Case forming dies/extended shellholder for the .17AB are available from RCBS-and you will need them. They're pricey, too. The drill goes like this: Run the cases thru the forming dies, then the full length sizer die. ANNEAL THE NECK/SHOULDER! An absolute must. Load a 25 Rem ahead of 12 grains of 4198 (You can also use the "Cream of Wheat" method, but it's not as reliable) and fireform. Resize, turn the neck square, and you've got a case. On my rifle, I've not had to turn the necks for thickness, but all require squaring up. Work up loads slowly- a tenth of a grain at a time. Sticky cases will tell you when you've gone to far, the extraction power of the Martini action is not all that great. If your cases split the necks after a couple of firings, re-anneal. My rifle really loves 12.7grs of RE7 behind the Hornady 20gr VMax, CCI BR4 primers. I have not chrono'ed this load, as my Chrony is "on the fritz".... The Bee will move 'em right out, but it's obviously not in the .17 Rem class...
Never heard of a .17 Mashburn Bee, so no comment.
