thats no good !! , if it twist in easly then its not seating up well and will give you head achs later down the road. This is pretty common actualy , I like the dual dove tail desgine as it is very strong by desgine but I make some modifications to mine.
Afte I get my rings set in the base I unscrew the base leaving the ring attached to the base and and I'll make a smlaa tig weld on the inside of the base welding the ring to the base. On my own guns I go a step further and run one small bead all the way around the outside of the base when the ring seat down on it , this make for a one piece ring/base setup.
In you situation , you might be able to shim it like you did with the aluminum but you better use some sort of steel , aluminum is a bad shim material , I have used a set of feeler guages (found at any auto part store) a few time to make a small shim for somthing their make fron strong steel and will last alot longer. I would also fill the recess in the bottom of the base with some sort of good bedding material like Devcon liquid steel.
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