Although I have no experience with the m77, I do have two liberty number one rifles in 7x57, and have owned others of that vintage and even earlier. I have found all of them to shoot very accurately. I shoot nothing but handloads, but have put quite a variety of loads through these rifles. As one would expect, some rifles like some loads better than others, but I have never had one that would not shoot to my liking, and none of the rifles were inaccurate with any of the loadings I fed them. I have fired lots of the lighter 140gr bullet loads and they have performed as well as the heavier bullets. Assuming that the M77 was chambered to the same specs as the number one of that era, I would not have any hesitancy in purchasing one of the former. It is an outstanding caliber - probably the best all-around for N America.
I had a Liberty in 257 Roberts. Without a doubt, the worst rifle I have ever owned. I floated the barrel, then glass bedded the barrel, went through a plethora of bullets and powders, played with seating depth, you name it. The best accuracy I could get was slightly more than 1 1/4" at 100 yards. finally got tired of wasting money on it, and sold it on the auction block. No doubt it had to be the barrel. couldn't have possibly been anything else. I have seen a few NIB Liberty's in the same 257 Roberts as recently as today, and they're asking $900. Wow!