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Posted: 8/21/2006 8:24:57 AM EDT
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I picked up a used stripped lower to build another rifle with. so i buy a parts kit and install the parts. then i begin to test everything to see if the lower will function properly. i cock the hammer back and test the safety, which checks out fine. I then move the selector to the fire position. I pull the trigger and hold it down. the hammer falls as expected. now while the trigger is still being held down, i recock the hammer as the bolt carrier will do when firing. the hammer stays back. now here is when the problem begins. keep in mind that my finger is still holding the trigger all the way back, and now the hammer is cocked back, being held by the disconnector. when i release the trigger, the hammer falls again, which is a big no-no. either the assembled rifle will blow up or perhaps even shoot full auto when i try to fire. i tested the rifle out by chambering a single round and then shooting it without a magazine in the mag well. after firing one round, i remove the upper to find that the hammer has indeed fell again as expected. I put in my trigger and hammer group from my other working rifle, and the new lower does the same thing. I tried the trigger and hammer group from the non-working lower in my working rifle, and it functions properly. I'm thinking that the holes that the pins go in to hold the trigger and hammer are slightly wallowed out just enough to cause this issue. I bought the lower along with a stock the seller called a "socom M4" stock for 160 bucks. here is a pic of the lower with my 16 inch upper installed(my properly functioning lower is sitting next to it): http://www.hi-upload.com/uploaded1/newlower.jpg what do you guys think? is there anything else that could cause this problem except for the holes being wallowed out? the guy i bought it from was using this lower with a .50 BMG cal upper, so i'm pretty sure the lower has been put through a lot of stress. thanks for any help, Mike |
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i dont know what the problem is, but here is how i would go about fixing it. when the FCG acts nutty i get scared and throw the whole set out. i hate worn fire control parts. i would try a new factory set and see how it works. if it still gave me trouble, i would switch to something like a RRA 2 stage trigger group. if that didnt work i would be pretty well convinved the receiver is out of spec. |
the trigger group is a brand new set from a complete lower parts kit made by RRA. they group works perfect in my known-good bushy lower that i have had forever. the bushy parts have the same symptom in my newly acquired used lower. im guesing the lower is FUBAR? thanks for the input. |
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the lower is a stinger arms lower and also has mega machine shop inc stamped on it. the trigger does have a hell of a lot of play side to side while the trigger in my bushy has virtually no side to side play at all. i did happen to find this upon a google search. would this help fix the problem and allow me to use the lower?
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Before you get into turning the disco, check to make sure that you have installed the FCG correctly, including installing the disco spring with the large coil side down into the back of the trigger slot. Once confirmed that the FCG is installed correctly, retest. Now if all the parts are installed and correct, then it's time to reset the disco. The photo you listed is the area that you will remove metal on the disco to time it for the rifle at hand, and the free gap is the distance between the tip of the disco sear, and the hammer's disco sear with the trigger at rest (not pulled). To find the free gap, you will lower the hammer down and once the disco and hammer rear sear get as close to each other as they can, you will note the distance of the two apart. By removing metal from the front/bottom edge of the disco, this cam’s the disco forward (closes the free gap) and allows the disco to better/ longer retain the hammer at trigger rest (read allows the primary trigger sear to get into the correct needed position to catch the primary trigger sear at disco release). Note: Since this is a used FCG, you shouldn't need to dress the sear edges (using a light stone and remove any production burs without rounding the edges (left sharp)), but on a new FCG, you want to clean up the sear edges before retiming. |
everything is installed correctly. if i install the parts the exact same way on a known-good lower, the parts work correctly. the trigger moves side to side with a lot of play in it. I'm pretty sure the excessive wear on the lower has caused the trigger and hammer pins to be farther apart than they normally are in a new lower. what do you guys normally use to remove material from the disconnector? I planned on using the flat side of a bench grinder. |
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The bench grinder is going to take off metal too fast, and better yet, you will not be able to maintain the correct angle of the bottom of the disco. I have a jig and a wet stone that works very slowly, but since you may not have such, a flat surface with 200 sandpaper will be a better option to draw sand the part on (will give you more control to take off a little metal at a time, and maintain the correct angle). Remember, remove a little metal, then put the parts back in and recheck the free gap. As for the side to side movement of the FCG parts, it shouldn't mater, and about the only time I shim a FCG is when it's being Match tuned. Now on the other hand, if the holes are just egg'd out, then there is the option of making the holes .170 for a Colt fire control group (if the holes are not egg"d larger than this to begin with). |
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One other thing that could be tried is a set of the KNS pins with the dogbones on the outside of the receiver. KNS Pins They are a little ugly, but they will absolutely stabilize the trigger pin locations if the holes are egged out for some reason. If the holes are just flat-out drilled in the wrong relationship to each other, the KNS dogbones MAY be hard to install due the the length between holes being long. This would also be a clue... 
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| I got a lower that is not to spec. I have purchased 2 lower assembly kits now, and installed both of them correctly. I am having problems with the trigger releasing the hammer. I measured the lower and compared it to the blueprint and the hole of the hammer is off by a few thousands, and the trigger hole may be off, but it seems to be on the money. I don’t have a working lower, so I don’t know how the trigger mechanism is supposed to look, or work. or cycles Any info would be gratefully appreciated. |
If you are in the greater LA area, come by my place one evening or weekend and I'll get it working for you. If not, look for someone nearby you with a functioning lower to compare with. Might also check in the "build it yourself" forums and read the tacked thread on assembling a lower. |
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