Hi there i had a sig sauer model 970 in 2506 that i had it rechambered to the ackley version.Useing 120 grain noslers i got roughly140 fps faster from the improved case.From barnes 100gr x bullets it was 185fps faster.My groups didnt change both were around 1 to 1 1/4 for 3 shots.I picked up my dies used so they were cheap but you can find dies for sale on alot of the auctions or at gun shows.
It is a great conversion---Go For It.
I have had 3 Ackley improved rifles and all shot better or the same but all were alot faster.I love speed so its just my 2 cents worth,
PS my brother still has my 2506 AI AND WONT EVEN SELL IT BACK TO ME IT WORKED GREAT FOR HIM LAST FALL .
All the best.
Frank S.