Barrel

[Dgjr]

NOTE:  A lighter weight bullet will mostly not work for this.  The reason is that the bullet needs to be loaded long enough 
so to lightly stick the bullet in the lands on bolt close.  I do mean slightly into the lands.  The bolt needs to just feel a bit 
of friction when closed.  The cartridge needs to be held in place by the bolt face and the bullet in the lands just a bit or it 
will move too much in the chamber to fire.  The lighter bullets I have would not allow seating deep enough to work correctly.  
They seat too shallow and basically will simply fall out of the brass at the long load length needed in my rifle's chamber.  
So, get some cheap longer heavy 6mm bullets to do this.

I figured this out awhile back. Yup, the shells I loaded with the 55gr Varmagedden bullets rattled around in the chambers of both my bolt gun & AR. I got this tool  to check my bullet seating depth. Sure enough, like you said, the light 55gr bullet wasn't long enough to seat in the case when set out to touch the rifling. I used another light bullet to replace to 55 grainer, a 70gr Varmagedden bullet. It seats a hair more than half way into the case neck. That's far enough in to hold the bullet firmly along with a slight taper crimp on the case mouth.

In my original post, I didn't have either of my rifles yet & was just making busy work while I was waiting for my shootin' irons. At the time, I didn't think about the shoulders of the 220 Russian brass being too short to allow for proper head spacing in the chamber. That light bulb came on about a month ago. All my 220 Russian conversions are now ready for fire forming... again!  

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Sounds like your on your way.  I have 200 of these formed.  Early on I purchased 200 factory rounds. I have not started any reloading yet. Used my Savage to fireform the brass into the basic shape.  Ordered a 24" barrel to replace my rifle's 18" barrel and I don't have it yet.  So, while I wait, I do a little bit with the Hornady brass and several different bullets that I have.  Like most, I am low on small rifle primers and the powder I would really like to use.  So, we wait a little while longer and keep searching the internet almost everyday for the stuff.  Glad to know I am not the only one that made some 6mm ARC brass the expensive way and I still have to fireform it in the intended rifle yet.