I'm having the same problem with my M305. I'd like to write it off as cheap norinco crap, and maybe even sell it replete with all mags and mods. I saw this thread but there was another M305 FTF thread from further back last year relating to the op rod spring guide not held properly by the locking pin, which isn't the issue for me. It used to get through a 5/20 round mag without any failures about 95% of the time. A friend I met at the range advised I take square pliers and slightly widen the lips of the mag to "show more of the next round's primer". I started thinking it was the odd rounds as mentioned above (1,3,5) but it would completely fail to strip #2 almost 100% of the time now. I tried loading the chamber but keep 5 rounds in the mag, and it wouldn't strip #1 at all. The gas port is open fully, and I cleaned the gas piston and everything and went back to the range, and nothing addressed the problem. It doesn't seem to matter if I use an "unmodified" 5/20 or any 5/20. I haven't tried the original 5-rounder that came with it, that was the next plan and I'm kicking myself for not packing it when I went last. I'm guessing this thread died without a solution, or a solution was found without a followup.
I installed the bolt buffer and a "match" op rod spring guide as the only internal mods, and had a fun rifle working 95% reliably (read: not reliable at all!) for several years. For testing I removed those parts and replaced the guide with the original guide... to no effect. I don't know if it matters, or what it originally looked like, but the gas block appears rotated about 5 degrees, ever-so-slightly counter-clockwise when looking at it when you shoulder it. But the rotation is only apparent when you look down the barrel from the muzzle and compare alignment with the stock.
It does eject and extract without issue. Some of those reliability issues way back was an extraction issue, and I believe the spent cartridges were bouncing off the 2-point scope mount back into the closing bolt, because those extraction problems started when I got that scope mount and never got around to adjusting the extraction path.
The next cartridge (no difference if it is left or right side of mag) gets clipped by the closing bolt but doesn't strip. There is a tiny bite out of the head of the brass cartridge, just a few mils of shaved metal. 80% of its primer is "showing" above the mag, whereas before it was less than half the primer and didn't have this problem as bad. I tried mags that also showed about half the primer and they're also showing shaved heads after a round. I looked at the bolt face that should be catching the round, and there didn't appear to be anything that looked wrong with it. It feeds 100% of the time when I release the bolt manually, allowing the operating spring to have its energy to seat the bolt into the extractor groove of the cartridge. I'll followup with photos later this evening.
edit: As I re-read my post, I thought I'd add that the cartridges touch the mag lips across their entire length in the modified mags, but the unmodified mags are only touched by the edge of the mag lips along their length. I'm thinking I should polish the inside of those lips and maybe try some teflon-based dry-grease if it is simply friction that is preventing the cartridge from stripping off the mag, as the bolt's energy is "bumping" the cartridge downward due to the slope of the cartridge head. Does that make sense? The ammo I've been using are early 80's SA milsurp, FMJ ball. I've tried new FMJ and it doesn't make a difference.
