I've been reading (both online and in this forum) about various combination safe locks, and I keep reading references to the different groups (1, 1R, 2, 2M, etc).  Unfortunately, I haven't been able to find anything that explains what the groups mean or the differences between them.  All I've been able to find is that there are large price differences between the groups, but no explanation as to why.  Could someone please explain what the groups mean and what makes one better than another?  Thanks!

The ratings pertain to the manipulation resistance of the lock.  Regular group 2 locks are perfectly fine for gun safes unless you're planning on having a professional spy get in and out of your safe without your consent.

Group 2 is moderate manipulation resistance, 2M is 2 hours resistant, 1 is 20 hours resistant, and 1R is 20 hours and x-ray resistant.