The gauge is for checking head space. Too little head space could result in a slam fire with a rimfire firearm. Too much head space and the firing pin won't strike the rim hard enough for reliable ignition. Really has nothing to do with OOB discharges. OOB discharges are usually the result of out of spec ammo, fouled bolt / chamber or a broken part.
There are other options for a "box mag gun",the Tippmann Arms and CMMG dedicated uppers are just 2. Heck, the Ruger 10-22 with a 15 or 25 round mag would fit the bill for a box magazine.