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Old September 1, 2008, 12:14 PM   #1
Txhillbilly
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Trouble with new AR lower

I built my first lower yesterday and put a new upper on. When I test fired the gun the 1st round fired fine,second round chambered but there was no trigger pull,like the trigger didn't reset.But when I extracted the second round I saw that the firing pin had contacted the primer on the second round and the round had not gone off. I thought there might be a problem with the new bolt assembly so I took my upper off my RRA AR and put on this new lower and same thing,first rd fires,second rd chambers but I have no trigger and the firing pin has contacted the second rd.
What did I do wrong,and how do I fix it?
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Old September 1, 2008, 02:04 PM   #2
Harry Bonar
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AR

Sir;
Just a guess on my part but it looks like your hammer isn't staying cocked on the cycle - it shows that it's following the assembly down and making a small indenture in the primer (supprised the round didn't fire).
Check that trigger for engagement.
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Old September 1, 2008, 02:16 PM   #3
Scorch
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Like Harry said, it sounds like a case of hammer follow-down. The hammer should be cocked as the bolt opens. My guess is that the hammer is out of spec (worn or defective). Try a different hammer to see if the same thing happens. If so, the problem would be the lower receiver itself. If the area on the bottom of the bolt carrier that cocks the hammer does not move the hammer far enough, the hammer could be getting close to the point of cocking but not quite far enough. Same thing could be caused by a worn bolt carrier, but since you tried the same lower on two different upper receivers with the same result, I will ignore that possibility.

Doesn't surprise me that it didn't fire the round on closing, though, because the firing pin is restrained by the firing pin retainer pin and pulls away from the bolt face when the bolt opens. On closing with a follow-down, the bolt closing is a gradual process and does not allow the firing pin to slam into the primer.
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Old September 1, 2008, 08:52 PM   #4
kraigwy
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tighten up your recoil buffer assembly, when its loose you often can't get it to fire.

Take the trigger out and re-install it. That worked for mine, just a burr or something.

Also if you are using reloads, run them through a wilson or simular case gage. Excessive headspace on the cases will cause both problems.
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Old September 12, 2008, 08:21 PM   #5
44 AMP
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Suggestion

Before doing anything else, cycle the gun empty. Dry fire, hold the trigger back, and cycle the bolt. Release the trigger. Will the gun (dry) fire or is the trigger "dead"?

If the trigger is "dead" (like what happened when you tried to fire the second round), push out the rear takedown pin, and hinge the action open. Look at the hammer. Is it down? or still cocked? If the hammer is "following" the bolt when it closes, it will be down.

You can also check this with just the lower reciever. With the hammer cocked, pull and hold the trigger (easing the hammer down, so it doesn't snap against the reciever). Then recock the hammer with your thumb. It should be held cocked by the disconnector (as long as you are holding the trigger back). Let go of the trigger, and the hammer should fall slightly, and be caught by the sear nose of the trigger. If the hammer falls all the way, then either the hammer, the trigger, or the spacing of the pin holes in the lower reciever is bad.

If you have another AR lower available, put the questionable parts in it and see if they work properly. IF they do, then your problem is probably the new AR lower. If they don't, the parts themselves are at fault. Be careful not to confuse which parts go to which lower when you have them apart!
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Old September 16, 2008, 12:15 AM   #6
andyandmaryk
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Another simple thing to check that is often overlooked. If your hammer spring is on upside down it will cause there to be less pressure on the hammer. This will often cause light primer strikes.
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