View Full Version : Midlength acting up again, bolt fetching up, failures to feed first round.
So awhile back I posted about this midlength abruptly failing to cycle.
https://www.m4carbine.net/showthread.php?202487-Previously-reliable-AR-suddenly-not-cycling&highlight=
The issue turned out to be a sheared gas key screw. BCM replaced the entire BCG.
However since then the replacement bolt carrier has exhibited a tendency to sort of "fetch up" in the receiver when it gets hot. It doesn't affect the gun when it's firing but when using the bolt release on a reload when the gun has heated up it usually feels like the bolt is just barely going forward with enough velocity to chamber the round, and on some occasions it doesn't, requiring the forward assist to push the bolt the final half inch or so into battery.
Working the bolt with the charging handle when the gun is hot it feels as though there is an unusual amount of friction. Even when cold the BCG makes a more noticeable than usual "rubbing" sound when manually cycling the action. I have since tried using a different carrier and charging handle and get the similar results. The audible "rubbing" sound isn't as pronounced with the alternate carrier but there's still a lot of friction when cycling it by hand when it's hot and it exhibits similar issues feeding the first round in a new mag. I tend to run my BCGs quite wet do lubrication shouldn't be an issue. Since the issue manifests when the gun is hot I'm assuming that some kind of metal expansion must be at work here, but I never experienced this issue with the original BCG I had in it.
Any care to hazard a guess what's going on this time?
My layman's guess would be that there's a burr or something in the receiver and during firing the metals in the receiver or BCGs are expanding just enough for this to cause a problem. But, like I said, I never had this issue with the original BCG in this gun.
Jwknutson17
03-31-18, 08:07
If there is that much friction you should surely see some wear on something. I would take the BCG and CH out, and brake clean the inside of your upper completely clean. See if you have any noticable abnormal wear. Take out your spring and buffer and inspect the inside of the RE. Inspect the carrier and see if you see anything. Maybe some debris entered in your upper? And how old is that carbine spring?
I've looked over the BCG and the only real wear points appeared to be on either side of the gas key, and it didn't look particularly severe. The wear I could see inside the receiver didn't appear significantly more pronounced than in my other uppers. However perhaps hitting it with the brake cleaner will reveal something.
The buffer spring is new, as I recently replaced it thinking that may have been part of the issue, although the length of the old one was within spec.
It *seems* like the friction is occurring somewhere in the receiver itself but behind the ejection port. I haven't been able to detect anything out of the ordinary in there as of yet. When I get home from work I'll pick up some brake cleaner and give the receiver and inside of the RE a more thorough cleaning and check.
I've looked over the BCG and the only real wear points appeared to be on either side of the gas key, and it didn't look particularly severe. The wear I could see inside the receiver didn't appear significantly more pronounced than in my other uppers. However perhaps hitting it with the brake cleaner will reveal something.
The buffer spring is new, as I recently replaced it thinking that may have been part of the issue, although the length of the old one was within spec.
It *seems* like the friction is occurring somewhere in the receiver itself but behind the ejection port. I haven't been able to detect anything out of the ordinary in there as of yet. When I get home from work I'll pick up some brake cleaner and give the receiver and inside of the RE a more thorough cleaning and check.
What do the ribs on the carrier look like? Anything that would suggest they are rubbing against the forward assist slightly? Also, what does the interior end of the FA look like? I'm not an expert, but given you say it "seems" like the friction is inside the receiver yet behind the ejection port my first impression is to wonder if perhaps the FA might be slightly out of spec/a hair too long. Either way, I'm curious to see how things pan out.
No real sign of wear on the the FA notches. The FA is probably worth checking out more closely but it seems odd it would really only be a problem when then gun is hot.
Do you have a fancy trigger in the rifle?
Since you said you run the rifle wet could it be a buildup of carbon/grease that is making it feel sludgy when cycling? Could it be creating a gritty feeling on the upper?
Try the upper on another lower.
Have you changed out the barrel...or otherwise cranked down on the barrel nut recently?
No real sign of wear on the the FA notches. The FA is probably worth checking out more closely but it seems odd it would really only be a problem when then gun is hot.
As I said, I'm not an expert. My thinking based on what you've posted is if the FA is slightly out of spec/longer it might not be making contact when "cool". As things start to heat up, they start to expand at a certain, possibly enough that they start to make contact with the FA when "hot". Granted we are talking very small measurements, but if I understand things correctly (and quite possibly I'm not) I wouldn't totally rule out the FA yet.
As others have mentioned, I could see it possibly being an "upgraded" trigger being slightly out of spec as well.
Try the upper on another lower.
I have. Early on I thought maybe the mag was sitting ever so slightly too high due to maybe an out of spec mag catch or something, and the carrier was perhaps dragging on the mag and getting slowed down as it went into battery. I had real trouble seating loaded Pmags on a closed bolt with this particular lower too, which I thought may have given credence to that theory, so I tried a different lower. No more issues seating Pmags but the same issue of the BCG dragging on something persisted, however......
Do you have a fancy trigger in the rifle?
I have an ALG ACT in the gun, when I swapped lowers I took the ALG ACT out of the old one and put it in the new lower. I hadn't considered the problem might be related to the FCG somehow.
Also of note is way back when I first got this gun and ran it through a carbine class with no issues of any kind, it had a stock trigger in it. I didn't put the ACT in it until last year before I started shooting it regularly again for the first time in a while.
I have. Early on I thought maybe the mag was sitting ever so slightly too high due to maybe an out of spec mag catch or something, and the carrier was perhaps dragging on the mag and getting slowed down as it went into battery. I had real trouble seating loaded Pmags on a closed bolt with this particular lower too, which I thought may have given credence to that theory, so I tried a different lower. No more issues seating Pmags but the same issue of the BCG dragging on something persisted, however......
How did it shoot?
It fires fine, the only issue hiccup I've ever had with the gun in any of it's various receiver/BCG combinations while actually firing it was the whole episode where the gas key screw sheared and BCM sent me a replacement BCG. So my current issue doesn't seem to be something that affects the gun while it is firing, it just creates a problem or potential problem of the gun not going into battery when reloading, and then having to stop and remediate that issue.
It fires fine, the only issue hiccup I've ever had with the gun in any of it's various receiver/BCG combinations while actually firing it was the whole episode where the gas key screw sheared and BCM sent me a replacement BCG. So my current issue doesn't seem to be something that affects the gun while it is firing, it just creates a problem or potential problem of the gun not going into battery when reloading, and then having to stop and remediate that issue.
I'd suggest making a quick list asking the following: What are the constants in each situation this occurs? What are things that have changed (both before and since the issue arose). Once you've identified those, go back to how it was before the issue and see if the issue is still present. If not, make 1 change at a time and check for the issue to be present.
A time-consuming, non-scientific process, but by the end you should know what is causing the issue. I've used it at work trying to track down failed electronic equipment amid a mess of spaghetti wiring and literal miles of extension cables and components that no one can remember the purpose of/reasoning for.
Have you stripped your carrier down and run it into the upper to see if there is binding with the gas tube and carrier key? How many rounds are through this upper now? I would also consider contacting BCM directly about the entire issue to see if they will address it.
Yeah, I took the bolt out and ran the carrier up and down the upper, the friction seem to be occurring well aft of the gas tube. The carrier almost makes a soft "zipping" type sound different from other carriers but doesn't really seem to hang up too badly on anything. However what I haven't tried, is doing that while the gun is hot, because that's when the problem really starts to manifest.
I think I'm going to take the brake cleaner to the upper receiver and the BCG, re-lube and try actually *shooting* this upper on a couple more different lowers, probably another basically identically configured one with a rifle RE/buffer and a different ACT trigger, and then a carbine lower with an H-buffer.
Even though it's currently on a different lower receiver than it originally was, the RE and buffer, and the FCG were transferred over from the first lower.
ETA: IIRC, when I had the gas key screw fail on the original carrier in this gun I had around 2500 rounds through it (I've had the gun since 2009, but after shooting it in a class it kinda became a "spare" AR for a number of years and saw very little use up until last fall). Since changing out the carrier I've probably only put another couple hundred or so rounds through it as I've spent most of my range time in January, February and March shooting some other guns.
So here's what I did this morning so far, I took another upper that I wanted to get at least somewhat sighted in anyway and stuck it on the lower from the midlength. I also put in the carrier and the charging handle that were in the midlength when the problems first started manifesting.
No issues, including after the gun had been heated up quite a bit. So I guess I can rule out the lower, the carrier, and the CH as the issue. So it's time to dig into the midlength's upper.
bamashooter
04-01-18, 19:09
Assuming the rcvr is dark I'd take a silver sharpie, light-colored fingernail polish, etc and paint the crap out of the rcvr. Evidently you see no signs of wear otherwise?
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