Check out this burn rate chart. 1680 is #84.
You didn't say what type of rifle you had. If you have a bolt action or AR with the gas port in the pistol position, you can use anything between about #63 thru about #88. If your gas port is in the carbine position, the action won't fully cycle below about #74 or so.
Hope this helps.
http://www.reloadbench.com/burn.html