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Old 11-29-2012, 6:50 PM
drg-racr drg-racr is offline
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Default Quentin LR308 80% lower problems and solutions...

This is a follow up to this thread I started a month ago:

Machining a 80% LR-308 is not like machining a milspec AR15 so I expected a few issues here and there. This thread is to help anyone that purchased one of the QD 80% 308 lowers with some issues that may come up during machining/assembly.

1- Safety selector hole. I used the prints off of the Colfax website as they were the only ones I could find. They say they are the DPMS dimensions so I went with them. For some reason QD and Colfax use different DPMS dimensions and the predrilled safety selector detent hole came out .024 offset in the .375 selector hole I drilled. To fix the issue I drilled the detent hole out bigger, pressed in a plug and offset drilled the new detent hole in the right location. I also moved the hole towards the centerline of the receiver .010 to give a little more room between the selector and the side of the reciever...the safety selector scraping the side of the receiver was an issue on my lower and is covered in detail in the original post linked at the top of this thread.

You can use the dimensions off of the Colfax website if you move all the hole dimensions .024 back towards the rear of the receiver. After I machined my receiver someone involved with the QD group buy PM'd me and said the QD safety selector hole dimension is 6.160 from the center of the front pivot pin hole to the center of the safety selector hole. Colfax shows this dimension is 6.136 back. You basically need to make sure you center your safety hole over the existing detent hole.

2- Bolt Stop mods. I had a few issues with the bolt stop. It had very little travel and would not catch the bolt and it interfered with the DPMS upper I bought. To fix the travel issue I had to modify the plunger and spring and grind on the back of the stop. The plunger and spring would not collapse fully into the drilled hole in the receiver so rather than drill the hole deeper I ground the small end of the plunger to shorten it to .390 long...I also had to clip a couple of coils off of the spring. That helped raise the bolt stop a bit but not enough so I ground on the backside of the stop(shown in the picture with an arrow). I would grind a little at a time and reassemble until I got the bolt stop to rise as much as I wanted it to. I ended up with .474 rise..this put the bolt stop at about the same position as it does with my AR15's. At this dimension it has never failed to lock back for me when at the range.
The bolt stop also had to be ground for upper receiver clearance as shown in the pictures. It sits nearer the back of the cutout in the upper receiver. One other issue I had was with the little set screw for the bolt stop. The little cross hole was about .002 smaller than the small diameter turned on the set screw that came in my DPMS parts kit. Whether the QD hole is too small or my set screw diameter was too big I don't know...but beware, the set screw that comes in the DPMS kit is awful and will break very easily. I had basically zero pressure on the allen wrench screwing it in and it just crumbled and was stuck in the hole. I am still amazed I got it out..I was sweating big time on that one. I ended up turning down a stainless set screw as shown in the picture. I use plenty of anti-sieze on it and don't tighten it much at all.

3- Buffer retainer. This one I didn't catch until a couple of days ago after firing 80 rounds through the gun. The buffer retainer pin that stops the buffer from coming out when you separate the upper and lower is holding the buffer back too far. When you close upper down on the lower the back of the bolt carrier is supposed to just touch the buffer. Mine was not, so every time I fired the gun when the buffer followed the bolt back it would slam into the buffer retainer rather than staying in contact with the bolt carrier. It was denting up the buffer and the buffer retainer was bent over and ready to break. I cleaned up the dented up buffer with a file and was able to bend the little nub back straight. I wasn't really sure what to do here so I made a spacer and pressed it into the back of the carrier. It took .027 of thickness to give me the contact I wanted. I had to drill a .375 hole in it so the firing pin could still be removed/replaced. The other part in the picture is the slip fit setup part to find the thickness I needed. I'm not sure what could be off here other than the buffer retainer hole..I've never had to deal with this issue.

4- Upper/Lower slop. My DPMS 24" barreled upper fits very loose on this QD lower. Not sure how other uppers will fit but a DPMS upper is sloppy. I put one of the little red rubber deals under the rear lug and it tightened up fine.

Here is the finished product. I'm really happy with it now that it's all done. I ran into some problems but finally got them all worked out(hopefully). I am a machinist by trade so I am able to take care of some of the problems easier than most...hopefully the other 80% QD 308 lowers out there work out a little better than mine did for my particular build. I will be heading to the range tomorrow to put another 45 rounds through it and see how the plug works out in the back of the bolt carrier. I have been able to get a group slightly under 1" at 100 and I expect it will get better when I get a couple hundred rounds through it. If anyone has any more specific questions about machining their 308 lower feel free to post a question.

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Old 11-29-2012, 10:40 PM
ott1 ott1 is offline
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Great job fixing the little niggles with the AR platform.
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Old 11-30-2012, 5:11 AM
h0use h0use is offline
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Good post! Thanks
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Old 11-30-2012, 11:34 PM
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Seesm Seesm is offline
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Nice job ...I for sure have had to fix a few guys bolt catch but that bolt spacer I have yet to see that needed.
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Old 12-02-2012, 1:45 PM
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deac777 deac777 is offline
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Thanks for sharing your experiences and solutions on your QD AR-10 80% build. I'm about to work on a QD AR-10 80%, and the information you provided will really help me out.
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Old 12-03-2012, 12:30 PM
drg-racr drg-racr is offline
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I put 44 rounds through it Friday and the aluminum plug I put in the back of the bolt carrier looks like new still. The only issue I'm having sometimes is the bolt not picking up a round when I drop it on a loaded mag. I have an AR Stoner 10 shot mag that I used on my first outing that the bolt would not pick up a round out of when the bolt was dropped. I have been using a 20 shot P-mag since then and It occasionally wont pick up a round only when dropping the bolt. It always does when fired. Anyone have any thoughts on that?
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Old 12-03-2012, 1:43 PM
atdski07 atdski07 is offline
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I'm going to build one, what type of upper did you go with?
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Old 12-05-2012, 8:08 PM
drg-racr drg-racr is offline
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I went with the 24" slick sided DPMS upper. Nothing too fancy just something to get me going.
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Old 12-05-2012, 8:50 PM
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wash wash is offline
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Wow, it took a lot to get that thing working.

Nice job but I have to say the TM-10 80% and Dark Soul's CNC program makes a lower with far less issues. Mine just had an undersized bolt hold open plunger hole which I solved by sanding my plunger small enough to fit.

For anyone looking for an LR308 style 80% to build, you've got options.

Originally Posted by oaklander
Dear Kevin,

You suck!!! Your are wrong!!! Stop it!!!
Proud CGF and CGN donor. SAF life member. Former CRPA member. Gpal beta tester (it didn't work). NRA member.
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Old 12-05-2012, 10:37 PM
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For anyone working on a QD10, PM me and I can send you some drawings of the proper hole locations and where to measure everything. Since talking to the OP about the problems he had I will be including this drawing with every QD10 order.

Thank you drg-racr, for documenting and sharing these problems and your solutions.
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