Then put it sideways in a smooth jawed vice. Done.
I have not used my action blocks on flat top uppers in years.
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Put some small scraps of wood between the receiver and the vise. That will protect the finish and the wood will absorb the pressure so you dont deform the receiver.
or,
you could do it the right way and not cry later....
whats it going to cost you to borrow the blocks?
Just sayin.