Their are at least 4 ways to firearm 220 Russian to 6mm ARC. & yes I suppose Hydroforming may be a good option as well.
While I waited nearly 6 months for my Pacnor barrel for a CZ 527 action I purchased a Savage AXIS in 6mm ARC largely for fire forming.
A few fire forming options summarized below. May recommend looking up fire-forming 6ppc if you want more info on fire forming.
1.) Work up a load with a faster burning powder & load with a light inexpensive 224 cal bullet & fire form away. Then trim to length & debur.
2.) Work up a load with a faster burning powder & load & somehow contain with wax & fire form away. Then trim to length & debur.
3.) Trim length a bit. Neck up to .243 . Work up a load with a faster burning powder & jam 6mm bullet in to lands & fire form away.
4.) Trim length a bit. Neck up to .264, neck back down to .243, creating a small false shoulder Work up a load with a faster burning powder & load 6mm bullet off the lands & fire form away.
I tried 1.) & 3.) above with fine success. Found 1st option worked out best in terms of getting OAL correct when trimming brass. BTW the Little crow trimmer was very handy for trimming after fire forming. https://www.midwayusa.com/product/101285306?pid=741578
While I waited nearly 6 months for my Pacnor barrel for a CZ 527 action I purchased a Savage AXIS in 6mm ARC largely for fire forming.
A few fire forming options summarized below. May recommend looking up fire-forming 6ppc if you want more info on fire forming.
1.) Work up a load with a faster burning powder & load with a light inexpensive 224 cal bullet & fire form away. Then trim to length & debur.
2.) Work up a load with a faster burning powder & load & somehow contain with wax & fire form away. Then trim to length & debur.
3.) Trim length a bit. Neck up to .243 . Work up a load with a faster burning powder & jam 6mm bullet in to lands & fire form away.
4.) Trim length a bit. Neck up to .264, neck back down to .243, creating a small false shoulder Work up a load with a faster burning powder & load 6mm bullet off the lands & fire form away.
I tried 1.) & 3.) above with fine success. Found 1st option worked out best in terms of getting OAL correct when trimming brass. BTW the Little crow trimmer was very handy for trimming after fire forming. https://www.midwayusa.com/product/101285306?pid=741578