I am working with the 5.7 with a T/C using a barrel made by MGM. Rather than trying to neck down the larger delicate 5.7 case to .17, it is preferable to work with the better 17CCM. This cartridge is excellent; preferable, as it is designed for 50 BR competition. In the Contender 14" bull barrel AA4100 and VV3N38 will burn 99% of a 17CCM full powder load with acceptable pressure for 18 and 20 gr bullets, as determined by Quickload calculations.has anyone tinkered w/5.7-28 fn down to 17 cal yet? always thought it would be interesting. have to be stronger than hornet cases. have asked dave at ptg if he has any prints on one, no reply yet. any info appreciated. thanks.