Sam54 .300 or 30 cal. is the measurement across the lands of the bore it also translates to 7.62 mm,the .314 is the groove to groove dia. of your bore.
As mentioned the .314" measurement is the one that is critical as it will determine which bullet dia. jacketed or cast to use to get the best results.
As donor mentioned you need to take another measurement of the inside case neck dia. of the once fired commercial brass you intend to use for your reloads to make sure that the jacketed or cast bullet you intend to use will chamber after it's loaded. You also need to measure each brand of brass if your using more than one due to different variances in case neck thickness.
For all intensive purposes a .311 to .312" dia. jacketed bullet is going to be your best option as I don't know anyone that makes a .314 jacketed bullet. As long as the once fire brass inside case neck dia. measures larger than your bullet of choice you shouldn't have any chambering issues. My Mosin bore measures .300 across the lands and .312 across the grooves,I shoot the .311 dia. jacketed bullet with excellent results.
I tend to get the best results with Jacketed bullets that are slightly smaller than the groove dia. of the bore and you should never use a jacketed bullet that is larger than the groove dia. of your bore otherwise you can get into some dangerous pressure issues with you loads. Cast bullet that are larger in dia. are a different thing all together they squeeze down much easier as they enter the leads of the bore and cause much less friction than jacketed bullet with an equal load.
Cast bullets might be a different story in your rifle depending on the inside case neck dia. In general a cast bullet that is about .002" larger than the groove to groove dia. of your bore will work fine but it may not chamber that large of a dia. bullet. As I stated my Mosin has a .312 groove bore so I have to shoot a .314 dia. cast bullet for best results but it chambers fine due to the fact that the brass I use and the inside case neck dimension allow me to use a bullet that large but a .314 dia. cast bullet is the largest my rifle will chamber.
