I already had a load worked up for my .357 rifle, but since I have this new chronograph, I thought I'd do a ladder test tosee if it matched the load I already had. .357 mag, standard primer, 158 gr xtp, 300MP.
17.4 gr - 1749 fps
17.6 gr - 1797 fps
17.8 gr - error
18 gr - 1815 fps
18.2 gr - 1855 fps
18.4 gr - 1817 fps
18.6 gr - 1817 fps
18.8 gr - error
19 gr - error
Here's what I find interesting, the last two loads dropped in POI, whereas I would have expected them to continue up. Unfortunately, I got errors on the chrony, so I couldn't verify that the velocity dropped. Anybody have an idea what's going on? On the target, what I labeled as 18.7 should be 18.8.

What is the prior load you had settled on?

18-18.6 is clustered, with velocity of 1815, 1855, 1817, 1817. If you take out that 1855 that is a huge flat node. I would be inclined to re shoot that make my loading as consistent as possible.

Typically, as you increase powder charge, you increase velocity and you see your POI rise on paper. Shooting only 1 round at a time only gives you data on pressure and velocity, which is also what I do on my personal rifles to begin dialing it in. If you were to shoot 3-5 round groups with each one you will go in and out of accuracy nodes. The one's not in an accuracy node will open up groups. So you get more random POI locations, which is probably what you are seeing. With 300 MP, it is a great powder and shoots best right at max pressure. So, I would concentrate on those and see what it likes.

Also, at some point, you reach max velocity and pressure. And adding more powder will decrease your velocity. When this happens, you have reached max (or over max actually), and will need to back the load down some.

Previously, I had shot groups of 3, and settled on 18.2 gr as the best group. These results seem to verify my last tests.

I would agree.

I also think like chad mentioned about velocity dropping as you go over max. I don't know what a max load is for this chambering but it would be nice if your chronograph had functioned for the last two.

according to Alliant's website, Max load is 18.6. I did not notice flat primers or sticking bolt on 18.8 or 19 grains. I would gather from the POI dropping, that maybe Chad is correct and 18.6 is Max. This was the 1st time I used a chronograph, so I may have had it set up wrong. Maybe I had it too close to the muzzle...don't know. I had it about 6 feet from the muzzle.

The errors could be from increased muzzle blast from increased charges. I’ve saw my 454 throw a flame front farther than 6 feet.

Yes, 6 feet is WAY too close. I set mine as far as the cord would let me. At least 12-15 feet away is what you need.

Now, I will say, all the 300 MP data that I have used, the Alliant data was very conservative. I almost always went over book max to find the best accuracy and consistency. So, I would go above book listed max and see how it does. As you know, just watch for pressure signs. Also, what rifle is it? If it's a lever gun, like a Marlin, I'd be cautious going too far on your powder charges.

It is a Ruger 77/357 bolt action with 20" barrel. I assume this is a pretty stout action. Probably stronger than a lever gun. But, I do want to use the same load in my Rossi lever gun and ruger revolvers.
So, with the data shown, would you have another go at it and see what it looks like at 18.8 gr, 19 gr, and possibly higher? watching for pressure signs along the way?

Yes. The Ruger will certainly handle pressure better than the Rossi lever gun. So, what may be safe in the Ruger may be over pressure in the Rossi lever gun.

I would also take both rifles and load up the same rounds to test in each at the same time. You will know when you reach pressure in the lever gun. The action gets rough to work and the lever will/can pop open. You will find pressure sooner in the Rossi, so just be careful. I don't like to hot rod my lever guns.

If you have a good load for the Rossi and it shoots good in the Ruger I would just go with that one.

Also what primer are they sing in their data, standard or small pistol Magnum primer? That and Crimp can affect the burn of the powder.

Alliant calls for standard primer, so that is what I use.

I've seen hotter charges drop low on ladder tests MANY times. And every time, that is a charge area to avoid. Not because it is unsafe, but because that charge area is going to look like a shotgun hit paper, when shooting a group. Because that area is not in an accuracy node. When you get three or four charges grouping, that's your node. Yes, they should have similar velocities, but an effective ladder test can be shot with no chronograph involved.

Not disagreeing or arguing just looking at it from a different perspective. I think it can be done to a certain extent with only a chronograph as the nodes theoretically originate due to stable velocity over different charge weights. Obviously I think its necessary to test your findings with actual groups also. I like having the chronograph as it can help take human error out of the equation. An issue im sure I have more problems with than others.