Failure to feed for a rookie.

Powerman · September 19, 2013

I don't necessarily want to diagnose a specific problem, but have a discussion on the topic. I am new to ARs, let alone .308s.

On the surface, I can certainly understand being under gassed, or high friction. I do not see any such "common" problem of .308s being under gassed. Most complain of over gassed. I have yet to hear of a particular brand/combo/setup that is in fact under gassed. Having an adjustable block and having it turned down too much is sort of obvious. And of other note, if the gas block is covering the port, or some sort of fouling is present to provide restriction for the gas... well ya, under gassed.

Any other restriction to movement, not enough lube, sand/grit/mud/fouling.... maybe too much weight in the buffer.... I don't know, not moving right. That is easy to understand.

So what are some other causes, and what are the symptoms? I know weak springs in the mag can cause FTF. I have been looking into 3" 1911s and can read inherent problems with shrinking the design. I guess I"m asking, what are symptoms of specific FTF events, that are caused by other specific problems of the action not working properly. Educate me... or of course point me in the right direction.

Boerboel guy · September 19, 2013

From personal experience I have had a disconnector cause short stroking. My 300blk upper behaved exactly like an under gassed gun. 556 uppers on my lower ran fine but wouldn't lock back reliably. After lots of cussing and head scratching it was determined that the slot the disconnector spring sits in was slightly forward of where it should have been causing it to be extremely stiff. New disco and it runs flawlessly w all uppers.

blue109 · September 19, 2013

Just so I understand....when you say FTF....are you saying the round is chambering properly but not being ignited? Not chambering?

Powerman · September 19, 2013

Failure to Feed

blue109 · September 19, 2013

Derp. Got ya. Way thinking FT fire in my head.

unforgiven · September 19, 2013

Could be a few things.In 5.56 it was mag related for me.Follower was hanging up.

survivalshop · September 20, 2013

I have a CMMG lowered 308 build , that will have FTF with P-mags & P-mags only.

I believe most FTF are mag related.

When building a AR , using different manufactures for upper & lower receivers , tolerance differences between them can cause many problems , including FTF.

washguy · September 20, 2013

Powerman I guess ive been lucky on ftf's....been putting them together since 1992....never had a problem on ar15's or 308's until 6 months ago. built up another 6.8 and it had ftf's...there was no solving this problem...took the damn thing fully apart twice....still did it....it locked back on function check,properly gassed...you name it....it was right on the money...but still ftf after the first one or two shots....so after picking up the parts after they had hit the garage wall I checked the inside dimension of the no name adj buffer tube cause i knew it was a cycling prob of course...the damn buffer tube was not machined right and it was way too short on the inside.put a new buffer tube on it and the rifle ran like a clock!

the garage wall was beat up and missing paint,but hey we moved lol Wash

Rsquared · September 20, 2013

Don't you just love cycling problems? <laughs>

seasprite · September 20, 2013

the garage wall was beat up and missing paint,but hey we moved lol Wash

So when you moved did you leave the parts stuck in the wall like a modern art master piece <laughs> <lmao>

washguy · September 20, 2013

So when you moved did you leave the parts stuck in the wall like a modern art master piece <laughs> <lmao>

hahaha the carrier did the most damage....it hit the sheetrock between the wall studs ..lol

the out of spec buffer tube really got out of spec after the 5lb mallet pounded the snot out of it!

:) Wash

blue109 · September 20, 2013

On my old 24" factory DPMS upper, the cam pin hole in the bolt was a tiny little bit off. Caused it to run really tight and have lots of issues including stovepipes and FTFs. Took me a while to figure it out. Like wash's extension, one tiny part can ruin the fun. Might take some trial and error to find it.

Powerman · September 20, 2013

So this is cool... lot's of different things. And I want to know other things... but obviously, I have a specific FTF problem I am trying to figure out.

I think everything is fine until it does not feed. If I look at my brass, you can see scrape marks. I'm assuming it is the bolt lugs. When it gets bad, the the brass is gouged and jammed by the bolt not going all the way back and picking up the round at the end.

This round was a little different. The brass was caught up top and inside the receiver, and the mouth was jammed up against a barrel lug I guess and was sort of split open.

So can brass help diagnos certain problems?

shibiwan · September 20, 2013

Perhaps pics of the damaged brass would help?

Edited September 20, 2013 by shibiwan

Robocop1051 · September 20, 2013

I just fixed a FTF issue for a friends 300Blk. He was an easy quick fix. Luckily I noticed he ha an A5 buffer and spring in a standard carbine receiver extention. A quick order from DSGArm got a VLTOR A5 tube to my door in a few days. 10 mins later I had him up and ready to shoot.

Powerman · September 20, 2013

Perhaps pics of the damaged brass would help?

I knew you were going to say that... I didn't keep them. I know, stupid. At leas the different ones. I have plenty of the gouged up ones.

If you are looking at the barrel from the breech... then when a jam did occure, the spent brass was at like 10:00, with the new round trying to get in the chamber. The spent brass was inside the reciver to the inside of the new round. The spent case was jammed into the barrel lugs which dented the mouth of the case in. (I assume) But this was new... never saw this before. And I thought it odd that the brass was inside... meanig was this more failure to eject which caused the FTF?

Any other time though... brass is thrown a good 4 feet to 4-5 o'clock position. All land in the same place.

shibiwan · September 20, 2013

Could be an ejector/extractor combo issue. Do you have one (or two) of those nitron O-rings around the extractor spring?

Powerman · September 20, 2013

Could be an ejector/extractor combo issue. Do you have one (or two) of those nitron O-rings around the extractor spring?

I have a JP bolt and never took it apart. So I do not know.

washguy · September 20, 2013

Powerman so without pics i believe its a stovepipe condition...one round coming in and the other round still inside or trying to eject...right?

when it happens like that its a fubar condition right?....sooo what kind of ammo first off? does it lock back on the bolt catch with only one round in the mag and fired? Wash

so lets go back to basics....store bought weapon what brand? did someone build it? car type stock or rifle stock? what ammo? barrel length? have you taken anything apart? gas block exposed or not? what brand of mags? break the rifle open...muzzle to ground...drop the bcg into rifle observing what happens as the carrier comes home to chamber....gas key or gas tube could be off a lil

Edited September 20, 2013 by washguy

MikedaddyH · September 20, 2013

As crazy as this sounds, I think your BCG is moving too fast. Slow it down by adjusting the gas or by adding weight to the bolt carrier.

survivalshop · September 21, 2013

Could be bolt bounce , from too much gas or short stroking , for that matter .

That jam ( FTF ) sounds just like my CMMG when I use PMags . Almost bends the case in half some times,other times just some deep gouges.

Could also be binding in the action & don't leave out the fire control group . Weak buffer spring or its binding .

Powerman · September 21, 2013

OK, so I'm not the thread police... I don't mind them going where they go. But is there some sort of check list... things to look for to start pointing to a direction?

I know you guys know plenty and it isn't as simple as just saying A, B, an C. I troubleshoot plenty in my line of work... it "depends".

Obviously I could go over my build and we could try to get to the bottom of my problem... but where do you start? If you have a feed problem... or probably a better term would be a "jamming" problem... where do you start to untangle the tangle?

shibiwan · September 21, 2013

Take it one step at a time:

Try a mag with one (and only one) round in it. See how it feeds, shoot it and see if the bolt catch catches the BCG in the right place (i.e. the bolt face). Do it a few times to make sure it's consistent.

Next, try two rounds in a mag... see if the rounds feed consecutively or if they jam. Again, repeat several times to make sure the issue isn't there.

Then try 3, then 4 rounds per mag...

As you go through this process you'll figure out where it's hanging up. That should give you a better idea what's causing the problem and make it easier to eliminate the issue.

-S