Obviously, you bought used. From what I have always heard the 78 is very accurate. Have you cleaned the barrel? Also, check to see if the crown is damaged.
I have an old Westernfield 822 (22 mag) that's accuracy fell to something like 3" @ 50. So bad that I put it up for yrs. Finally, upon reading-up I realized that as a kid, I alway would stick the cleaning rod from the end of the barrel. This damaged the inner rim. Finally, I took it to a gunsmith and had him lop-off 2" of the barrel. At 24" I always thought the rifle was too long for my hunting purposes. --- Anyway, after the cut and new crowning, she shot consistant groups 1/2" @ 50 and 1.25ish @ 100.
You may want to check this out.