That meter goes to 130 dB and firearms register considerably higher:
I have no idea how far away the firearms were from the test instruments, but if it were me I'd keep this in mind. I'd get some distance from the muzzle of the gun, at least the first times I tried using the sound meter.
And if you haven't already done so, I'd study up on the nature of sound measurements. A 10db difference in sound level is considerable.
Please let us know how it all works out. I'd be curious to hear how a .357mag and 45colt compares as measued by that meter.