If they're falling off the jag then make them a little longer.They'll bunch up under the head of the jag and come back out.

90's a good place to start. I tend to just try 100 from the start then play with it from there. If you want cheap shooting buy two bags of sabots from MMP, one HPH-12, one HPH-24, and then buy a box of 250 gr XTP's. Try both sabots to find out which your gun likes best and then buymore of them.

If I knew your exactbore diameter I could recomend one over the other. But for 2 bags of sabots and a box of XTP's you'll pay about $40 for 100 shots. That's about as cheap as it gets for a saboted bullet.

http://mmpsabots.domainhost.com/store/page6.html
http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=776746&t=11082005

EDIT: Oops, forgot to comment on powder. I use FFG Triple Seven in my 50 Cal rifles.