Like many of us, I have more bullets than powder available to load them as I shoot my stash of reloads.
I want to substitute powders on the shelf for ones I run out of.

Question: what are the critical factors in choosing a substitute powder NOT listed in the loading manuals for a powder that IS in the loading manuals? Variables commonly available are case capacity, powder burn rate, powder type, and bullet weight/diameter/ballistic coefficient.

Is there a SAFE way for a person unskilled in internal ballistics to figure out how to do this?

Case in point: powders available - BL-C(2) and TAC, desired use - 140gr SP in 6.5x55 Swede
EX: There is load data for H380 in 6.5x55 Swede for a similar 140gr bullet; H380 is right next to BL-C(2) on the burn rate chart, case, primer, etc are all similar. Can I drop back the H380 data 10% and use that data as a starting point for the BL-C (2)?
Harder EX: there is load data for a similar 140gr bullet for TAC in 308 Win. Case capacity is very close, same primer, bore diameter .308 vs .264, same powder. What percentage of the 308 load is a safe starting point for developing a 6.5x55 Swede load?

Is the methodology: (1) find a case of similar case capacity, preferably in the same bullet diameter; (2) find load data for same weight bullet for the substitute powder; (3) identify experimental starting load x% below listed data?

(That faint noise you probably hear in the background is qualified ballisticians screaming at their computer screens!)

You can find powder burn rate tables on the Net.

They can help you decide.

Saeed is right. And the "qualified" ballisticians will be silent because what you suggest is the way to do it. Pick a powder with a known result; substitute one as close as you can get on the burn rate chart. Safest on the slow side. Start ten percent low. Or find a load for a heavier bullet with the powder you have. Lots of ways to skin this cat. Now, don't stray too far from the one you have firm data for. The deal is off then.

Your on the right path with your first example. Your second example has the potential to get you in trouble. Don’t tread there.

Here is a good resource that discusses what you are attempting to do.

Burn Rate Chart

Also, the QuickLOAD interior and exterior ballistics software can be useful. It costs about $150, IIRC. If used properly, it can be helpful in choosing powders and determining rough powder equivalencies.

quote:

Originally posted by dpcd:
Saeed is right. And the "qualified" ballisticians will be silent because what you suggest is the way to do it. Pick a powder with a known result; substitute one as close as you can get on the burn rate chart. Safest on the slow side. Start ten percent low. Or find a load for a heavier bullet with the powder you have. Lots of ways to skin this cat. Now, don't stray too far from the one you have firm data for. The deal is off then.

This

I would NOT use a load from a caliber that is a larger bore than the cartridge you are using. ie, .308 to 6.5.
I would however use a load that was a close caliber to the one you have on hand. ie, 6.5 Jap to 6.5 Carcano. A case capacity close to the one your working up to as well.
I've done this too many times to list over my 57 years of reloading. With a little common sense you can find a very safe starting load and work up.
Remember not to "underload" the cartridge as this can be as bad or even worse than a too hot load.
All of the answers above are good to know info.

going down in diameter doesn't work so well.

going up does, but the smaller diameter barrels will generally use a much slower powder to compensate for the pressure rise.

Load density and burn rate will be two of the most significant factors if what you are trying to do is find a substitute that is a close duplicate.
Research and having alot of data from different sources will definitely be important.

In the examples you provided you can find developed and tested loads on Hodgdon.com and go to reloading data. All the powder manufacturers have data sites like that.

Start with data developed on a ballistic lab before you go looking for other sources.

Why do we make things difficult and create our own problems, Use a "Reloading book", forget the substitution...The only real substitution I have found is H414 and WW760 as they are the same powder in a different container..H and IMR are close in some powders but no ceeegar! The book has powder burning rate, but that just about worthlesss in most cases, the load data is where the answers are.

I think he can't find any of the powders listed in his books. It's a sign of this year.

I 've reloaded obscure calibers where load data is very dated, from a time when fins on cars were a big thing. I wouldn't trust any powder amounts mentioned in the reloading manuals of that time. If I can find a cartridge of a similar size using both powders, I note the relationship between them , bit slower, or bit faster. More data I could find, the more secure I was in my starting load. Here is where a Chrony is a necessity. While they are not the best pressure guage, they are by far the best we have. I look for my expected velocity. I don't trust case head expansion, I have one brand of cases so soft they, the primer pocket expand after only three reloads. Where as another brand goes 20 and never expands. Same, Same.