When I began loading shot shells I went only with published data. As I gained more knowledge, the substitutions came. Some times in search of that "better" load but most times because of what I had on hand. Baltz is correct, there are many "clone" and "OEM copy" wads currently produced. As these become popular I'm seeing the yearly publications including them. Mild primers such as CCI, Win, NS, Cheddite, Fio, Rem, etc... would likely test safe in loads were one in particular gave pressure data at 9000psi or less. In fact, the Hodgdon manual once said substituting Win and NS was fine. Alliant's manual also once said to use Clays data for their Clay Dot powders. Just to be clear, I am not recommending anyone stray from published data, I'm just relating what I have read and experienced.
I treat magnum primers such as Fed209's and CCI209M's differently and have never subbed them without published data. I will however use the copy/clone wads freely when using them.
I think Downrange has a nice chart out detailing wad substitutions. I have/had one at home. they are probably available online.
Labs are available that will pressure test loads for you. I have read many threads on other sites about the process and some of the results. It doesn't sound overly expensive or burdensome as I recall. Personally, I doubt I'll ever do it. There is plenty of useful data out there for my needs. I look at chrono data and pattern board then try them at the range. If I were to send off some test loads I suppose a starting point might be using something similar to published data not pushing pressure too hard (8-10000psi).
