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Making .300 AAC Blackouts
Making one cartridge into another is the next skill step up from basic handloading. It can be as easy as creating Ackley Improved cartridges, which are often created merely by fire-forming the parent case, to the more complex, multi-step, processes required to make the dreaded .17 Squirrel.
Somewhere in the middle in terms of difficulty is making .300 Blackout cartridges from .223 Remington cases. So far, 300 Blackout cartridges are still kind of rare compared to the .223/5.56 cases that often litter public shooting ranges. Using those, you can make your own .300 Blackouts for free; all it takes is a form die, a hacksaw, a file and bit of spare time on your hands.
The .300 Blackout is based on a .221 Fireball case necked up to .30 caliber. With its 1.368″ case, Blackouts can be made from any of the .222 Remington family of cartridges. Because the.223 Remington/5.56 Nato is by far the most common of this cartridge group, it’s the one most often used as a parent case, but any of the others will work as well. The Fireball just takes a lot less trimming.
The first step is to reduce the .223 from its standard 1.750″ to 1.760″ length to the Blackout’s shorter 1.368″ dimension. To do this, lightly lube the case body with sizing wax and run it up into the sizing die. The neck and shoulder portion will protrude above the top of the die. Using a hack saw, remove this portion of the case. Follow up by filing the cut down smooth with the top of the die.
Chamfer the case mouth and rim with a de-burring tool and then measure the neck dimension. When forming one case from another, especially when the case body is being re-formed into a new case neck, it is important to measure case neck thickness and compare it to either a loaded factory round or to brass that has been fired in your chamber. The danger here is that thicker brass from deeper in the case body may change neck dimensions on loaded rounds, hindering the bullet release because there is no room for neck expansion inside the chamber.
The SAAMI approved .300 AAC Blackout owes its development to J.D. Jones and his trademarked line of “Whisper” cartridges. In order to get around Jones’ trademark, various reamers were made with designations like .300/221 Fireball, .300 Fireball and 7.62X35mm. There are neck diameter variations between reamers which may create pressure problems for cartridges with larger neck dimensions.
The best way to check any non-standard Blackout/Whisper chamber is by measuring the cartridge neck of a round fired in your chamber. Remove it as quickly as possible after the rifle is fired and use a set of calipers to get a good measurement. Once you have that dimension add .001″ to it to account for brass spring back. This bit of arithmetic should provide a good idea of your neck dimension.
For a clean release, a loaded cartridge’s neck dimension should be at least .003″ (.0015 per side) smaller than the chamber dimension. So, if your case neck measures .334″ fresh from the chamber, including the addition of .001″ to account for spring back, a loaded neck dimension at or below .331″ will give a clean release in your chamber.
A quick check of a Barnes factory round showed a neck dimension of .330″ which seems a good standard for handloaded cartridges in this caliber. Our loaded rounds that were made from Winchester brass (WCC NATO 08) came to .330″ without any outside neck turning. This is not always going to be the case. Make sure that you check your case neck dimension frequently, especially when loading with mixed headstamp brass.
In all, making the .300 Blackouts is a simple job given the right tools. We would like to thank Redding for their fine Form and Trim Series D die #83432. It made the job a snap.
Take care and Happy Loading,