Has anyone ever compared pressures between 257+P and 257 AI loads?

Well the only data I have for comparison says the 257R was 45,000cup the +P is 50,000Cup. As a comparison the same source shows the 6mmRem at 52,000cup. Other data shows the 6mm at 65,000psi. Using that ratio it gives 62500PSI for the +P. I would see no reason why the AI couldn't be loaded to the same 65,000 as the 6mm.

there are very few folks with pressure barrels that can honestly say that they have.
The AI group don't have a pressure they work to....the +P might actually be a SAAMI credited round with an official pressure.....the AI is not...so there's actually no comparison to actually be made.

As far as performance between the .257 Roberts and it's AI modified design you'll get as much info as you like...truth be known the AI don't actually generate more than about 100'/sec more than the parent round loaded to the same pressures as the AI gets loaded to.

The largest number of AI conversions are truly over stated and overrated. There are a few that might be worth the effort.....but very few!

I agree with Vapodog 100%. People will talk about the 300+ fps gain they get on the 257AI. They compare to factory loads at 54,000PSI and then load to 65,000+. I did a lot of playing with a pressure barrel and the 280 taking it from 280 to RCBS to AI to JRS and then a Gibbs+. At the same pressures the gain is 1% velocity for 4% powder.

257 Roberts Ackley Improved, 75 gr Vmax, 26" barrel, IMR4895:

1) powder charge #1: 3757 fps and the extractor groove grows .0005"

2) lesser powder charge #2: 3736 fps and no extractor groove growth

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end of reality and begin calculatiing
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Quickload thinks the halfway point between those loads is 56693 psi

Quicklaod thinks that at the same pressure, bullet, and powder, a 257 Roberts +P would produce 3719 fps, or ~30 fps less.

Did I go to the trouble of case forming for 30 fps?

Are you sure of your pressure? Loadtech requires around 65,000+ to get those velocities. Plus 57,000 should not give expansion. Is it CUP by any chance?

Yes you will get around 6.5-7% capacity increase. So if you can increase your powder by the same amount you will get about a 1.7% increase in velocity. At 3700 that is 62FPS. Everyone claims huge increases in the 257AI most of it comes from higher pressure.