Since no one helped you with the stability part, That bullet is on the edge.
Remember that stability is about RPM for a length of a thing, it's weight is almost irrelevant. The 75gr Amax is very long, noticeably more than the 77gr SMK.

MV*720/Twist = RPM

That is your formula, remember the Hornady bullets (NOT SXSP) has a RPM limit of about 290,000.
It is completely normal for a barrel to be a 1/2-twist faster or slower than advertised. Also how smooth your barrel internals are plays a BIG role. Lucky for you you have a Ruger, smooth stuff typically.

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Use the links on the left to find bullet lengths, then input your specifics.

I'm a firm believer in the theory of " If it bleeds, I can kill it".