I wouldn't recommend buying a cheap torque wrench unless you are prepared and except it to break and are extremely careful.
I bought a cheap bike one from Trade me knowing if it was carp I could return it, and it was carp and I got my money back.
I tested it on a torque tester at work and it seemed ok accuracy wise, the click mechanism was pretty dull though.
Used it on my mountain bike being cautious and it failed to click at the set torque.
The problem is for scopes 20-40 inch pounds is very very low torque and if the mechanism fails it's very easy to over torque the fasteners, either damaging the scope or the rings.
Keep a look out on trade me for a used branded one, but I wouldn't trust a cheap torque wrench.
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