Get the inertia sensor that you attach to your rifle. Mine would miss shots and was more finicky than I thought when I first got it. After getting the inertia sensor it doesn't miss shots, unless I forget to arm it.
I use the JKLPrecision inertia sensor. Both of them work fine. Find the most convenient method of attaching it to your rifle. I think JKL sells a picantinny adapter, but the didn't have any in stock when I bought it so I ended up just using velcro.