Let me say this on crushing your own, GOOD LUCK. A friend of mine gave me a bushel of cherrystones. Which I devoured and also made clams casino and put in the freezer for later use but barely lasted a week. So I got this lame brain idea that I would dry the shells out and then crush them. Far more work that it was worth,at least for the chickens, although I did get one heck of an aerobic work out. What I did was take an old feed bag and put the shells in and ran them over with a home made roller full of concrete to pre-break them, then did the rest with a bottom of a sledge. Took a hour or more to get them down to a edible size with about 3lbs of shell. For $9 I can get a 50lb bag and not break a sweat. Now bear in mind oyster shells are a lot thicker than cherrystones shells. Next time I might try steamers(soft shelled clams for you mid westerners).
As far shell as grit goes, it does serve as that to a degree but they still need some real grit to break down the shell. Depending how fine your sand is you might have to give them a little something with some fine stones in it.
Edited by duckinnut - 8/8/12 at 11:51am