I have a brand new cz 455 american that came with the synthetic stock. I want to order a boyds but it list 3 options. Clip feed, any barrel- mag feed, or mag feed with bull barrel. I know I currently do not have a bull barrel but it may change later. But whats the difference in a clip feed and a magazine feed? Also, if it is magazine feed will i be doing anything wrong by ordered the one clearances for the bull barrel. Thanks