I am working up a load for my 300 H&H using 150 grain Sierra Boat Tails with IMR 4350. Fed 215 primer.

I started at 69 grains and chronographed 3212, 3212, and 3214 on three shots. Book says max velocity is 3400 at 74 grains.

I jumped to 72 grains and this is where my question starts. 3 shots generated velocities of 3237, 3257, and then 3414. These velocities don't make sense being so spread out and the first two rounds you would think would be significantly faster than they were with 3 additional grains of powder.

Any feedback will be appreciated.

Is the 72 grains a compressed load? I wouldn’t think so with a 74 max but that’s all I’ve got.

What type chronograph?

No sir.

Caldwell.

I would repeat test probably bogus chronograph readings.

Don't hear much about that caliber. I almost bought one instead of my wby back years ago. I didn't remember seeing that kind of performance from it in load data back then, but haha I know what I remember is probably suspect at best. Hope to hear an update on what you find.

This.

Not that you're needing to do this, but even with higher end chronographs than what you're using, I still don't totally trust them. I find MV by running down to the end of my range. Point being, chronographs are a reference, but not gospel.

What book shows a 74 grain 4350 load for the 150 in an H&H?

I just bought a magneto speed and kinda of love numbers and such (as stated before, engineer) and I think I'm coming to the same conclusion. I'm still teaching myself what is and is not important but right now things don't exactly line up with the supposed numbers. I know I what I am seeing on paper in real life so I'm more inclined to go with what I see. For example, supposedly the best groups would come from the low variations in speeds, ie low ES. My small sample set of 2 has shown the opposite.

The best group at 100 yards with a higher ES will look like crap at 500+. Sometimes a group that’s not quiet as good will outperform a better group at distance.

In my defense, i was shooting at 300 yards. And it wasn't just that either. I used Strelok to see what the drops to at least 400 yards was and I I needed more adjustment than the calculator suggested. Oh which btw requires MV as a key input. I'm still on the learning phase of this whole journey so I often just shoot groups without too much concern to point of impact so it wasn't a big deal but it will; be eventially. In fact, I couldn't help but think about the rule of thumb (Fireman i think) someone posted for elevations out to some distance. I couldn't remember precisely but I remember it being intervals of 0.5mils.
100 - 0
200 - no data
300 - 1.5 (vs 1.3)
400 - 2.5 (vs 2.2)

Again, I don't know what exactly is causing the discrepancy. While as a layman I can fully refute the calcs (from the calcs themselves to the input values), I can'y argue with where it was exactly landing.

That said, part of my over-thinking wonders if the bayonet style chrono itself is skewing my results.

P.S.
Another example of numbers vs real life. Hornady manual states 41.3gr of Varget yields 2400fps. I'm getting 2620fps with 41.2gr. However, I have a 26" barrel and I'm seating my bullets 0.055" shorter (compressed).