[MadKaw]

There's no way you're going to burn that much HP38 in a 4" barrel. What length barrel did Hodgdon use?

 

[MadKaw]

MadKaw, why do you say that much HP-38 can't be burnt in a 4" barrel. I think of HP-38 as a fairly fast powder, and expected pretty full burn even up to low .357 Magnum levels. Hodgdon lists 8.5 gr of HP-38 as max for .357 Magnum, and I was planning to try some rounds in the 6.0-6.5 gr levels. They don't say what length barrel, and I don't think I would try that much in a 2" snubbie, but 4" is pretty respectable.

What powder would you recommend for a bit hotter .38 Special (+P or +P+) in the 4" Model 66? My hands don't really like full house .357 Magnum loads but every once in a while something a little louder is fun at the range.

ol' hit many of my points. Thanks for jumping in.
The mfg's max load is solely based on safety. It is the largest load that stays within SAAMI pressure limits in a SAAMI specification pressure barrel under SAAMI specified testing conditions. That has nothing to do with best performance or efficiency. It is quite possible for that load to be unsafe in any individual gun because of manufacturing variability.
HP-38 is a moderately fast powder. It is between AA-2 and WW-231, but it is 28 on Hodgdon's list of powders by speed. It's not even Hodgdon's fastest powder.
Burn Rate
I have no personal recommendations for you as all of my load development has been for accuracy, not pop. I would just have to peruse the loading manuals.
That said, I've been partial to AA-2 in the. 38 case for more than 20 years.

 

[MadKaw]

If you are interested in learning about pressure testing:
http://www.saami.org/specifications_and_information/publications/download/205.pdf