I am pretty sure I know the problem but I wanted to ask people far more intelligent than I am.
Today I took my SBR to get it sighted in before my LAV class. The range is an indoor range with more rules than common sense so I was not able to do much to diagnose issues on site.
3 or 4 times over the course of the day I had the bolt slide over the top of the round and trap it between the bolt and the feed ramps. I thought it may have been mags since I was using a couple USGI 20s, however my friend had no problems with the same mags and I did have the same problem with my single Lancer 20 rounder.
The gun is the following.
Daniel Defense 10.3 inch LW barrel, Upper and Bolt. Colt Lower with a H buffer.
Ammo was PMC Bronze 55 grain.
I am sure it is something pretty basic but I wanted to run it by the smart people. :jester:
From waht you said, it sounds like the carrier speed is too fast and the magazine spring isn’t acting fast enough before the BCG slams home. Do you have an H3 buffer and maybe an extra power spring laying around so you can swap weights around to change the buffer weights?
I am having a little bit of a hard time understanding the problem when you state that the “bolt [slides] over the top of the round and trap it between the bolt and the feed ramps”
failure to feed? Does the round leave the mag?
What’s the history of this rifle?
I would not take a gun to a class unless I knew it was reliable, hopefully you can fix this and put some serious rounds down range before the class, you don’t want to be “that guy”
Is this the first time you have shot PMC 55gr? My experience with it that it a little under powered. When I chroned some the average velocity was quite a bit less then most.
Yeah. thinking along the same lines. Originally I was thinking the bolt was moving to fast but it is also possible that it is short stroking. I just hadn’t thought about that since the ammo has worked in the past.
I am just going to have to put a few more rounds down range tomorrow to troubleshoot.
For gas leakage. If the ammo was working before, it sounds like something changed.
From reading the description of the malfunction, it sounds like the bolt is traveling rearward enough to eject the spent casing but not enough to catch the rim of the round in the mag. The bolt, as it returns forward, catches the case body to push it out of the magazine. Since the case is under the bolt, it cannot be fed into the chamber
Just to repeat the above- ALWAYS check to be sure the bolt will lock back with a single round in the magazine
My 11.5" and 12.5" both have eaten up the PMC Bronze Line with no issues. I fired about 600 rounds of it during the Magpul Dynamic Course.
For shits and giggles use a heavier buffer and see what happens. I would also do the lockback test as Tweak mentioned. As a matter of fact that should be one of the first things to check.
The spring in a magazine is under a constant load. As a cartridge is stripped from the magazine and chambered, the next cartridge in the stack is moved up in the magazine and is trapped by the feed lip well before the chambered cartridge can be fired. That is to say that it is “on deck” before the recoil phase of operation begins.
The problem isn’t the bolt out running the magazine before it can chamber and fire the next round, the problem is raising the round into place before the bolt moves forward to strip it from the magazine.
As the bolt moves rearward, it pushes the top round down. If the bolt moves forward again before the magazine can raise the round into position, the bolt will pass over it. It may be possible that with a fast bolt and/or weak spring, momentum will carry the rounds down further, exacerbating the problem