Posted: 9/8/2009 9:18:15 AM EDT
I have been burning up the search button, but have come up empty. I have a malfunction/FTFeed with a home built DMPS LR-308 frankengun. The upper, lower, and mags are DPMS, the barrel and bolt are Fultan Armory (20" Titan barrel). Total rounds through the gun so far are about 300+. All parts were ordered brand new by me and assembled lovingly in my basement.
The malf has two parts. First is that the rounds sometimes (about 1 in 20) hit the front of the magazine which sets the bullet back into the case and makes my velocities go all over the place. I'm worried about pressure problems. I use my own handloads, but they're pretty mild (43.4 grains of Reloader 15 in once fired Winchester cases pushing 165gr SGKs, or 155gr Nosler CC; velocities at 2,585 fps for the 165 grainers). I have tried crimping the cartridges but it does not matter at all. The bullet is jamming in there way far. I have had this happen with the 4 round DPMS mags, as well as the 19 rounders.
The other problem is that the bolt sometimes (hard to tell the quantity as I often load one round at a time into the mag. Load development and all) fails to pick up the next cartridge at the case head, and instead catches it about two thirds of the way to the shoulder. This drives the round up towards the chamber at an angle, not all the way in, and the bottom two lugs of the bolt press down into the case, ruining it. I'll post pics tonight if I get the chance.
My initial guess is that the bolt is cycling faster than the magazine can present the next round. The mags are brand spanking new, but I loaded one of the 19 rounders up full and left it for a week to break it in and it still does it. The buffer and spring were for an "AR-10", which, correct me if I'm wrong, the LR-308 doesn't need the extra length required for the Armalite? Am I using the wrong buffer and spring? The buttstock is the standard fixed buttstock, not a collapsable. I am considering ordering a new buffer and spring from Fultan Armory; maybe it will work with their own bolt? Am I in the right ballpark, or is there something I'm missing?
Can any AR jedi out there set me straight? I would greatly appreciate it.
As far as the magazines catching the bullet and driving it into the case, I have filed the front of my mags for this exact reason. I use a half round file and lower the front of the mag so I can see the full diameter of the case mouth when looking from the front of the mag. I probably removed 1/8" from the front face of the magazine.
The other modification I have done is to remove the followers and clean off any casting flashing. I use a fine file to smooth off the rough corners, etc. It's a possibility that your followers are hanging up and not allowing the rounds to feed up quickly enough for the bolt to catch them.
Thanks for the tip, it sounds like it would work well. I'll give it a try. Did you file away material only in the center of the mag, or did you take away from both sides underneath the rounds?
I have also heard others with a FA Titan complain about the same sort of jams with DPMS mags, but no jams with C-Products mags. As promised, here are some pics:
That's my frankengun.
Here is a picture of the chamber with feedramps. Notice the shiny copper coating on the bottom of the feedramps. I'm wondering if the rounds are hitting here as well; it sure looks like it. I've ejected rounds out that appeared to feed properly, and measured them with my calipers and found them set back slightly.
Here's a look at some of the rounds in question. The one on the left got hit with the bottom two lugs of the bolt that run through the feedramps, but is otherwise at the normal seating depth. The one on the right hit the front of the magazine. Twice I've had the rounds visibly stuck against the front of the mag.
This whole thing is wierd.
I had this with my first .223 build where the bullet seemed to jam in the feed ramps. I pulled the barrel and used a bic pen tip (the perfect tool for a 223) and sandpaper. I started with 220, then 320-400-600-800-1000-1200. I skipped 1500 and 2000 grit, after all its not a Grand Torino. Besides I went to the polishing compound and a Dremel. It was about an hour or two in front of a TV and well worth it. Never a jam with any mags since. Not even the steel mags I had to file the front of. I should point out that you don't try to reshape at all. I only rounded the edges and widened it slightly. The main focus was on making that surface smooth as glass.
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