need that high a primer pressure, to ignite that small a charge.
That primer, the 6-1/2, will also work very nicely in many of the .222-based cartridges, where one is using a slow cast bullet or a reduced velocity jacketed bullet load, that likes a small charge where powder position in the case may not be important (Titewad and Titegroup powders come to mind) a primer such as the 6-1/2may be just the ticket for a low-noise situation where there's no need to get the neighbors excited or the police interested.
Now, when the .222 became popular for purposes such as BR and Varminting, guys started to run into primers getting pierced, and then Remington answered the problem with the heavier cup on the 7-1/2 primer. If you want to see some graphic photos of primer performance, go to German A. Salazar's website, "The Rifleman's Journal", and look for his article on primers, both large and small, HERE:
http://riflemansjournal.blogspot.com/2009/06/primers-small-rifle-primer-study.html
This will give you a better idea as to the power of the 7-1/2 and 9-1/2 primers. I particularly like the 9-1/2s if I have to load for the .243 Win.
Hope this helps.