WARNING: This is a controversial subject. If you don't understand the following, then don't attempt it.
I've been doing this for the last thirty years with NO problems in bolt actions and semi-auto firearms. The following is the procedure I use to keep me from having problems.
A slow powder for the cartridge is one that usually isn't on the reloading manual listing below the slowest powder listed. It gives inferior velocity, leaves powder kernals in the barrel and gives low pressure. I mainly use the surplus 50 cal. and 20 mm powders for this....... mainly in 7.62x51 but also in the 5.56 to 35 cal. range. Let's take the 30 caliber as an example. In the 308 I use WCC 867 with a fast rifle powder as a booster/duplexing charge. I noted from the reloading manuals that 4350 was about the slowest powder listed in the reloading tables for the cartridge. Powder burn charts are accessible on the internet for these types of powders, so use them. 867 is much slower than 4350, so I loaded a full case charge behind a 175 gr. cast bullet and fired it in a strong rifle......... a mid range report, recoil and a fouled dirty bore was the result. I substituted 2 gr. of the fast rifle powder of the 867 and tried again, in creasing the booster charge and lowering the main charge for a full case condition until the barrel was basically clean and a normal report ensued. I have found, that regardless of cartridge, you will likely find the maximum booster charge shouldn't go over 7 gr. and the small cartridges will be no less than 2 gr. to get clean bore results. I use that loading in several 308/7.62 rifles to get about 2,400 fps (that depends on barrel length of course) with a 175 gr. cast bullet of my own design with approximately 1 MOA accuracy well past 300 yards. Various booster charges can be used doing this..... most work OK, but the main difference is accuracy... find the right one and you get excellent accuracy. The wrong one gives poor accuracy. It may seem I'm not being forthcoming here, but I've seen that people seldom follow instructions. You get a basic safe procedure established for the way you think and do things.... then you can do this also. I've taught this procedure to others, but it usually involves several hours of phone time and some instruction on reading manuals and powder burn charts. Questions?
