the modern cross is a 4 way hybrid bred from about 60 years of selection. (from the best research i can find). this same question gets asked probably 10-15 times a week, i have never found a direct answer. the breeders who developed these strains guard their secrets closely.
you can breed cornish to fast growing plymouth rocks and get a descent chicken, but chances are you wont get anywhere close to the feed to meat conversions of the cornish rocks. however you will get better livability. in my own experiments i found it best to use cornish hens with a plymouth rock rooster. cornish roosters can be a little ornery. after about 4 generations of very selective breeding i did come up with a barred rock over dark cornish cross that had excellent breast qualities. the growth rate was a little slower than the cornish rocks with about 15 weeks grow time. some were a touch faster, maybe 5.5 lbs dressed at 12 weeks, on high protein feed.
i encourage you to try it, but i will warn you; it is much cheaper to just go ahead and buy the crosses if you want meat birds. the expense in raising a fast growing plymouth rock breeding stock alone can quickly reach well into the thousands. i was lucky and found some locally that a local breeder/judge was working with. i still ended up with about $2,500 in the experiment including cages, feed, supplies, and breeders.
