Q: What can you tell me about a mysterious phenomenon that affects handloads with reduced charges of slow burning powders, and blows up guns? It doesn’t make any sense to me that less powder can produce more pressure?
A: Alas, while it may not make sense, it does happen. In fact it may have happened to me once with my .25 WSM, but although the bolt lugs were set back, the rifle held together and I couldn’t prove anything. Nils Kvale of Norma and P.O Ackley both confirmed this “pressure excursion” does happen, but said they were unable to duplicate the phenomenon in their ballistic laboratories. However, in more recent times S.E.E has been reproduced at will in at least one ballistic laboratory. It is the main reason why some ultra-slow burning powders have never been sold in canisters to reloaders. If not used precisely right, there is a very real chance, slim though it may be, for a pressure excursion. There have been several different theories advanced about what happens to cause a pressure excursion, but they’re academic, for now. We know that S.E.E occurs when reduced charges of very slow burning powders are loaded and only in large capacity cartridges that have a relatively small bore size. The danger can exist in any of the magnum cases from .257 up through about .300 and in many non-magnum cases of the sort usually described as being “over bore” capacity. Theoretically, it could happen just as easily with a .25-06 case that has too much air space behind the bullet. Case shape may play a role, but this has not been confirmed. The best way to avoid S.E.E is to strictly avoid loading reduced charges of slow burning powders that are 10 percent or more below the recommended maximums in any small bore cartridge which has a large case.