, then place a second barrel and repeat as soon as parrrly is up again. If there is a cannon minion hit him with your passive blade in between barrels to insure he dies too. The weakness to this method is it is very scriptred, and thus easy for the enemy to mess up.
Another way is to let the minions duke it out, occasionally poking one of them to insure they all get low, and then throw a barrel in the middle of them. This requires much better timing, but it far safer.
However, going out of your way to farm Silver Serpents tends to be a waste. It is better to use your barrels to harass the enemy away from the minions, and try to time your parrley to hit both the enemy champion and some of his minions. While this only gets you a few minions at a time, In the long run this tends to get you more serpents, as you will spend far more time in lane.