What to do with significant lot-to-lot powder variation.

[justindad]

I have a batch of Ramshot Silhouette smokeless powder that seems to burn significantly slower than the load data would imply. I had abysmal performance in a .45 ACP with cast, and now I am also getting very low speeds in .40S&W with a 4.7” barrel (load manual uses 4” barrel) with a 155 grain Hornady XTP. My measured speed at max book charge was 1,043 fps compared to the books data giving 1,170 fps. The speed I measured at max charge (7.4 grains) was slower than the book’s data at min charge. Primers were far from flat, and the cases went through the bulge buster die effortlessly.
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In July I loaded some of these to 7.2 grains and measured 1,070 fps (125 ES). Today I got 1,043 fps (58 ES, 15 SD, 9 rounds). Accuracy & recoil we’re good for me.
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This is not a slow gun. I loaded some 200 grain lead boolits with True Blue and got the 1,000+ fps to match max book data. Those cases were difficult to push through the bulge buster and primers was much flatter than the Silhouette rounds. I also measured 1,041 fps with the starting charge of HS-6 and a 175 grain lead boolit, compared to the book data of 976 fps.
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I suspect that I can assemble a few test rounds at 7.5gr and 7.6gr (this is over max book data and is therefore not something I would ever recommend someone else do). Or, maybe it’s best to conclude this particular bottle of powder is too slow for the bullets I have and try something different? I know what the safe answer is, but I figured I’d ask folks with experience. Would you give up on this bullet/powder combo?

[dverna]

Before getting too far down the rabbit hole shoot your loads over another chronograph.

[BLAHUT]

I shoot each lot over a coronagraph; so I can adjust load, so, I can get each, to 1200 FPS for what my rifle likes .........

[MT Gianni]

Change chrono batteries and find a friend with another one to compare. I would not be at all concerned about the difference in reading in July and now unless the outside temperature is the same. The fact that your starting charge of HS6 is over the book by 75 fps might be reason to suspect the chronograph, our your screen set up.