All guns are as individual as you and I. What load does good in one may not do well in the gun made right after or before it. Each of us has to take his gun to a range and shoot multiple loads to find the best for that rifle.
Shooting to 300 yards is going to require accuracy not velocity. Then Practice. Im sure it can be done.
Start with carefully measured mid range powder charges and work up 3-5 grains at a time. When Im helping others with this process on CF guns I tell them the gun speaks to me. As you shoot different charges your groups will get bigger or smaller. Your gun is telling you what it likes or dislikes. If you cant get the group you want you may have to try different powder, primer, bullet, sabot, lube, combinations. Its a process especially to get good groups at 300 yards. Take your time, keep notes, and if it doesn't all happen in one range session dont be discouraged. Its likely your gun will do everything you want it to. Just need to put in the time.
If you want more specific info PM with some questions.