Don't try it unless you *really* know what you're doing
Common sense says not to risk a $500 gun for $20 worth of powder, but...
I know it's wrong to encourage the use of mixed powders and I would definitely not recommend it to anyone who is not absolutely sure of what he was doing, but I did the same thing a few months back when I was making test buckshot loads with 1# containers of Red Dot and Unique. On my metallic reloading bench I make it a habit of only one powder container on the bench at a time and dump the powder back into the container after each session. This happened on the shotgun reloading table which is a lot smaller with no space for the powder container. After seeing what I did
, I blended the powder really well, calculated the density from the 8# jugs from which the powders came from and figured I had a mixture of 2:1 Unique:Red Dot, about 2/3 of a pound of the stuff.
From my loading experience, I knew that the volume-energy of Unique and Red Dot were almost identical in my 40 S&W loads. The powder measure setting for 5 grains Unique was nearly identical to 4 grains of Red Dot and they both gave me about the same MV in my M&P 40. I knew that the Unique gave me lower pressures, but the Red Dot was a little cleaner burning because it produced higher peak pressure (23,000 psi compared to 17,000 psi with Unique according to Quickload). I plugged in the numbers to Quickload using numbers weighted 1/3 Red Dot ant 2/3 Unique to see what range I could load this mixture with.
I started low with 4 grains (worst case 100% Red Dot) and worked up to 5 grains (worst case 100% Unique) and found that the mixed powder performs pretty much like pure Unique but burns cleaner with fewer unburned flakes and less soot probably due to slightly higher peak pressures.
I typically load my 40 S&W in the region of 75% of SAAMI max pressures and never get anywhere near the peak so even if Quickload is flawed in its calculations, it's not off by too much. It usually predicts MV's within 10% so I typically load to 75% max for an extra margin of safety. I usually just make sure I am above 10,000 psi for a good clean burn.
If you have an unknown powder but are sure it is meant to be used within the load range of what you're shooting, you can work up a safe load. It may not be optimum but if you stay well below max for either powder, a mixture of both will probably be usable. After all, most powders are blends and if you buy surplus military or pulldown powders, they typically give you a "start" point and the rest is up to you. So long as you stay conservative, you can use the powder safely.
Now if it were a fast and slow pistol powder like W231/W296, Red Dot/Blue Dot, or a rifle and pistol powder that got mixed, it would have gone into the flower bed.
