SR-4759 is the sole survivor of a group of powders which date from the very earliest times of smokeless powders. These were the Bulk powders, which could be loaded bulk-for-bulk with Black and gave you similar ballistic performance. SR-4759 has gone into and out of production so many times since I started handloading that I can't even keep count of them. It always goes out of production because it is 'obsolete'.... and it is always brought back because it is necessary. A point to be aware of: SR-4759 is NOT a Bulk Powder PER SE.... but it is awfully close. There are NO true Bulk powders remaining
You will note that I refer to it as SR-4759 rather than as IMR-###X. IMR powders were designed originally as progressive powders for military use. The powder we are talking about it a Sporting Rifle (SR) powder and it was designed for home-rolling ammo for your Black Powder rifles with Smokeless Powder.... without wrecking the original rifles. Black Powder cartridges commonly operate no higher than 15,000 psi and very often a lot lower than that. The IMR powders are designed to produce the far-higher pressures demanded by modern military and MODERN sporting rifles: pressures up 'way past 60,000 PSI. With the super-bulky Sporting Rifle (SR) powders, you can't get enough into the case (leaving room for the bullet) to destroy your rifle too seriously, although you CAN overload with it, just the same as any other powder. It just isn't as critical when you start talking about tenths of grains. It is very forgiving and it has to be: many of the old-timers who originally used these powders didn't even own scales. If their 25-cent powder/shot measure didn't give the exact amount they wanted, they would beg a fired shell from a neighbour or something and use that for a measure: straight across for a standard load, heaped for a heavy load... and the powder all came out of the same tin because "that was the tin that the hardware had an' it costed dang nigh a dollar an' they ain't no-one a big enough fool t'waste money on TWO kinds of powder for ONE gun so that's the kind she uses!". Very practical, but it didn't ALWAYS work, so some slack had to be designed in just so nobody blew themself up too badly.
After Black powder had become an academic topic of conversation, many of the old-time Black Powder cartridges remained in production as Smokeless rounds. This included the .41 Swiss, .45-70, .45-90, .38-40, .32-20, .44-40, .32-40, .38-55, .50-110 and a whole yardfull of others. Thirty-two rimfire, .25RF, ,25 Stevens, .38 RF, .44 Henry, .43 Mauser and a host of others (including .303 British, to my dismay) began their careers as Blackpowder rounds and made the transition to smokeless. The smokeless loads were very much appreciated by everyone for a whole raft of reasons: less cleaning to do, don't have to clean your gun right now, cartridge cases don't corrode to nothing, cartridges don't corrode from the inside when not fired.... things like that. It is useful stuff. And so everybody wanted to use it. SR-4759 was just one of the most useful of this class of propellants and has lasted the longest. It is a very bulky powder and used to be sold half a pound in a one-pound-size tin. I see that the most recent lots are being sold in plastic jugs and it seems to be denser than the old stuff, but I have to actually break down and spend money for a tub of the new stuff and weigh it against what I still have from the 1960s, to make certain. I will do this and I WILL post the results here.
The one really good thing about it is that this 38%-by-weight formula will hold, just so long as you keep to it. It should give you VERY close to identical performance AND pressure to the original Black Powder loading but without the muss, fuss and corrosion.
As to burning speed, it is quicker than IMR-4198: no question about that. My Remington .50-70 digests SR-4759 very well, whereas it simply lines the bore with half-burnt IMR-4198 and gives me a squib shot no matter HOW it is wadded. Best way to use the SR-4759, I have found, is wadded lightly, just to prevent it from rattling around in the case, but you usually don't have even to do this: the great bulk of the powder, combined with the lesser internal capacity of modern casings, gives pretty consistent ignition 'as-is'.
It's pretty good stuff.
You did a terrific amount of work to derive your formula for using smokeless in Black rifles. So did I, many years ago. And then I saw a really old book...... and there was my formula, written down and printed when my Grandfather still was swapping guns for furs at the Hudson's Bay post at Fort Edmonton!
This hobby is a LOT of fun!
Take care, keep 'em smokin'..... or not!
PS: when you buy your powder, be sure to ask for ESS ARE FOUR SEVEN FIVE NINE. Asking for it as an IMR number is like asking for Ford parts at the Chrysler dealer! Keep shooting!
