If you want New Hampshires for a meat bird, the source is really important. I made this mistake once. I got New Hampshires from a hatchery, and they were quite small, though the pullets lay well (I kept one girl and gave a friend the others, culled all the boys). I then got some New Hampshires from a breeder (in my case, Eight Acres, cpartist on BYC). Completely different story - HUGE birds! Wonderful! They were a bit slow to mature egg-laying-wise, which is not a surprise with "heritage" types, as I understand. As I recall, they got a lot of their mass later than the time you propose to cull. As far as what they'd weigh, again, totally depends on the source/line.
Here's Earl, one of the boys from the breeder. BIG boy.
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May I suggest considering Naked Necks if you want a backyard meat bird? They are friendly, hardy, and lay well as well. You can get good ones from a hatchery for that purpose if you want (at least mine were great - from Ideal).
But punchline is, if you are thinking of getting a big batch of NHs from a hatchery as meat birds, you may be disappointed in their size. If you get them from a breeder, you can keep the best boy and all the girls and then hatch your own meat birds! I'm actually crossing NHs and Naked Necks - it's turning out great so far.
(Perhaps someone else will pipe in - that's just my two cents.)
- Ant Farm