So I’ve taken my 733 clone SBR out to the range twice now and have been experiencing disappointing intermittent failures to feed.
Thus far they have been pretty straightforward failures to feed as far as I can tell. It’s occurred with a couple different Okay GI 30rounders which have functioned fine with other guns, including different uppers on this SBR lower (back before I Form 1’d it). The FTFs are happening about midway through the mag.
Gun has a Fulton C7 upper with M4 feedramps, a Spike’s 11.5 lightweight barrel (Spike’s was the only one offering a chrome lined 11.5 pencil barrel as far as I could tell), and a SOLGW BCG, BCM gas tube and Colt charging handle. Buffer is an H, and the spring is a standard Colt buffer spring.
Gun was lubed with Slip EWL prior to use. Temperature on both days was 20-25 degrees. Ammunition is Federal XM193. Seems to eject mostly around 3 o’clock or so, occasionally more like 2 o’clock to 2:30.
I’m wondering if perhaps I should be using a different weight buffer here. I don’t know the gas port size on the Spike’s barrel (I was told that was “proprietary information” when I emailed them, I really wish someone else offered the barrel profile I needed sigh…) So the gun could either be under- or overgassed for all I know. Or maybe the issue is with the feedramps?
Another odd thing is this:
This case is mangled. I would assume that this sort of case deformation would have caused a malfunction but the only malfs I had were straight up FTFs where the offending rounds fed, fired and ejected normally after remediating the FTF. This case looks more like one that failed to eject and got crushed by the BCG coming forward, and I didn’t have any FTEs. So I didn’t notice this happening while shooting. It is possible that this isn’t even from my gun, but it has the same headstamp as my other brass and was in the same spot as my other cases, where I don’t believe there was any pre-existing brass on the ground.
So I don’t know what relevance, if any, this has, but I thought I’d mention it. But like I said, perhaps this wasn’t even from my gun.
I got this thing home a day took some close up pics of the feedramps.
The right feedramp does look a bit “off” to me. It looks slightly misaligned and also isn’t quite flush with the ramp on the barrel extension (the ramp in the upper appears as though it is ever so slightly deeper, creating a tiny, tiny “lip” where it meets the ramp in the barrel extension.) But I don’t know if this would be severe enough to cause the issues I was having or not.
I was having failure to feed issues with a Sionics 11.5 barrel and Mega Billet receiver. The issue you describe above was the same issue I was having. Bullet point would catch the overhang and push the slug into case, and bend the case slightly. It only happened with GI (okay and DH) type mags. I presume this is account the shallower feed angle vs a PMag. Receiver was the culprit. When I changed upper receivers, this issue went away.
ETA: In your bottom photo, on the right side feed ramp, you can see where it appears the bullet is hitting the feed ramp right at the junction between the barrel extension and receiver.
I think your assumption of needing a different buffer weight is correct. Any time I take a new upper to the range, I bring a carbine, H and H2 buffer, as well as a normal weight recoil spring and a higher tension spring for tuning purposes. I start with the H2 and high tension spring and work my way down, decreasing tension/weight until I can make it through 3 full mags with no FTF/FTE’s. I would start there.
The ramps aren’t perfect but few are. Picture 1 and 3 answer your question. The bolt is on top of the next round up in pic #1. In pic 3 the spent case was crushed by the bolt because it just dribbled off of the extractor and got in the way of the bolt closing. It’s short stroking. It doesn’t like the combination of spring, buffer and/or gas.
I think the angle of the first photo makes it look like the bolt is on top of the stopped round, but the brass visible at the rear is the base of the next round down on the left side I believe.
EDIT: this angle shows the position of the bolt relative to the cartridge a bit better.
As far as the mangled case goes, how could that case have gotten crushed by the bolt, then flung all the way over to where the rest of the brass landed though, all without causing an additional FTE malfunction or me noticing? I didn’t find the crushed case until after I finished firing the mag and went to pick up my brass. I’m still not ruling out that case was from someone else’s gun and was already there.
I only had one malf in that particular mag, the one where the round went nose-first into the feedramp and stopped. I pulled the mag, pushed the round back into place and rechambered it and it fed, fired and ejected normally. Im not trying to sound like I’m doubting your assessment, I’m just trying to work through how it all happened in my head.
The crushed case didn’t come out of your rifle by itself. It got crushed by a bolt trying to close on it. Either you cleared it or it came out of another rifle. Discount pic 3 and go back to pic #1. What could possibly cause that? How could the round be jammed into and hung up on the feed ramp if the bolt is on top of it? Think about it.
Well if it is an issue of being undergassed, I can stick a CAR buffer in there and see how that does…(which is actually more “correct” for the gun this is a clone of actually…)
I will say that I feel like the gun was somewhat softer-shooting than my 14.5" and 16" carbines, which I wasn’t expecting, I was assuming it was going to be a fairly harsh shooter (by AR standards of course. This is my first and only SBR) so perhaps there is a gassing issue.
The gas port may have been sized for suppressor use…
I started to write a response earlier this morning, but got sidetracked, are the fail-to-feeds all on the right side of the mag?
That would tend to point me at the right feed ramp. The right ramp in the picture seems a little proud, but really not that bad.
Carbine buffer, known good spring and see how it shoots. Use various mags. If you are still having the same problems, I’d see about smoothing the feed ramp.
ETA - Just a thought - are you sure the gas tube is entering the carrier key smoothly? I could see where that might slow things down a bit - wouldn’t necessarily impact ejection, but might feeding.
Circle 10, did you perform a lock back check? Without doing a lock back check, you can only guess whether or not your AR is short stroking.
If it is short stroking, do not assume it’s under gassed. Your gas system could be leaking or partially blocked.
If you’re ejection seems erratic, take a close look at the extractor spring.
I doubt the crushed case is from your AR. Cases are crushed like that from a stovepipe.
A lighter buffer will not fix short stroking. At best, it can only mask the problem until whatever is causing the short stroking gets worse.
Yeah, all the FTFs were on the right side. I probably wouldn’t have looked twice at that right feedramp otherwise. I have shot this exclusively with GI 20s and 30s so far (the FTFs were only with 30s, but that might just be coincidence) but next time around before doing anything else I may try Pmags because they basically cover the feedramp extension in the upper receiver anyway. Although I want the gun to work with GIs too, it might be informative to see if I have any issues with Pmags.
I don’t think the interaction of the gas key and gas tube is the issue, as the FTFs are all like the one pictured, where the BCG is stopping well before the key reaches the gas tube.
I didn’t do the “load one round in the mag, fire, repeat…” lock-back check but I didn’t have any failures to lock open at the end of firing the last round in any of the mags.
Ejection seems fairly consistent, mostly about 3 o’clock from what I can tell.
I’m kind of leaning toward the crushed case being from someone else at this point. Just odd that it’s the same headstamp (not that 2018 Lake City is rare ammo or something) and was sitting on top of the ice with the rest of my brass.
One thing about the gas system being blocked…I will note that in order to get a preliminary zero with the gun, I had to dial the windage farther to the right than I normally like, not all the way, but the aperture is pretty noticeably over to the right. I considered that the FSB might be slightly canted but with the high winds both days I figured I’d need to try to re-zero on a calm day to really be sure what was up with that.
If the FSB is canted though, could that be causing a partial obstruction of the gas port, or would the FSB really need to be way off to cause that issue? (because like I said, the rear aperture is off to the right, but not actually maxed out yet)
The hole on the underside of the FSB is so much larger than the gas port, I can’t imagine a slight cant choking off any gas.
I’m thinking if it were canted that far, it’d be WAY over, probably wouldn’t even be able to zero.
That’s kinda what I figured but I thought it was worth mentioning just in the off chance that there was in fact a connection.
I’m strongly suspicious of the right-side feedramp at the moment.
So, if the rounds are indeed failing to feed due to that slightly “off” section of the right feedramp, could the reason that the rounds can’t overcome that slight imperfection in the ramp be because of a gassing issue (Either legit undergassed, leaking, or obstructed somewhere)? Or could that minor of an imperfection in the feedramp cause issues on its own in a properly gassed gun?
The ejection pattern seems healthy but my understanding is that ejection trajectory can be symptomatic of a gassing problem but is not, by itself, diagnostic of one. So presumably “proper” ejection doesn’t always mean proper gassing.
I’ve had failures to feed that were because of barrel feedramps standing slightly proud of the receiver M4 cuts, so I would personally be looking at that.
If worse comes to worse I can pull the upper apart and see how that barrel mates up to the ramps in some other receivers too I suppose.
One reason why we polish feedramps and check their interface with the feed cuts in the upper. Get a Dremel and some Cratex and polish both sides first. I think this will solve your issue.
Thanks! I can do most work on my ARs myself, but anything involving a Dremel may be something I let a professional handle just to be safe. I do appreciate the info though.
Is it possible there’s some other issue besides just the ramp, and the FTFs are just a symptom? Because technically, on semi automatic, even with GI mags, shouldn’t the cartridges not be feeding at a low enough angle to even catch that “lip” between the barrel extension and receiver cut in the first place?
Is that where an H2 buffer might come into play?
That’s what I’ve always thought, until I had the points snag the extension when dropping the bolt on a fresh full aluminum 20 rnd mag.