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27 RPM reloading dilemma

Mandrels should work fine in steps. Anneal the neck-shoulder junctions well beforehand, and use a neck lube.
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Maybe you could get a 6.5 RPM bushing die, then just have the die modified to accommodate the larger neck diameter of the .270 version. It would only be a small amount of material to remove, and anyone with a lathe should be able to handle that. Then you would just need to get the appropriate size bushing.
As pointed out by @Mikecr, annealing is your friend with this project. I have done it without and it can be quite the fail.

As suggested by @Poor_choices, bushing die. I have assembled many bushing dies to alternative caliber. It works very very well.

When expanding the neck for a bigger diameter bullet, sometimes the previous neck shoulder junction brass is thicker and makes that lower part of the neck different from the rest. So, since you are going up in bullet diameter, there is also neck turning to consider. While not strictly required it can be helpful.

When I was making 277, 308 and 338 "26 Nosler" from Nosler brand 26 Nosler brass, I found a thicker portion of the "new" neck. I skimmed and uniformed the necks. The difference was ~0.002 from the lip of the neck to the shoulder. I made them all 0.0145 and that works great for me.
 
Thank you for that info. I had forgotten that the ex-neck jct. might have a bump in it. I had to trim when I created a .338 version of the 300H&H (named 330HNH) some years back.
 
Thank you for that info. I had forgotten that the ex-neck jct. might have a bump in it. I had to trim when I created a .338 version of the 300H&H (named 330HNH) some years back.
I look forward to reports on how this works out.

I have 3 Mark V 6lug that could/may become xx Weatherby RPMs. 6mm, 6.5 and 277 are the most "logical" in my mind. The rifles at the moment are 243 skinny OEM barrel, 270 skinny OEM barrel and 308 Accumark.
 
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