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As I understand it, the 7 1/2 was originally developed specifically for the 17 Rem. It has or had a much thicker cup to avoid piercing (the 17 Rem can create odd and unpredictable pressure spikes).
Later, Remington morphed it into their benchrest primer under the logic that they had to make it extremely consistent anyway for the 17 Rem, so why not just call it their match primer? It still is.
The 7 1/2 is quite hot, has a very thick cup and is the primer of choice for rounds like the Fireball, 17 Rem the new 17 Fireball and most loadings of the 223. For milder cartridges, the standard and very mild Rem 6 1/2 is a better choice.
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