Will the 300gr Sierra cycle through the Marlin 1894 action @ 1.730" OAL?

If your talking about factory rounds then yes. If your talkin handloads I am not at my bench to check my OAL but I can tell you I shoot plenty of the Buffalo Bore hardcasts in heavier weights through my 1894 with zero problems. I loaded some 325 grainers this year and I think they were right about the same length as yours.

Hey Stetson, thanks for both responses.

The 300 gr Sierra shoots really well in my Ruger Super Blackhawk, and I was hoping it would cycle through a 1894 .44 I have on order.

The only ones I know of that won't cycle are the 340 grain buffalo bores and the garrets. Other than that you should be good to go. The 44 is a lot of fun.