The impact of regional mb and MS formulas on regional MS/mb discrimination is investigated using a large number of Western United States earthquakes and explosions. Comparison of NEIS mb values with regional mb values shows a systematic error of 1.2. Additionally, a simple analysis of variance shows that the variance of the magnitude estimate is reduced when log(A) replaces log(A/T). These changes, along with a refinement of the distance correction, yield a new regional mb for the Western United States given by mb = log(A) + 2.4 log(Δ) − 3.95 + cj, where A = 0 to peak amplitude in nanometers, Δ is the distance in kilometers, and ci is a station correction. Usage of this formula improves the performance of regional MS/mb discrimination by a factor of 2 to 6.