What exactly do you mean by 'dual purpose'?
I only grow layer breeds, but use plenty of them(cockerels, old hens) for meat.
How do you store 80 cockerel carcasses?
Splitting the hatch up, about 60 eggs set every 3 months, about 30 will be boys. I've been breeding towards width/size to get chunkier birds so that the boys are "worth it" sized by 6 months old. At 16 weeks they're only about 3 pounds dressed. We can go through 3-4 of them a week for the early 16 week old culls. So far we haven't been able to fill up the little deep freezer we have.
The Marans line I'm working on carries fast feathering females and slow feathering males, the boys get chunky early on, about 60% of them. I've been keeping boys from the chunky group. The Bresse line is starting to get a consistent width going on and I've been holding onto the meatiest ones. It's been neat watching the changes from one generation to the next.
Our first year on the farm we had 16 cockerels, mostly EE's. I'll be alright never eating them again, built too narrow and there was a difference in the meat texture that almost turned us off on the whole idea. Crockpot birds is what they were.
I've noticed that there are a LOT of different shapes a chicken can be, even within the same breed. A narrow, knock-kneed bird with a super long keel bone can't sit flat on it's back in a baking pan without the support of potatoes all around it. That bird might be of a dual purpose breed but it doesn't have the structure that makes it a decent dual purpose bird.
We tried Black Copper Marans twice, from two different lines. Scrapped them both, it would have taken SO long to get them into being the same type as the Black Silvers I have.
Chicken tastes like chicken. Differences between them is their shape/structure, where and how the meat lays over the bone, skin type/texture, growth rate, and some variations to the meat texture and how/where they develop fat.
especially when the hens join in
