Failure to Feed on Last Round

[ViperTwoSix]

Hey Everyone,

I’m hoping someone can provide insight on a recent malfunction.

While using a C Products Defense 30 round magazine with Winchester .223 FMJ, the magazine performs without issue. I switch to Winchester 5.56 FMJ and the bolt catch locks open with one round left in the magazine every time. The bolt catch looks engaged as if the mag was empty, but the last round still sits in the mag untouched by the bolt. Pressing the bolt catch release closes the bolt, which then properly strips and chambers the round. Switch back to Winchester 223 FMJ and the magazine fires all rounds and locks the bolt back on empty.

What would be causing the bolt to lock open with one round left in the magazine, and only with 5.56? I tried to manually replicate the malfunction but couldn’t. Loading the magazine with one round, locking the mag in with a closed bolt, and using the charging handle to chamber the round doesn’t cause any issue. The issue only occurs on the last round with 5.56 when live firing from a loaded magazine.

Thanks!

[mack7.62]

Bad magazine, replace.

[26 Inf]

I think the first question I have, is does the rifle lock back like this on any other magazine?

If not, I'd mark the magazine and not use it for serious stuff.

Are you holding the rifle in the same manner? Are you pulling back on the mag when firing 5.56 and not when firing .223? (Don't know why you would be, just asking) Bolt Catch sprin and detent in place and functioning? Catch moves with no side to side wobble?

That's all I have.

[ViperTwoSix]

I will definitely mark the mag and use for practice only.

I did hold the rifle in the same position. I even put it in a bench rest to make sure it wasn’t something I was doing. Bolt catch functions fine, spring and plunger in place.

The rifle functions normally with other magazines. It’s just really odd to me that this one mag has an issue with 5.56, but seems to work flawlessly with 223. At first I thought maybe the follower was the issue, but it doesn’t lock back when manually using the charging handle with only 1 round in the mag. I thought maybe the pressure difference was somehow vibrating the bolt catch up, but it only does this on the last round. So now I’m at a loss for what the specific cause would be.

[georgeib]

The last round is on the bolt catch side of the magazine. Could be that the mag or it's follower are slightly out of spec and are causing the round to hit the catch. Just speculating.

[GSPatton]

The only thing that comes to mind is mag spring, since the last round has the spring at its least compressed length. However it's also when the mag spring has the lightest load on it, so who knows. But yeah, gotta test out a number of other mags to isolate the issue to specific mag(s) hopefully.

[LDM]

The OP's description with different results for different ammo is beyond my limited experience.
Something I have experienced with failure to feed last round was a magazine spring issue.
I replaced magazine springs in several magazines (another story in itself) and was having failure to feed last round resulting from the last round being loose.
The issue was the last loop of the magazine spring was rubbing the ribs of the magazine enough to catch and prevent the follower from reaching the top of the magazine. I noted this after a failure to feed upon removing the magazine after a failure. Examining the magazine from the top, I could clearly see the top loops of the spring in contact with the ribs of the magazine. I tried it a few times and it repeated. So I removed the spring and squeezed the top loop to make it slightly narrower (just enough to eliminate the interference). This solved my problem.