SBR failures to feed

[Circle_10]

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.

[Circle_10]

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.

[nightchief]

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.

[dwd1985]

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.

[GH41]

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.

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.

[Circle_10]

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. 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.

[GH41]

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.

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.

[Circle_10]

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.

[Rascally]

The gas port may have been sized for suppressor use...

[26 Inf]

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 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.