Antique or no, still a fine rifle, of which I own several.
I have an early 77 round top 7mm mag, took it to Colorado this year and it did the job on a good 5x5 bull. Mine shot well with 160 gr Nosler Partition handloads, but that's the only bullet I shot in it because that's what I wanted to shoot. I used Reloder 25 powder in Federal cases with Federal magnum primers, got 1.25" groups, a few a little better than that. My groups opened up if I shot very rapidly. As has been suggested, let your rifle cool between shots when sighting in and developing loads. When you have your load like you want it, see what kind of 3-shot groups you can produce waiting one minute between successive shots. As has also been stated, some 77s are finicky about forend pressure, others not so much. Mine shoots well with a little pressure so I have not floated it.