The RCBS X Die Full Length Die Set for .300 Winchester Magnum (RCBS part no. 38912) tackles one of handloading’s most repetitive chores: trimming brass every time it’s fired. Made by RCBS, this two-die set pairs a specially engineered X Die Sizer with a Standard Seater Die so a reloader can full-length size and seat bullets for .300 Winchester Magnum straight out of the box. The sizer’s mandrel controls how much a case grows in length after that first trim, which is the whole point of the X Die system. It is built for rifle reloaders who are tired of running brass across a case trimmer before every loading session and want consistent, repeatable results without the extra step.

Key Specifications

• .300 Winchester Magnum is the chambering this die set is machined for, so it sizes and seats bullets for .300 Winchester Magnum brass only.
• Two-die set pairs the X Die Sizer with a Standard Seater Die, covering full-length resizing and bullet seating in one box.
• Integrated mandrel combines an expander ball and decapping pin on the sizer die so the case is expanded and deprimed in the same stroke.
• One-time trim is all this system needs; reloaders take 0.020 inch off the case’s maximum length once, and the design keeps it there.
• Growth control comes from the mandrel contacting the case mouth during sizing, which slows how fast the case grows compared to a standard full-length die.
• Neck-wall consistency is preserved because the extremely close tolerance between mandrel and neck wall keeps the wall from thickening as the case is pushed back.
• Stabilized case length results after a few thousandths of initial growth, with no discernible loss of accuracy or case life on later firings.

Frequently Asked Questions

Do I need to trim my brass every time I reload? No. After the first trim of 0.020 inch off the maximum case length for .300 Winchester Magnum, the X Die’s mandrel controls further growth so most reloaders can skip trimming on subsequent firings.

What comes in the box? This set includes the X Die Sizer with integrated mandrel, expander ball, and decapping pin, plus a Standard Seater Die for bullet seating.

Buy the RCBS X Die Full Length Die Set .300 Winchester Magnum

Order the RCBS X Die Full Length Die Set for .300 Winchester Magnum from our Lubbock, Texas gun shop for fast, insured shipping and add RCBS to your bench alongside the rest of our reloading dies lineup. It’s a straightforward way to cut trimming time out of every future reloading session.