RCBS Full Length Die Set for 300 Remington Short Ultra Magnum (300 RSUM)

The RCBS Full Length Die Set for 300 Remington Short Ultra Magnum (300 RSUM) is a two-die reloading set purpose-built for handloaders working with bottleneck magnum cases. Whether you're loading ammo for long-range hunting, precision rifle competition, or building a dependable supply for your 300 RSUM, this set gives you the dies needed to resize, deprime, and seat bullets to consistent SAAMI specifications.

The Full-Length Sizer Die is where case prep begins. It brings fired cases back to minimum SAAMI cartridge dimensions, resizing the case body and neck while decapping the spent primer in one pass. Engineered vents prevent air traps and excessive lubricant buildup that can damage cases during sizing—a detail that matters when you're working with premium magnum brass. Headspace is held tight at minimum tolerances, so the die won't inadvertently change case body length, which is critical for maintaining consistent chamber fit and performance shot to shot.

The included Seater Die handles the final step: seating your bullet and crimping the case neck simultaneously with its built-in roll crimper. This combined operation keeps your rounds secure and uniform, reducing the variables that hurt accuracy and reliability downrange.

Key Features

• Two-die set designed specifically for 300 RSUM bottleneck cases
• Full-length sizer die with integral expander-decapper unit
• Case vents prevent air traps and lubricant buildup during sizing
• Headspace held to minimum tolerances for consistent case dimensions
• Seater die with built-in roll crimper for secure, uniform bullet seating

Specifications

Frequently Asked Questions

What's included in the 300 RSUM die set?

The set includes two dies: a Full-Length Sizer Die with an integrated expander-decapper unit, and a Seater Die with a built-in roll crimper and bullet seater plug.

Can I use this set for other cartridges?

No. These dies are sized and configured specifically for the 300 Remington Short Ultra Magnum. Using them for any other cartridge will not produce safe, accurate ammunition.

What does the roll crimper do?

The built-in roll crimper in the seater die secures the bullet in the case neck as you seat it, creating a uniform crimp that holds the bullet firmly in place and helps maintain consistent bullet-to-case contact.

Why do the dies have vents?

The vent holes in the sizer die prevent trapped air and excess lubricant from building up inside the die during the sizing stroke. This reduces the risk of case damage and ensures more consistent sizing results.