Reading the description of the Wrangler I envisioned it working like this:
You have to be in the vicinity of your sentry gun, so within sight of it. Pull out the wrangler, now the sentry aims where you point.
For example, on Badwater, one sentry could be set up on the roof, pointing the Wrangler at the roof door to ensure it's covered, while another engie builds on the lip of the roof to protect the cart.
I don't think you will enter the sentry gun's view and also doubt you will be able to shoot at distances beyond the regular sentry gun's distance, as some here are suggesting.