Years ago I used to load 150 grain, 30-30 bullets on top of the 30-06, they worked fine. I'm not so sure about the performance of your bullets. If they are in fact 30-30 bullets, and designed for the 30-30 velocities and you are sending out the barrel at .308 speeds they might want to open up very quickly and not give you the penetration that you would want. Calvin45 gave you a great place to start with the Marlin Express load data. If you are using load data that will not over-pressure your loads (powder load/weight to bullet weight), and you are using the proper caliber/diameter bullet, as in this case 30 caliber (.308 thousandths) it doesn't matter what the grain is, your gun is not going to "make a loud bang". Depending on the twist in your rifle and the jacket thickness/design you might not get the performance that you are looking for. I would be quite surprised, that unless your 300WinMag has a twist to accommodate the 140 grain bullet, you will not get great accuracy or performance out of your bullets. Personally I would not waste time, powder and primers shooting the 140gr bullets in a .300 WinMag, unless that twist in you barrel is designed to shoot light bullets. I hope this makes sense to you.
