LBL,
I have had more than a dozen Ackley Imp chambered rifles over the past forty years. My experience in 25 cal has been that the 25-06 AI is one of the easiest Imps to load for.
I have had two 250-3000AIs, one 25 Souper and two 25-06 AIs. All of them worked very well, but the 25-06 just sang. 120s with H870 drives them close to 3300 fps. The powder fouling was bad, however, so switched to Re22 and that was taken care of. A stomach full of H450 will get 100 gr bullets going past 3650. Unfortunately, this powder is hard to find today, but H4831 does almost as well and burns cleaner.
The best way to work up loads for your Ackley is to start with max loads listed in your reloading manual for the standard 25-06 and go from there. Do not use reduced loads for fireforming brass. Doing so will just create headspace problems. The 25-06 Ackley is as accurate fireforming as it is with formed cases. I have hunted with fireforming loads, and never gave it a thought. Just be sure to keep your brass!
My last 25-06 Ackley had a 27" Shilen barrel One of the CGN guys here is shooting it today. It gets about 100 fps more than the earlier one at 24".
My latest excursion into this arena will be a rechambered Stevens with its 22 inch factory barrel..... just to see how it works. I have a feeling it is not going to be much of a disappointment at all.
Ted
We'll let you know when it gets back from the 'smith.