308 failure to feed issue

[cmann250]

I’m having a failure to feed issue that’s puzzling.

Here’s the specs on the rifle: Magpul 20 rounders, Aero Precision BCG, DPMS barrel, and a LaRue MBT.

Now for the ammo, my reloads: TAA 07 brass (Taiwan I believe) and a 168 grain SMK. RCBS sizer die, Lee seater die, Lee shell plate.

I’m having an issue with the bolt coming completely into battery. This only happens when feeding from a magazine. I never have any issues dropping the bolt with a single round already in the chamber. The issue probably happened 8 times in 30 rounds.

Factory ammo functions normally. Mysteriously (but confusingly) I never had an issue before the MBT.

I’m blaming the sizer die, although I did measure some rounds beyond 2.800” OAL (2.803” was the largest, still “should” chamber) Rounds that try to chamber but can’t come out with carbon on the neck.

What say you, INGO? For what it’s worth, I did sub-inch groups at 100 yards with 44 grains of RL15.

 

[indyjohn]

There are so many possible causes.

I don't know if 44 gr of RL15 is a light load or not, but it is pushing the bolt carrier back far enough to grab the next round, so that's not likely it.

I too would suspect the cartridge shoulder. But you really can't determine that being the cause unless you have an appropriate gauge to measure them. Either this: https://www.midwayusa.com/product/1016251573/le-wilson-case-length-headspace-gauge
Or this: https://www.midwayusa.com/product/1012939524/rcbs-precision-mic

As far as OAL, I think that if it fits in the magazine it should chamber.

 

[cmann250]

Thanks for the reply.

I don’t think it’s the charge weight because I was doing ladder loads and others did it too.

I forgot to mention that all rounds went through an EGW Chamber Checker.

 

[Gaffer]

On the RCBS sizer dies, are you using the Small Base or Full length? RCBS recommends the small base die for auto loaders.

ron

 

[indyjohn]

Thanks for the reply.

I don’t think it’s the charge weight because I was doing ladder loads and others did it too.

I forgot to mention that all rounds went through an EGW Chamber Checker.

That says to me your issue is with your firearm.

I am not an expert.

 

[cmann250]

On the RCBS sizer dies, are you using the Small Base or Full length? RCBS recommends the small base die for auto loaders.

ron

I am using a FL die. Thanks for pointing that out.

 

[55fairlane]

If factory loads work just fine, it' your reloads........I'm guessing it's your powder, not enough power to cycle the bolt properly

Try varget,bl-c2,imr4895(my choice) or something with a burn rate closer to these powders

Aaron

 

[cmann250]

According to my Hornady manual, 45 grains is the max charge. I stated above that this issue is not exclusive to the 44 grain charge. It also occurs at 44.5 and 45 grains.

I believe Gaffer is on to something with a small base die.

 

[cmann250]

Update- problem solved? I ordered a small base die for good measure.

Today I reloaded the same 30 brass I had trouble with yesterday. I did 15 with the RCBS FL die I used before and 15 with a Lee die I had setting around. I had no issues which is puzzling. However, I was diligent in checking COAL and using the Chamber Checker.

 

[halfmileharry]

I don't know if your problem is solved or not. That's not many rounds and only one quick ammo run to say you've got it fixed. Especially when you don't know what you did??

 

[bigedp51]

When the case chambers when feeding a single round when feeding a single round into the chamber the magazine is not slowing down the bolt velocity.

Meaning you do not have enough shoulder bump and the case headspace is longer than chamber headspace. And the weight and velocity of the bolt is slamming the case shoulder back and pushing the case shoulder back when chambered.

And when feeding from the magazine the bolt velocity is slowed down and can not force the cartridge into the chamber. Meaning your cases are slightly longer than the chamber in headspace length.

Below is a fired case from my AR15 carbine and I then adjust the die for .003 shoulder bump from this measurement.

Bottom line, bump your case shoulder back .003 to .006 shorter than its fired headspace length. Right now your case shoulder location is a few thousandths too far forward. Meaning the case is longer than the chamber headspace.

If this is once fired brass or range pickup brass it is a good idea to size the cases with a small base die. This will size the cases to minimum SAAMI dimensions and shorter in headspace length and diameter vs a standard die. Sometimes using a standard die and pausing at the top of the ram stroke for 4 or 5 seconds will help reduce brass spring back in lieu of using a small base die.

Below the case grows in length during sizing until the dies shoulder pushes the shoulder back down below chamber length. Meaning for a semi-auto .003 to .006 below the red dotted line.

 

[cmann250]

Biged, thank you for the informative post. This is all once fired brass. I have a small base die on the way and I will pay attention when I set it.

Is my Chamber Checker not checking head space? The rounds that caused a problem went through the checker.

 

[Broom_jm]

I never have understood buying a "chamber checker" when you already have the actual CHAMBER your reloaded ammo will be fired in. All you have to do is know how to measure it. Strange!

Anyway, a bottle-necked rifle cartridge is two-tapered cylinders (body and shoulder) under a straight cylinder (neck). One of your cylinders is out of position; most likely the shoulder. A small-base die is designed to squeeze down the other tapered cylinder, the case body. It's no more likely to solve your problem than your existing FL sizing die, which, based on the information provided so far, you have yet to adjust properly.

In other words, running your existing FL die down another 1/4 turn, even if that means shaving a little off the top of your shell holder, is the answer to your problem, as presented.

 

[Robert Richardson]

I never have understood buying a "chamber checker" when you already have the actual CHAMBER your reloaded ammo will be fired in. All you have to do is know how to measure it. Strange!

Anyway, a bottle-necked rifle cartridge is two-tapered cylinders (body and shoulder) under a straight cylinder (neck). One of your cylinders is out of position; most likely the shoulder. A small-base die is designed to squeeze down the other tapered cylinder, the case body. It's no more likely to solve your problem than your existing FL sizing die, which, based on the information provided so far, you have yet to adjust properly.

In other words, running your existing FL die down another 1/4 turn, even if that means shaving a little off the top of your shell holder, is the answer to your problem, as presented.

 

[cmann250]

In other words, running your existing FL die down another 1/4 turn, even if that means shaving a little off the top of your shell holder, is the answer to your problem, as presented.

Tolerance stack up got me. Now I understand. I also understand that after I fire this brass, it is fit to my chamber. That’s why I never had an issue the second time around.

Case closed. Thank you all.

 

[red_zr24x4]

I didn't see any mention of trimming the brass, but I might have missed it. Check to make sure the brass isn't to long

 

[cmann250]

I didn't see any mention of trimming the brass, but I might have missed it. Check to make sure the brass isn't to long

I’m at 2.010”, max is 2.015” according to my book