I don't have the 6mm version, but I do have a 25-35 Ackley Imp. and I had similar fire forming problems.  I solved my problem with a RCBS form die set ($$$) making the cases from 30-30.  Always use new brass and never lose any.

Sir: You should not be seating your jacketed bullets into the lands as it increases the thrust and obviously pressure. Putting them into the lands is for use only on lead bullets.

Anneal, anneal, anneal! Use stout loads and absolutely seat the bullets into the land. You need to prevent ANY possibility of case jump or you will end up with dangerously short cases.

My two cents, I have a 6mm Bullberry Pistol from where else, Bullberry in Utah (Fred Smith).
So thats a 6mm-30/30 Imp for the record.

I lost cases immediately because I didn't follow Fred's advice right from the start (not that you have made any mistakes!)
Once I landed on doing what he said explicitly, I lost NOTHING! Here is his to-do list.

NEW virgin 30-30 brass. (I have sub-saami chamber so fired brass is not useful)
Size 30-30 FL.
Trim for case mouth square (as little as possible)
chamfer lightly
Anneal neck/shoulder
Slowly size in 6mm/30-30 die, short stroke, lightly lubed, really lightly lubed!
Likely to have shoulder dents. Neck wrinkles will be the only potential for failure.
That was from Freds mouth on the phone when I ordered the barrel.
Here is the text from his website, which excludes several steps.

"Brass was first full length re-sized in a standard 30-30 Winchester die. The 6mm cases were then necked down using a tapered-shoulder 25 caliber die to a point slightly below the position of the base of the neck on the finished case. Cases other then the 6mm's were reduced in one step and by sizing in a tapered-shoulder sizing die for the appropriate caliber to the correct neck base position. Improved cases were then sized in the appropriate full length sizing die. All cases were trimmed to the correct length, those not requiring trimming were simply trimmed for uniformity."
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The powders used for fire forming varied by whatever was on hand, but the lowest reduced load of any loads shown will fireform without excessive pressure.
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What I didn't do was the 30-30 FL and the trim of the case for square. The slightest deviation from concentricity and uneven neck length axially causes the brass to flow non-uniformly and thus changes the radial pressure being exerted on the brass in one spot or another and bang, you get scrap.

Fireforming was using light bullets at mid loads, just kissing the lands. I did not use 6mm heavies, use varmint bullets. I used WW brass because it was handy, for no other reason. My loses have all been in forming not fire-forming.

I don't think it takes much in the realm of misalignment from brass flowing oddly during FL forming and prep, to cause a fine wrinkle or other defect that will generate a neck or shoulder failure on Fire-Form.
A second anneal might be in order only on the neck region to ensure that massive flow didn't harden beyond useful limits.

Again, my 2 cents.