Load Data For .375 H&H (P.O. Ackley Improved)

[Paul Homsy]

I believe that round is very similar to the.375 Weatherby.

It is, you can use the Weatherby loads for it. I have a similar round of my own design, between the Weatherby and Ackley loads I achieved substantially faster speeds than in the original H&H. Because it becomes a straight walled case, it's important to not use absolute top loads for ease of extraction and variations in temperature. The Weatherby loads are available in both older Barnes and Hornady manuals.

 

[Nevada Mike]

@wswolf
...Sorry folks even the 280AI only shows slight gains over a properly loaded 280. But dang they are sexy looking cases.
Bob

Bob

The 280 AI definitely handles heavier bullets better than the standard 280 Remington. I have a 280 AI being completed for delivery next month and looking forward to load development and seeing what it will do in my rifle.

 

[jwp475]

Bob

The 280 AI definitely handles heavier bullets better than the standard 280 Remington. I have a 280 AI being completed for delivery next month and looking forward to load development and seeing what it will do in my rifle.

If both are loaded to the same pressure there is only about 40 FPS difference between them. The 280AI is SAAMI'ed about 5,000 PSI higher than the 280 Remington. I'm getting 3132 FPS with 140 TTSX. I used 270 Winchester 130 gr. data since they are about equal in SD.

 

[fsrmg1]

Most of the .375 H&H AI data is fairly old and powder characteristics have changed since then, even if it labeled the same brand and type. I would recommend using newer .375 Weatherby Mag data (since they are similar), like on the Hodgdon website and start low and work up.

 

[fourfive8]

Some of Ackley's "data" was based on his building a rifle, loading up until the rifle or case blew or action locked up then backing off a tad and calling it good. Iffy territory trying to extrapolate from that. I think I would approach load workup for any individual AI the same as the standard parent cartridge, starting at the parent cartridge suggested starting load then working up and not expecting miracles. The beauty of the 375 HH design is the body taper that helps mitigate sticky case extraction if unexpected high pressure excursions are encountered due to high ambient temps or exposure heat gain. The advantage of the extra body taper is lost with the AI version so extra care is wise when working up loads. I can't find much advantage In trying to push a 300 gr bullet a little faster than 2500 fps (normal vel of standard 375 HH) at the expense of higher pressure or increased risk of sticky case extraction. Also, the points mentioned about freebore should be heeded. Variablitly of chambers, including freebore, makes it impossible to extrapolate data and predict results with high confidence.