I picked up some 125 grain JHP "Seconds" from Midway. They appear to be Hornady so I pulled out my 9th.

I ran the entire window that Hornady lists for 300MP. I made twice as many as a normal window because I wanted to test in my 4" 686 and my Coonan classic.

All loads ran great in Coonan. Huge flash. Perfect cycling. Relatively clean. I have run cast and JHP heavy bullets before with 300MP and H110 with similar results with this both pistols so I had a good comparison and was quite pleased with results.

Then I ran them through my 686 (S&W 4" revolver). The lower charges were a mix of big bang and light pop. I double checked the barrel to make sure there was no stuck bullet. 686 was filthy.

I produced the loads the exact same time. Essentially, I made two boxes reproducing the same 10 loads for each charge for each box. 100 rnds total. 5 doses. 10 rnds x 2 guns.

The 686 didn't start producing consistent bangs and good accuracy until I got to the hot doses. In fact, the top dose (according to Hornady) ~20gr was the best in the 686.

I later noticed that this was the starting charge for a Speer bullet of equal weight according to Alliant's data.

I haven't loaded the Speer GDHP 125 but based on pics it looks shorter (perhaps the reason it can run hotter?).

I was impressed enough with the high dose in Coonan and 686 to load a bunch more but if anyone has a good explanation of my results, please chime in. I should note that I was using a pretty heavy crimp with once fired starline brass and CCI magnum primers and bullet seated to mid cannelure.

Posted update in thread with chrono results: Summary: Don't trust Hornady data.

What primer are you using?

Is brass all same head stamp?

CCI LPM in Starline brass

I know H-110 needs max charge and magnum primers to function correctly. Maybe the shorter barrel isn't allowing full burn of the powder? Never used 300mp.

I just called Hornady to get their opinion. Interesting reply...

He said they only considered velocity as a max. For whatever reason, they didnt have the incomplete burn at their low end in their test setup. But once they reached 1600 ft/sec (maximum velocity for the bullet), they stopped and considered it max charge.

From a pressure standpoint, he believes the maximum could be higher.

I suspect the issue at low end I am having is same density issue you can get with H110.

I may run another batch a little higher and see if I can chrono them.

Guessing the gap between cylinder and barrel in a revolver lowers the pressure a little resulting in incomplete burn compared to the semi auto coonan.

I'd try switching primers or at least run the loads over a chronograph looking for the differences in the es's of the 2 firearms. If 1 firearm has a es of 50fps and the other firearm is all over the map with a es of 150fps+. That's a clear sign of inconsistent ignition.

A chronograph and 2 firearms testing the same loads is what I consider an ideal setup.

I went ahead and loaded up another 100 loads. This time, I went from 20.5 to 21.8. Max on Alliant site is over 22gr for 300MP with 125gr bullet.

I had the chrono out but light levels were not optimum as I was out pretty early.  The cells with a * didn't have enough data points to get a good value and STD. THe other cells have at least 4 shots worth of data (out of 5).

Grain:S&W 686        Coonan Classic
20        1042.4 STD 114.4No test
20.5       1085.6 STD 49.4~1490*
20.9         --------------*      1520.2 STD 21.9
21.2       1225 STD 38.21542.5 STD 23.2
21.5        1201 std 76.21583.3 STD 4.9
21.8        1254 std 44.41622 STD 18.2

What I found is pretty clear. The 686 is not efficiently burning the charge. The higher dose is better. I ran a low dose of 20gr for 686 only to replicate last week a little. Very dirty at that value. I don't believe the difference in barrel length is enough to account for the large increase in velocity.

I did note that the values over 20.5gr start to create the same full magnum effect that I am accustomed to when shooting my 158gr H110 loads. The most accurate rounds were 21.8 in the S&W.

I find it odd that Hornady said they didn't go any higher because they they were afraid to go above the maximum velocity rating of the bullet. Their manual indicates they tested with Colt Python 8". I don't see 4" adding 5-600fps in velocity.

My opinion... their results are flawed for whatever reason. I am going to run in the high 21.x region since that is where I got good accuracy results with both (although it becomes a pretty heavy load in the Coonan).

My only worry is that if I use the high velocity rounds in my 16" rossi, I might see going over max velocity of bullet. I might just stick with cast/coated bullets in the Rossi if that is the case.

Thanks for everyone's advice.

I am sitting on a ton of Alliant 2400.. won't get the velocity but it might burn better if you are right about a faster powder. Try that?

Switching to federal primers should clear a lot of the issues up in the 686.

A revolver looses +/- 10fps per 1/1000th's of cylinder gap.

There's huge amounts of flame cutting with the 125gr/h110 combo. H110 is at it's best with heavy loads/heavy bullets.

2400 is a better choice, easier to ignite and doesn't burn as hot as H110. You will also find with 2400 there's what's called diminished return. When you get near max loads there will be less velocity gain per .2gr increase in powder charges. A chronograph is your friend.

FWIW:
The jacketed 125gr bullets are extremely hard on the 357 revolvers. Hard on the forcing cones, a lot of flame cutting with the extremely large amounts of hot burning powders it takes to push them. Any 150gr to 170gr bullet doing 1200fps is impressive in a 357 and is very doable in 4" bbl's. 8.0gr of power pistol and any (7 different 158gr bullets I cast/coat) 158gr bullet I cast/coat will give me 1200fps+ in 2 different 4" bbl'd 357's. Actually you ought to give power pistol a try with your 125gr bullets. 1/2 the powder, burns clean with around 40fps velocity loss compared to H110.

Save your H110 for the 170gr bullets.