The regular .300 Win Mag casing Peterson makes is the traditional SAAMI spec’d version that headspaces off the belt. This longer casing version different is different however, as it headspaces off the shoulder.
Traditional .300 Win Mag casings tend to show signs of case head separation after only 5 to 7 firings. This is because SAAMI specs for any calibre, list a minimum and a maximum tolerance for each dimension on the casing and the rifle’s chamber. With most calibres, the max dimension of the casing for length-to-shoulder (L-T-S) has the shoulder of the casing right up against the chamber wall.
With a .300 Win Mag, unlike most other calibres, if your rifle’s chamber is cut to the SAAMI minimum for L-T-S, and your casing is at the max length-to-shoulder dimension, the casing shoulder is still .0095” away from the chamber wall. With the same minimum rifle chamber, a casing at nominal L-T-S is .012” away from the chamber wall. In a worst-case scenario, if the casing was produced at the SAAMI minimum L-T-S, and the chamber was cut at the maximum L-T-S dimension, the casing shoulder could be up to .026” away from the chamber wall. This is actually quite a a lot. That large gap is what causes the casing to stretch so much when it is fired. And it’s that stretching that causes premature case head separation.
These .300 Win Mag- Long casings are made with a longer L-T-S dimension. (All the other dimensions are SAAMI spec.) Its longer L-T-S is what prevents the casing from stretching excessively on the 1st firing. The casing gets more support, which means less stretching and ultimately, a longer case life.
Warning: Only use Peterson Cartridge Co. casings in firearms in good condition, designed, marked, and chambered for this cartridge. Do not use Peterson Cartridge Co casings for “fire forming” or any other purpose other than what they were designed and tested for. Peterson retains no responsibility for the enclosed casings if they are used outside of the manufacturer’s recommendations.