You dont need a small base die for M1s normally, what is more important is to get the shoulder set back to specs. Most people mistake this as needing SB dies.
What happens when a gas gun is fired, you have gas pushing against the inside of the shoulder as it expands the brass. As the gas pushes the bullet down the barrel some is beld off thought a port, pushing the piston/operating rod back, which caused the bolt to start pulling on the rim. While the bolt is pulling, you still have some gas pushing the inside of the shoulder.
Two oposing forces.
This causes the case to stretch from the shoulder to the rim.
So what needs to be done, is make sure the shoulder is set back while resizing the brass. To do this one needs a Case Headspace gage, sold by wilson, RCBS, Dillon and other, or if you have the equipment, you can make your own. They are easy to use.
Failing to set the shoulder back propertly cause: Faliure to chamber, failure to extract, or in some cases slam fires.
SB dies sqeese the case above the rim to factory specs, it dosent help setting back the shoulder any more then FL dies.
No one should load any brass for a gas gun with out using a case gage to set up the sizing die.
