The thought of mixing different lots of very same powder to increase volume to just one "blended lot", has crossed my mind many times..... But then, I ask myself, "how do I know with certainty that the powder is completely mixed to one homogeneous mixture" (consistent lot)? It will appear mixed, because the powders appear exactly the same.... But is it perfectly mixed to a consistent blend of the ingredient powders? If it is not, well then, the effort has missed miserably short of my objective....
If anyone has a proven method for mixing powders, please chime in......
Many of us have been doing this for decades, without issues.
Some folks, such as Ganderite once became the recipient of several hundred pounds of powder that came out of a lot of Santa Barbara produced surplus 7.62x51nato.
The ammo was sold by International Firearms out of Montreal and apparently some of it had ''pistol'' powder put into one of the hoppers by mistake.
There were several Kabooms, which International took responsibility for, but I don't believe there was anything serious, other than some damaged beyond use rifles.
I have a friend that got some of those disastrous rounds, which were all in a couple of specific lots, indentifiable by the code on the boxes. We both bought a half dozen crates at the same time. One of his crates was in the ''infected'' lot. None of mine were.
Anyway, to make a long story short, International had all of the ammunition from those lots broken down for components.
Not sure if Ganderite was involved in that or not, but he took all of the powder and blended it all together to make up a very large and useful lot of powder for his reloading endeavors.
That isn't something I would recommend for the average handloader to pursue, unless they have some indepth knowledge on how such things work.
As for the speed between different lots of the same powder, as long as none of it has broken down, there isn't any danger.
As a rule of thumb, the powder will burn at the rate of the slowest powders mixed.
To give you an example of mixing lots of powder, when 4831, first came onto the market, it was surplus powder, bought up by Bruce Hogdon and marketed to the plebes at very affordable prices. A lot of that powder came out of surplus 20mm shells, broken down for their components, then put into large mixers and ''blended'' into a huge very extremely consistent lot of several hundred tons.
There was also a bunch of that same powder that had never been loaded and was stored in large paper drums, 50 pounds each.
Powder controls in the 1940s weren't nearly as stringent as they are today and it was a common practice for post war hand loaders to ''blend'' same designation powders so that they could get a decent lot of consistent burn rate powder.
Right up to the early eighties, it wasn't unusual to go into certain outlets and purchase different powders directly from their original drums and weigh it out, then dump it into paper sandwich bag or maybe a box, depending on the amount.
There were a lot of foolish experimenters back in those days that tried to blend powders of different burn rates and it ended badly.
There were lots of other experiments, such as duplex loads, which utilized a fast burning powder covered by a slow burning powder to get a more uniform and efficient burn. Some went so far as to install extended primer tubes to burn the powder from the top down, with the hope of extending the pressure curve, rather than having a high pressure spike, more akin to a small contained explosion.
Go ahead and blend away. Just DON'T MIX different manufacturers or burn rates listed on the containers.
I have a friend with appx 40 pounds of very early 4831/H4831/IMR4831/H4831sc all mixed into one large lot and he's been using it in his 30-06, as well as that both son in laws and several grandchildren for the last 20+ years. Every time he comes across a part can or a cheap can of H4831, he adds it to the blend and stirs it in well. He uses different loads for each of the different rifles, but not out of anything but being careful to tune the load to the particular rifle. He never exceeds any of the loads in the latest Lyman manual.