tcp,
Here's what I do when working with an "improved" case where there's no pressure-tested loading data:
First, I measure the case capacity of the standard round. This is most accurately and easily done with a fired but unsized case. Weight the case empty, then fill it full of water and insert a bullet in the neck, causing the excess water to overflow. A cannelured bullet is easiest, because you simply insert it to the cannelure.
After removing the bullet, wipe any water off the outside of the case and re-weigh it. (The water has enough tension to stay in the case, if you don't try to knock it out.) The difference between the empty case and the water-filled case is the "functional" case capacity with that bullet.
Next, divide the capacity of the improved case by the capacity of the standard case. Let's use the .257 Ackley as an example, since you already mentioned it. Usually the capacity of the improved case with a 100-grain bullet will be around 57 grains, and the capacity of the standard case around 52. Divide 57 by 52 and we get 1.096.
Now we us the 4-to-1 Formula for calculating the potential extra velocity. Dividing .096 by 4 results in .024, which means the improved case will get approximately 2.4% more velocity when loaded TO THE SAME PRESSURE.
If we look at +P data for the .257 (not the old-fashioned low-pressure wimp data, since nobody has any idea why it's still used, or ever was), we find the top velocity loads for 100-grain bullets get around 3100-3200 fps. This means our Ackley Improved rifle is capable of around 3175-3275 fps AT THE SAME PRESSURE.
Now we can start loading ammo. Maximum loads for the standard cartridge will be perfectly safe, since the larger improved chamber reduces pressures. We start with one of these, using one of the powder producing the highest velocity in the standard case, and note the average velocity.
Hornady's latest manual, for instance, lists 49.7 grains if Superformance powder as producing 3200 fps with the 100-grain Interlock Spire Point in the .257. This is from a 22" barrel.
When we fire this load in our .257 Ackley Improved rifle (let's say it also has a 22" barrel) it won't get that much velocity, because of the larger chamber. Let's say it gets 3100 fps.
So we add more powder, a half-grain at a time, watching the chronograph to check the velocity. Eventually the chronograph will probably read around 3275 WITHOUT ANY "PRESSURE SIGNS"." I know this from using this technique with several rifles chambered for improved cartridges.
Now, we probably could continue to add powder without seeing any pressure signs, but that doesn't mean pressure isn't really high, since often they don't show up until 70,000 PSI or even more. And 3275 is plenty of velocity with a 100-grain bullet anyway.
Or, if you do have a .257 Ackley Improved and don't want to go to all this trouble, you could look up the data in any of several current sources that include pressure-tested .257 Ackley data. But the technique does work for improved rounds.
But somebody already commented that the big jump in velocities with improved rounds is due to more pressure. This is true, since most handloaders using improved rounds work up loads using "pressure signs." Since most factory cartridges are limited to 60-65,000 PSI, adding another 5,000-10,000 PSI results in considerably higher velocity.
If the factory cartridge has an even lower pressure limit (and many do) the difference between the velocity of the standard round and Improved ammo loaded with "pressure signs" is even greater. This is exactly why the .250 Savage AI gets so much more velocity than the factory ammo, since the SAAMI pressure limit is very low, and even +P .257 Roberts data is only 58,000 PSI.
But the biggie is that you can start with maximum powder charges for the standard round without any worries.
