I've had a 77/357 for a couple of years now but haven't shot it much. That said, I was doing some optics swaps today, and mounted a Leupold VX-R to it in plans to shoot some hand loads this weekend. The few times I have shot it, I recall it having trouble feeding from the factory mag so I tried to see if it would feed this evening. It looks like it only sucessfully chambers a round from the mag ~25% of the time. The remaining 75% you'll end up with a round shoved into the breech face; this seems to occur on multiple different projectile types.
I tore the mag apart but didn't find much of anything wrong; I ended up hitting the internals with CLP and putting it back together (using two turns of tension on the rotor). Upon reassembly it seems like there is a minor improvement, but it's still jamming, especially on the last 2-3 rounds out of the mag.
Anyone have this problem? If so, what did you do to resolve it?
I've had a 77/357 for a years and have shot a few thousand rounds. This is what I've found:
-- There needs to be a break in period. I have two of these carbines, and one took a little longer to break in.
-- Don't baby the bolt. Don't be an animal either, but work the bolt authoritatively, especially on rounds 3 and 4, when I've found that jams most often occur.
-- Take your magazine apart and reassemble it without the spring, then slowly spin the follower and see if it catches or scuffs at any place in rotation. The slightest drag could have an effect, especially on those numbers 3 and 4 rounds. The follower may need the removal of plastic burrs or flashing. Going over it once with fine sand paper also helps.
-- There are types of ammo that these rifles don't like to feed. The rim of a case can catch and prevent the round from chambering. Many brands load with a nominal or nonexistent crimp as with revolvers this isn't nearly as much an issue. The best kind are those that have a pronounced crimp curling inward towards the bullet. The best all around plinking round I've found is the PPU SJHP. It feeds extremely smoothly, is accurate, fairly high-powered, clean, consistent, and cheap at about $18 a box of 50 online. I don't know how well it expands, but getting the boost of an 18" barrel it should behave at least like a SP. Another good brand is the Remington HTP 158 gr SP, although that is more expensive at $26-30 a box of 50. I've also found -- which wouldn't be so apparent when shooting a revolver -- is that the point of impact varies wildly with .357 Magnum out of a long barrel. I've noticed as much as an 8" vertical difference of POI at 100 yards. Of course the hotter it is, the higher the POI will be.
-- One last thing I've found that helps, although one of my rifles doesn't need this tip. When pushing the bolt into battery, most people -- me included -- have a tendency to put a little downward pressure on the handle. Try putting upward pressure on it, pushing up slightly. This will help chamber the more stubborn brands of ammo. I suspect it's because this dips the bolt face down a little, and may cause it to scoop up the round at a more effective angle.
I hope you find a solution. Hang in there with this carbine. It's a great gun and one of my favorites.