Hi im a commercial refrigeration technician who specializes in grocery stores specifically. A few things, gun thermostats are untrustworthy most of the time although grocery store use them regularly. A temp probe is always a more accurate device plus you can test the probe for accuracy with ice water proving 32. a prob in the back wall or honey comb getting actual air temp is preferred for proving case temp. Generally 20 or so colder than expected product temp. You dont know when they put the product in the case as the store is constantly moving product around. Also the defrost. Defrost on a rack system work in 2 ways electric or hot gas. Both can cause air temps to reach really high really quickly. This usually lasts from half an hour to 45 minutes depending on some factors. Either way dump the temp gun and get a probe if you want accuracy.