A customer brought in a Double Star AR pistol chambered in 9MM that fails to clear the empty case before the bolt comes forward and jams the empty case up when attempting to chamber a new cartridge. Customer has returned it to the manufacturer who tested, replaced the hammer, doused it in oil then sent it back declaring it fixed. For the customer, it promptly jammed in the same manner. So he took it to a local dealer (not a gunsmith) who also doused it with oil and pumped 15 rounds thru without a jam and declared it ok. For the customer, it promptly jammed. Then he brought it to me.
Noted several things right off, it should function fine without swimming in oil! If there’s that much drag, something’s amiss.
Being chambered in 9MM, it has the typical blowback action used for 9mm ARs that included the cut down hammer with the large notch in the front. The bolt catches that notch in the hammer every time it cycles made obvious by the obvious finish that is missing inside the notch. Thinking that’s an obvious source of drag that would prevent the bolt from fully cycling back enough to get the case completely off the bolt face, put in a standard AR hammer to test with, also a slighter lighter buffer spring. Test firing with 115 gr cartridges yielded a jam every third round. Testing with heavier bullets (147gr) put 9 rounds thru with no jams and the cases going out about 10 feet.
Then refaced the extractor to ensure it was parallel with the bolt face and just a few thousandths deeper to ensure a good pivot point when the ejector did its job. Test fired 5 rounds with no jam, yet one case almost didn’t eject…fell almost straight down once out of the receiver…rest landed about 10 feet away. IOW, not satisfactory performance. Oh yes, wiped out most of the oil so it wasn’t swimming anymore.
Question is this, since the blowback action is strictly physics based, what else might help eliminate this jamming issue with standard 115 gr 9mm cartridges?
JR
Assuming the extractor geometry is correct, might check to see if there are any blockages in the gas system that might be slowing the action or not driving the bolt back far enough. Jeff
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It is a direct blow back action, so there is no gas part to check. Looks like extraction is alright. Ejection is the problem. The bolt doesn’t fly back far enough for the ejector to kick the empty. It is short cycling. It works ok with hotter loads is another clue. Try verify it with the tape method.
If there is no source of drag, I would lighten the recoil spring to correct. -TL
Question Follow Up
Thanks for the direction. Short cycling is what I suspected also. Hence, first tried the Dunlap method of stretching metal on the ejector for more length. Tried a lighter and shorter buffer spring, That induced a fail to eject on every round. Which seems to defy the physics of this blowback action system.
And, of course, the AR action design has lots of riding surfaces that induce drag. Suspect that’s why everyone prior to myself had it swimming in oil to get it to function without jamming. Yet, that is not a permanent solution. This should be more reliable that a standard gas fed AR…there are less parts that move or get dirty.
Somehow the physics isn’t right.
JR’s GunWorx
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Not familiar with the 9mm blow back system on AR platform. Is the ejector on the bolt face just like regular AR? -TL
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Don’t assume the extractor is correct. Make it correct. Good tension and spring out, positive angles where it needs it. Make sure the extractor claw is close to the bolt face as it should be. Then fit the ejector as need. Failure to work with hotter ammo usually indicates it isn’t a cycling issue. I believe the ejector is fixed in the frame, make sure it is up and as far to the right as it can be so as the bolt cycles the ejector fills the ejector slot. Keep us apprised. Ken
Question Follow Up
Thanks Ken. Haven’t had a chance to test the further extractor tweaking. Made the back of the hook a little more positive and deeper. Verified it holds the cartridge against the bolt face without shaking off. As far as I can tell, the ejector fills the slot in the bottom of the bolt carrier. Everything does as it should when manually cycling dummy rounds. That, of course, is not nearly the dynamics that occur when firing, so will test fire again on Saturday when I have daylight.
Oh, and yes, the ejector is fixed. In this case, in the 9mm adaptor that fits in the standard AR mag well. Yes, have verified it up as far as it should go.
Will post the results of the test shoot.
JR
Follow Up
Still a no go. With a Std 115 gr bullet the second round failed to eject. 🙁 So the deeper extractor pulled it out fine, just didn’t eject from the receiver when it hit the ejector.
JR
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What is the ejection angle? Are some cases hitting the ejection port? If the extractor is fit correctly you may need to adjust where and when the ejection is occurring. Erick
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Is it jamming the same way as before you worked on the extractor?
Question Follow Up
Yep, same way. Case comes off the bolt, yet doesn’t clear the receiver before the bolt starts forward. A high speed camera could be useful if focused into the ejection port to slow things down enough to see what’s going on. May have to try something like that, just to watch things move around.
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The extractor should hold the case onto the bolt face and as close to the bolt face as it can, and still have the extractor claw snap over the rim. Make sure the extractor claw is holding the case close to the
breech face and not simply keeping the case on the bolt face when you swing the bolt around. This is subtle but important. (Come take the extractor class 😀 ). Ken
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If I’m reading this all correctly, the gun runs with 147g bullets but not 115s? Seems like you eliminated the extractor and ejector as possible problems.
Like commented above, it’s just physics. You get enough recoil from the 147s to make the gun run but not enough from the 115s. So I would think that either the buffer is to heavy or the gun is over sprung.
In theory, shouldn’t the gun run with the lightest factory loads without the buffer “bottoming out” and when shooting maximum loading have the buffer bottom out to absorb the recoil not countered by the recoil spring? DC
Question Follow Up
I tried a lighter spring, well at least a shorter spring. 115gr rounds didn’t eject at all. So think I eliminated the over sprung issue also.
Ken, your comment on the case seated firmly against the breech face, thought that might be an issue also since the breech face opening is rather deep. By eyeball, somewhere around .125 inches. the extractor holds the bullet in that hole yet perhaps not against the bottom. Having it against the bottom would enhance the pivoting action of the extractor. Will check that out by attempting to move the extractor back some, yet may have to replace it to do so because of the way it’s retained by a pin in the bolt. JR
Question Follow Up
Tried to post a picture (file is too big) with this comment of a typical jam in a 9mm AR pistol. The empty is 180 degrees as the bolt comes forward and picks up the next cartridge. This one happened on the 3rd round out of a 10 round mag with 5 rounds in it.
Extractor holds the cartridge base firmly against the breech face and the ejector is as high and to the right as the bolt will allow. Not sure what the fix is. Further ideas?
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180 degrees, so the mouth of the case is facing the shooter. Is the case hitting the ejection port somewhere? If not then it sounds as though the extractor is still not doing its job correctly. Ken
Question Follow Up
I have to think it’s hitting something and bouncing back into the receiver. It does have a fairly large case deflector that takes up about a 1/4 of the typical AR dust cover. The dust cover is cut off that much and the hinge pin is what holds it in place at the rear of the opening. However, every 9MM AR I’ve seen has one of these deflectors. So didn’t think it could cause the case to kick back into the chamber.
Managed to get the picture small enough to upload. Hope it’s big enough to see the jam. Can also see the case deflector.
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Is the bolt behind the unfired case rim?
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Does the bolt come back too far so the spent case hits the receiver wall or deflector instead of the opening? That might indicate it’s under sprung. Jeff
Question Follow Up
The thought that it may be under sprung could also indicate too short of an ejector???? hmmm, perhaps not starting to eject soon enough? Would the physics of a heavier bullet that ejects ok prove that to be the case? I did try stretching the ejector, yet that was only a few thousandths at the most. Perhaps more is necessary.
And yes, the picture shows the bolt with the empty brass and a new cartridge in front of it. JR
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Check the inside edges of the ejection port to see if the case is catching on the edges. Make sure the ejection angle is not directing the shell into the bottom or top of the ejection port and not out the ejection port. Ken
Question Follow Up
The angle of the ejection not clearing the port? Would think to adjust that would require putting an angle on the ejector itself, without making it shorter. How much angle would vary by each individual gun and its unique physics, wouldn’t it?
Will see what I can do with this, starting with just a degree or two to bring it down some. At least after another good look at it.
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Watch the prerecorded monthly video by Ken. He shows how to find and change ejection angle this month. Jack R
Reply
Have again been busy enough to neglect posting the result of my last work on this AR pistol. Ended up with a happy customer as I fixed what the maker of the firearm could not. 😀 Was the ejector angle, was bouncing the extracted case off of the lower part of the port instead of out the port. A slight change of the angle and flattening the tip a bit was enough to raise the ejection so the case cleared the port each time. Thanks for the feedback and tips. They led to a solution! JR
Comment from Mark Foster:
I was reading the posts and drawing a mental picture in my head of the angle on the front of the ejector. Much like a 1911 ejector, you can make the case eject anywhere you want. I had one that I allowed to hit a lieutenant in forehead almost every time he shot. He still has a little scar!! I was finally forced to fix it… nobody else saw the humor like I did!!
One thing I want to add- do not alter the buffer springs. JR mentioned trying lighter and shorter buffer springs. You should not be shortening the springs. Most will do better with a slightly stronger spring and a normal weight 9mm buffer (which is already heavier than a 223/5.56 buffer).
The “physics” are correct, as they always are. It’s just that most folks don’t really understand what is happening. If the bolt opens too soon, due to a light spring and buffer, the blowback gas is at a lower pressure and exits the system faster. It is wasted. Keeping the bolt closed a fraction longer allows the gas pressure in the barrel to build (before the bullet exits) and exerts more force against the bolt. Gun will probably cycle better with the 115g bullet and thus the reason Wolff and Sprinco make extra power springs. Mark
