Thank you Beltfed and Heckler for the great posts.
I had my synthetic M1-9 out to the range for the first time yesterday and my results were the same. I only brought 250 rds with me. By the end of it, things were looking better, but at first, I was really worried I had bought a lemon. Strangely, my rifle feeds hollowpoint ammo the best and 124gr FMJ the worst.
When I got home I spent about 2 hours on the rifle. I noticed that the buffer seemed to be meddling with things, but I was really tired at this point and didn't want to over do things. I had already spent a lot of time cleaning and lubing everything. I didn't go crazy modifying things.
Two things I did do, came to me thanks to the Ruger 10/22. I think that cocking the hammer robs the M1-9's bolt of its rearward momentum even worse than in the 10/22.
So firstly, I polished the surface of the hammer that the bolt rides over on the rearward action. This is minor, but it helps. I chose this angle of attack because its really easy to get the trigger mech out and apart. Then I did just a little tuning on the hammer bent. This involved multiple assembly/disassembly processes. Be aware that when you do this, there is a tendency for the trigger/disconnector pin to work its way loose. Once its out of one side of the trigger, which isn't all that easy to see at a glance, your results will be all messed up. Make sure that pin is properly located. When I was finished, there was almost zero camming of the hammer on trigger pull.
Then I swapped out the hammer spring for a factory Ruger 10/22 hammer spring. With the addition of two small washers between the head of the hammer strut and the spring, this seems to work perfectly. I loaded 5 primed empty cases into a mag and all 5 fired. Only one failed to feed too.
Before this, my trigger pull gauge would bottom out at 9lbs before the trigger even BEGAN to move. Now it fires at 5.5lbs.
Cycling those empties, I was certain I could feel the buffer wasn't letting the bolt back far enough. The bolt face just barely gets in behind the top round in the mag. Maybe the buffer will compress some with use, but I think that if my rifle doesn't feed reliably after the next 100rds I am going to trim it as you did Heckler.
Thanks again!
One other peeve I have with this rifle is that it is really hard to seat a full mag with the bolt closed. The tab that engages the magazine is so tiny, I am leery about modifying that at all. Anybody have any thoughts on what to do here?