valin, Here's some H-110/W-296 information based on my experiences and guidance from other sources. These two powders are nearly identical so load data is almost the same. These slow burning powders take about 15" of bullet travel to burn up so don't be surprised when you see a 3 foot plume of fire puke out of a revolver barrel. Not only does the unburned powder put on a nice muzzle flash show, it also makes a louder thundering boom. Longer barrels (like rifle barrels) burn nearly all the powder so they have way less muzzle flash, less noise, and a much higher velocity.
Most manuals have a caveat for these powders being: "Do not reduce powder charges below the minimum charge weight", which is for squib prevention. The max loads for 357 Mags and 158gr bullets is 16.0 gr, 18gr for 140gr bullets, or 20gr for 125gr bullets, however most reloading manuals may indicate a little more or less, depending their bullet profile and how conservative they are. One of the odd effects with these powers is .... if powder charges are increased beyond maximum, chamber pressure will increase dramatically, however muzzle velocity increases just a little. Why? Because theses slow burners peak in pressure in about 1 1/2" of bullet travel but take up to 15" of bullet travel to totally burn up, most of the excessive powder will burn outside the barrel, leaving muzzle velocity almost the same with a hot load versus a normal load.
Back in 1993, SAAMI lowered the pressure limits for 357 Mag by 25% to increase revolver life expectancy. If you look in a pre-1993 loading manual such as Speer #11, it indicates a max powder charge of 17.8gr for a 158gr bullet. This is about 11% more powder and velocity but it increases chamber pressure to 46,500 psi, which is about 25% higher pressure than SAAMI's current max 35,000 psi. As you can see, there's no good reason to load these powder beyond their SAAMI limit.
I've had the best accuracy in all my 357 revolvers with 140gr bullets and almost as good with 158 gr bullets, both loaded at the high end of the chart. 125gr bullets develop higher velocity but accuracy isn't as good. I found this to be true in my 4" GP100, 6" GP100, and Blackhawks with 4 5/8", 5 1/2", and 6 1/2" barrels. My 2 1/4" SP101 didn't seem to care much about accuracy so I used 125gr JHPs that developed higher velocity.
Another quirk of these powders is ambient temperature. They don't ignite well when powder temperature drops below 30 deg F. For this reason. magnum primers should always be used. Besides the primer issue, these two powders don't burn as well when they are cold so my chronograph data went from 1200 fps at 70 deg F, down to 1025 fps at 25 deg F. I found a bad combination was a 125gr bullet with a modest crimp being fired in sub-freezing temperatures would produce squibs about 1 out of 6 rounds fired. Heavier bullets were no problem with the same modest crimp.
Crimps can be deceiving. If you apply a hard crimp on a jacketed bullet, chances are the case will accordion and actually grip the bullet looser. If you shoot lead bullets, you can apply a hard crimp without losing neck tension but not with hard jacketed bullets. Text book-wise .... a sized case should provide about 90% of the neck tension on a jacketed bullet and the crimp should provide the last 10%. I've found accuracy was best (no squibs) with a modest crimp. I actually made a crimp check tool that mounted in a single stage press. It was made with a Lee bullet seating die with a factory hammer spring from a Blackhawk installed in the cap of the die. A properly crimped bullet would not move when this tool was used. If I applied a lighter crimp or a heavier crimp, the bullet would seat deeper when the tool applied 45 lbs of pressure on the bullet nose. The reason why I brought this up was because many people get poor results with these powders and think it's the powder's fault when indeed it was a heavy crimp that caused the case to accordion slightly and loose its grip on the bullet. Point being, crimps are more important with slow burning powders because it allows better ignition to take place.
I do not recommend using bullets lighter than 125gr with these two powders. This can cause excessive top strap and forcing cone flame cutting. Also, accuracy with these light bullets using W-296 or H-110 is grim.
