IMO, if you hold over a few of your Rangers or Pioneers, and breed them back to each other, you've got nothing to loose in trying to develop your own line of meaties. Sure, these birds are hybrids. But, the gene pool is there for a nice meaty bird. So, I'd start there. In fact, I already...
You can always read up on AI. If you're determined to do a Cornish cross. While the red rangers won't breed true, my guess is that the second generation will give you a good meaty bird.
I don't have experience with them, but assume that they are similar (in their grow out time, feed conversion, and foraging ability) to the Pioneer. (alias Dixie Rainbow) I grew 10 Pioneers last summer, and was very pleased with them. I held back one pullet from the freezer, and she's a very...