I have a .280 Imp-Imp Handi, although it has the straight case and 40º shoulder of an Ackley, it's not an Ackley since factory ammo can't be fired in it. If your gunsmith can ream the chamber to include the 40º shoulder without changing the neck shoulder juncture which sets headspace, then factory ammo might work in it, but I don't know that anyone has had that done.
My smith, Wayne York of Oregunsmithing, reamed it to the traditional chamber depth, but since the barrel can't be set back as in a bolt gun, this ends up being an improved-improved, case capacity for a normal .280AI is 73.4grs water, my case hold 74.3gr water. I use Nosler Custom 280AI brass, when I drop one into the chamber, there's about .030"-.050" headspace!
He does this a lot for hunters in Alaska that want all the case capacity they can get in their 35 Whelen bolt guns.
To get around that, I have to create a false shoulder on the neck that eliminates the headspace, then fire form with start loads. I also loaded .005" into the lands to make sure the case didn't get pushed forward any at the pin strike. I also greased the case to allow it to slide completely to the rear so there's no case stretching, just in case. I did 50 rounds with no anomalies. Then reload as normal using a neck sizing die set and a body die if needed.
I have done the same thing with standard brass, forming a false shoulder is easy, use a .308" neck expander to enlarge the neck, then partial resize the neck to .284" just enough to eliminate the excess headspace and then fire form with full pressure start loads.
Tim
