Pardon if this is previously mentioned, but I don't believe there is a plant called 'chapparel', but you may mean, as some have talked about, manzenetta (pardon the spelling!) That's common to the brushland of California. There are a lot of other tough shrubs there too. (If I'm wrong about chaperell that's Ok- I'm wrong about a lot of things)
That is one of the hardest plants there is to chop. It is not only very tough, but it gives under a blow, bouncing away and not allowing the full force of the blade to develope. I've tried both fast and light blades and heavy and slow. Honestly, though I've chopped the stuff, I still don't have an answer.
I usually go medium. Too heavy and I'm swinging extra weight for nothing. Too light and it's not digging in enough- a little weight helps. There's a limit to how much weight will actually impact on this material. Kinda like a big bore rifle round not able to expend all of its energy upon a small deer.
So, If I were buying just one, I'd go medium. Heavy to me is anything past a medium weighted 27 to 29 ounces. Some of the khuks have their sweet spots in slightly different locations. That's the part of the blade that naturally is balanced for the blow- like the part of a baseball bat you use effectively.
Whether you go light or medium, try and choose one that has a longer sweet spot, as you won't be able to strike a narrow area because the plant is going to move when you hit it. There's a technique of 'bunching' the branches as you hit, and medium and light blades do this. The branches slide along the edge so you not only get a chopping blow but a slicing one as well as it whips away./ That curve on the khuk really comes in handy.
I'd choose a heavy sirupate, or a WWll. Make sure it's 18".
What you've chosen to chop is one of the toughest jobs imaginable. And it's going to be hard on the blade also, because when the edge is embeded in the limbs they'll move and put side to side pressure on the metal. Edges have been known to chip with such work.
Oh, chopping in Spring might give you a slightly more vulnerable plant as the juices are flowing and it's softer than in late summer or early fall.
munk