I'm lloking to make some kids arrows out of 1/4"dowels and put a bullet casing on the end. Then dill a hole insert a bolt and grind to a field point tip. My question is what caliber bullet will fit snug over a 1/4" dowel.  

Maybe a 25 cal. Why not go with a 5/16(ID is slightly larger than 1/4) shaft and they you can get glue on field points, and also BH's if needed.They do sell crimp on 1/4(OD is 1/4 and ID is a tad smaller) fp's and you can add steel bb to up the weight.

You can also get standard glue-on field points for 1/4" shafts. They look just like the big guy arrows and stick in the target well.
Mike at The Nocking Point has them.

If you want to use cases for some other reason, look at .25ACP. They'll be pretty light but I think they'll work, although they could be more costly to track down than field points.

Guy

thanks guys, Just looking for some quick easy options, though it could end up being one of those short cuts that takes a whole lot longer.

25 ACP   will work    BUT    Like mentioned above  Crimp on   1/4 field points are   available from 3 rivers  etc and I USE them even though I have a bunch of  25 acp brass around

I would agree, 5/16 is a good choice. Parallel points would require crimping, since a tight fit would not allow for glue, unless it were superglue or something real thin.

And how many people even shoot .25's anymore?

3Rivers has cheap 1/4" points that will fit right on the shaft. Really cheap if you buy by the 100. The 25 Cal ACP would make a nice blunt for cheap too if you already have the empty cases.