Yeah, I saw that--but one thing that complicates all this is the CCI 450 Magnum primer (which appeared about the same time as the 7-1/2 in the 1960s) was NOT a "hotter" primer when first introduced. Instead it simply had a thicker cup, to withstand the pressures of newer cartridges.
Until then, no American cartridge taking small rifle primers had a SAAMI Maximum Average Pressure much more than the .22 Hornet's--which is pretty mild. Today, of course, most SAAMI pressures are rated electronically in PSI, and even the .222 Remington's MAP is 50,000 PSI, only 1000 more than the Hornet's. Many other small centerfires, such as the .25-20 WCF, developed even lower pressures. Consequently small rifle primers had relatively thin cups compared to large rifle primers.
But when the .222 Remington Magnum appeared in 1958 its MAP was 55,000 PSI, as was the .223 Remington's when it appeared in 1964--and the .221 Fireball, introduced in 1963, has a MAP of 60,000 PSI. Plus, many benchrest competitors who used the .222 (the most popular BR round back then, along with variations) hot-loaded the round, to help resist win-drift. The standard SR primers of the day didn't hold up well at 55,000 PSI or more--which is why the first CCI 450 "Magnum" primers only had a thicker cup.
But eventually CCI added more priming compound to the 450, and in the early 1990s changed to an even hotter mix. So there have essentially been three versions of the 450. Which is why I started wondering about the 7-1/2.
I suspect the first 7-1/2s, which appeared around the same time as the CCI 450, simply had a heavier cup. But did later versions actually use more priming mixture, or a hotter compound? The typical photos of primer flames that some experiments have taken tend to show 7-1/2 with more flame than some other SR primers--though as one primer manufacturer remarked to me during an interview a few years ago, flame length doesn't necessarily have anything to do with flame temperature, which is more important when igniting powder. (He suggested chronographing the same load, in the same rifle, using different primers as a far better indicator of primer "hotness.")
So I'm wondering whether the 7-1/2 went through any actual changes after its initial introduction, or has only the name been changed? There hasn't been the same continuity in production management at Remington as there has been at CCI, and I haven't encountered any Remington worker who has any firm answers.
