5.56 and 7.62 Case Capacities
Date: May 01 2015
Due to a continuing run on handloading components, over the last couple of years, cases are difficult to find and expensive when they are found. Military surplus is currently about the only readily available source of cases, but military cases are thicker, which results in less case capacity than commercial cartridges and necessitates reducing powder charges. How much less volume and how much reduction of powder charges are required to compensate remain questions. Those uncertainties also made me think about the differences in capacities of various brands of commercial cases, so I set out to measure and test a variety of .308 Winchester/7.62 NATO and .223 Remington/5.56 NATO cases to find some answers.
Various Internet sites sell once fired military surplus cases. The best deal I found for 7.62x51 NATO cases, which can be substituted for .308 Winchester cases, was Everglades Ammo and Reloading (www. evergladesammo.com) that sells a box of 500 cases for $85, delivered. Four days later the box arrived, and I tore into it like a birthday present. Everglades had cleaned the cases, and they were as bright and shiny as new. The box actually contained 521 cases; only three were unusable. Some of the case mouths were dinged, but a sizing die rounded them out. The cases had a variety of headstamps, such as LC, PSD, RA, SBS and TAA, with the NATO insignia headstamp of a cross in a circle. Cases were from various years.
The headspace dimension of the cases after being fired in an openbolt machine gun was a concern. Using a Hornady Lock-N-Load Headspace Gauge, the military cases were 0.011 inch longer to the datum point of the shoulder than Hornady commercial cases fired in a .308 Winchester bolt action and autoloader. A sizing die set the shoulders back to the proper headspace dimension. Case necks lengthened quite a bit from that sizing and required trimming to bring the brass down to a length of 2.005 inches.
Sizing the cases also knocked out the crimped-in spent primers. To remove the crimp around the primer pocket, an RCBS Primer Pocket Swager Tool that fits in the shellholder slot on a single-stage press was used. Raising the press ram inserts a case into a die with a rod that holds the case as the swager irons out the crimp around the primer pocket.
There was no need for me to order any 5.56 cases. My garage is brimming with commercial .223 and military LC 5.56 cases gathered from years of shooting. To determine the total case capacity of these various brands of cases, each was weighed then filled with room-temperature water and weighed again. The difference was their capacity of water in grains. There was little difference in the weight and capacity of the assorted .223/5.56 brass. Four brands of .223 Remington cases and one 5.56 case varied only 1.6 grains in weight and 0.2 grain of water in capacity, but there was quite a difference in the weight of the 7.62 and commercial .308 Winchester cases. The five 7.62 cases’ average weight was approximately 8 percent heavier than the average weight of the two .308 cases. I sawed open a couple of military and commercial cases, and it looked like the military cases’ extra weight was located in the head and web. However, a capacity of 1.5 grains less water for the heavier military cases was not all that much different than the lighter commercial cases.