This link is for additional collets for the Shoulder Referenced Trimmer (SRT)
 
Please check Collet No. vs Caliber table
 
No.1-31 Collet material: 303 stainless steel
No.0 (blank) Collet material: 12L14 easy machining steel, suitable for gunsmith or DIY use, friendly to lathe turn or chamber reamer(12L14 steel is carbon steel and it WILL RUST)