Is there any problem with fire forming 6mm remington brass to 6mm Ackley improved using factory ammunition. I have been told that factory 6mm ammo may not be hot enough to fire form the 6mm Ackley Imp. properly and that I should load a hot load with faster burning powder from a reloading manual.

Factory ammo is fine. Remember one of the
advantages of the Ackley imp. version is that you can shoot factory ammo if need be.
I use standard loads all the time to form 30-06 Ackley imp. & .280 Ackley imp. with no problems.

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Factory ammo is fine for fire forming. That was one of the design considerations P.O. mulled over when he came up with the design.

A hot load isn't neccasarily the way the go fireforming. A heavy bullet with a mid towards max load of middle burn rate powder for the bullet you are using works about the best. I went to a little faster powder and had a few problems with cases pulling apart. After that irritating experience, it's back to my standby. A 100 gr Hornady, a Winchester primer, and about 34-35 grains of RL-15. It's even fairly accurate and shows no problems in my gun. Please notice I said my gun. Fireforming can be a little iffy as what some might call an Ackley my be a little off so developing a fireforming load should always be approached with caution.

There was the individual last year that figured he would give a shot at fireforming using magnum Winchester primers. He didn't blow up the gun but it was interesting. Then three years ago there was the genius who was only ten grains off (over) for his charge weights... And people wonder why I don't load with anyone else around and don't share a lot of data.

I would recommend that you anneal your case necks and shoulders after fireforming. The best way to figure out what a case neck should look like when hot enough is to get a 450 degree temp-stick. It's like a crayon and it is formulated to melt at a given temperature. Apparently Hornady is going to market a kit with all the materials included soon. Anyway, when the neck reaches the right temperature, immediately dump it in a bucket of cold water. Overheating burns the zinc out of the brass and weakens it.

Aren't you glad you asked , more information than you really wanted.

Thanks for the reply. That will make my life a lot easier.