The ARRSUM is the new name for the old 300 RSAUM arbor for use with 223 boresight. The arbor has the same fitment; however, it does have a different name for the arbor. This is the laser bore sight for the Remington Short Action Ultra Magnum and Remington Ultra Magnum calibers and wilcat variations built on this brass. RSAUM/RSUM/SAUM are all the same designations for the Remington Short Action Ultra Magnum calibers. 
 
Calibers it will fit:
Remington Short Action Ultra Magnums 
300 RSAUM
7MM RSAUM
Remington Ultra Magnum calibers
7MM RUM
300 RUM