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Posted: 6/23/2007 6:06:39 AM EDT
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Ok, here's a problem that has been stumping me for several hours now. I got a new 4 position stock in this week planning to install it on one of my DPMS lowers. The problems is after installing the buffer assembly and spring and tightening down the stock tube castle nut, the rear take down pin does not want to close properly. It acts like it is binding up. Whatever is happening, is only after the stock tube nut is completely tightened down. If I leave the nut slightly loose everything will function properly. At first I thought it could be an alignment mis-match with the upper and lower, but I have installed the same lower on several uppers and the problem is following the lower. Anyone have some thoughts? |
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is the pin difficult to push in and out without the upper installed? if so, is it possible that you installed the detent backwards? I'd look into that first. If it's tight only with the upper installed then it could be alignment issues (upper holes and takedown pin) |
No, the pin is fine without the upper installed. I thought about it being an alignment issues but I wouldn't think it would be the same on all of my uppers, but I guess it could be a possibility if the lower is that far off.
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Buffer tube spun in until it touches the center post of the buffer retainer pin (castle nut and plate all the way back), then backed the tube off the 1/4 turn needed to index the tube for the butt stock (read the end of the buffer tube still dam near kissing the center post, and retaining the buffer retainer pin over it's rear outer flat). Rear take down pin installed pointed side in with the rear take down pin in the locked/correct position, the detent spring loaded behind the detent, then the plate pressed forward* with the castle nut chased forward against the place. * The plate should be pushed tight against the back of the receiver with ease. If you have to fight it, then either you have installed the wrong spring, or it is too long. Receiver/tube held in index, and the castle nut tightened. _____________________________ Now her is the rub, if the tube had to be spun back more than 1/4 turn to index for the stock from kissing the center post of the buffer retaining pin, then the end of tube will need to be shortened to index correctly. Also, if you have to shorten the bottom of the tube for the additional need spin, then you will need to shorten the top as well (read the top of the tube can not protrude past the top receiver threaded section or it will block the back of the carrier as the upper is lowered into place. 
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Dano, As always you have some information that I was not aware of. I have installed many of these and have never run into this problem. I have had to back the tube out nearly a 1/2 turn after touching the center post. Do you think this is a large enough amount to allow the binding? Thanks, Cayenne |
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At a half turn, you are chancing that the buffer retaining pin will escape somewhere down the line. If this was one of the cheaper knock off units, then shorten the end of the tube to get another half wind (plus a hair) to index the stock butt plate accordingly (so the end of the tube is just short of kissing the center post). Regarding the take down pin, did it seem that the detent spring was bound up as you loaded the plate tight against the receiver. It could be that the spring is too long (clipping a coil or two will resolve such), or just the tip of the detent pin may be a tad too sharp in the take down pin detents, and the reason that you cannot slip the take down pin into the locked position from the open position (read without the upper in place, and just dulling the tip of the detent a bit may resolve such for the later part of the above). If the problem is only that the take down pin seem very tight as it slips through the upper rear lug to lock the home, then it could be just a matter of mating through wear/use. Since you will be pulling the receiver extension to shorten the end of the tube (both top and bottom), pull the take down pin and check the alignment of the upper rear lug to the lower receiver at the same time. As long as the upper lug and take down receiver holes are very close, then don't break out the file yet to enlarge the upper lug hole. On some uppers/lower combos, you have to tap the rear take down pin home/to unlock a few times until it gets to the point that you can achieve such by hand (read tight is good). Also to point out, everything as you assembly it should be lubed with CLP. Once together, the rifle gets a good cleaning/lubing (more CLP and copper solvent for the bore) before it’s madam range voyage. If you are installing the parts dry, you have new/rough anodizing against parkerizing (which is the same), and the two surfaces do not promote gliding (read like trying to rub two sheets of 600 sandpaper together). To add if you haven't figure it out yet, pull all the way down on the butt stock adjustment lever (not cam'd to one side) then pull the butt stock off the back of the receiver extension. This allows you to work on the tube without the extra weight of the stock, and when you go to install, can get the receiver extension dead nuts with the receiver as you use the bottom of the two to gauge. |
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Dano, I spent some quality time with my buffer tube and a grinder last night and all is well. I ended up taking a tad over 1/32" off of the end of the buffer tube and all of the binding disappeared. I think that the extra length on the tube was causing the upper receiver to cant forward just enough to misalign the takedown holes. Thanks to everyone for their input. |
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