The Norma version can be more correctly called the delayed ignition of powder when slow burning rifle powder is used in relatively large capacity rifle cases
I do think it is reasonable to assume that an unsymmetrical charge weight of propellant within a given case design refers to a relatively large rifle case filled with a charge of slow burning rifle propellant occupying less than 50 % of the loading capacity of the specific cartridge design.
Then an article appeared in Hand-loader Journal of Ammunition Reloading in June 1997 (number 187). Titled "Mystery Solved", the author Charles L. Petty came up with some pretty convincing evidence on why the secondary explosion effect occurred and in more specific terms why certain combinations of propellant in certain cartridge designs led to the catastrophic failure of guns using such ammunition. The main difference lies in the fact that a major ammunition manufacturer in America were able to reproduce the secondary explosion effect for the first time.
In his article Mr Petty relates on how a (unnamed) major ammunition manufacturer developed a load for the 6.5 X 55 Swedish cartridge using the copper crusher pressure measuring system. A quantity of ammunition was loaded using a quite slow burning (non-cannister) propellant and a 140-grain bullet.
Initially it was thought that under some (unknown) circumstances, a half charge plug of slow-burning propellant would be blown forward by the primer spark into the shoulder / neck area where it would wedge, smouldering, until it detonated just like a bomb.
For the handloader today it also emphasizes the importance of the burning rate of a propellant being used in relation to the loading capacity of the cartridge being reloaded.
Let's see.... hmmm....slow burning....slow burning....slow burning...slow burning... certain combinations of propellents in certain cartridge designs...
By golly, I think I see a pattern here!
Not trying to be facetious, but I think that study is pretty well known. It is very, very important to heed manufacturer's warnings about reducing charges with slow burning powders, such as Norma's MRP, the 4831 family, and a bunch of others. Much of the data is listed as "WARNING! DO NOT REDUCE!
The faster burning powders do not seem to display this problem. There are literaly thousands of tons of lead bullets fired in this country over relativly small charges of the 4198s, the SRs, both 4759 and 4656, Unique, 2400, Red Dot, and others. The message to me is to use a powder of appropriate relative quickness in relation to the case volume, bullet weight, desired velocity, etc.
The key word is apropriate. SR 4759 is a very appropriate powder for this use. All one needs to do is check the data generated in the last couple of years.