>Is there a problem using the 6 1/2 primers ?<
Well, actually there is. Remington's standard small rifle primers were designed for use in low-intensity cartridges like the .22 Hornet, the .218 Bee, and the .25-20. They are perfectly adequate for that purpose.
However, when the .222 Remington, the .222 Remington Magnum, and the .223 Remington first came out, the standard primers would burn through and give small pin-holes in the primers when fired. It was not a dangerous situation, but disconcerting nevertheless. Remington found that the high intensity and long burn times associated with the newer cartridges was the culprit and promptly designed and marketed the 7&1/2 primer as a cure for the situation. You have exactly the same set of circumstances with the .454 Casull and H110 powder.
Use 7&1/2 primers and be happy.
