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Posted: 5/29/2005 6:37:49 AM EDT
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I have about 200 rnds thru my RRA carbine and notice movement, slight but 'feelable', along the upper and lower junction. Normal? fixable? I recall an accessory item to reduce this but at a loss to where it is..... TIA docgary |
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Yes, the Accuwedge tightens this fit but it doesn't take out the play. John Feamster in his book Black Magic details a near permanent fix using easily obtained materials. I have done it to many uppers and while it requires some care, it is easy to do. What you need is either an Accuwedge or some kind of rubber shim to press up on the rear takedown pin lug of the upper. A thick rubber band, folded 4 times, worked for me and was free. You need some JB Weld (tm) filled epoxy for the filler, a good degreaser (carb cleaner workes perfectly as does brake cleaner), a home made brass knife (cut a cartridge case open, shape to fit X-acto handle and sharpen) and some wax, either sizing die wax, beeswax or old style furniture polish. Clean the mating surfaces of the upper with the solvent. This means all of the surfaces where the upper contacts the lower, including the rear, curved rear section of the upper. Apply wax to the lower, inside and outside of the upper and all other places that you can see when assembled. Apply a thin but uniform coat to the lower mating surfaces. Leave the upper mating surfaces CLEAN and bone dry. Mix the JB Weld according to the instructions. Mix well and carefully apply with a toothpick to the mating surfaces of the upper. You only need a very thin layer, not enough to sag over the sides. Pay special attention to the front pivot pin area, making sure a good coat is put ahead of the pin (but NOTHING in the pin hole) and behind the lug. If you use too little, you can always repeat but using too much will make a mess. Carefully assemble the upper to the lower without charging handle or bolt/carrier. Let it cure overnight upside down (mag well up). Next day, carefully drive out both pins and using a mallet, seperate the upper from the lower. Using the brass knife, carefully trim the squeeze out off the upper and trim any JB Weld off the lower. Some may get into the threads so be sure to clean this out. If you have a hard anodized upper, the brass knife will not scar the finsih but will leave behind some brass shavings. Use any good bore solvent to remove these marks. Make sure to clean out ALL squeeze-out and any epoxy that is not between the upper and lower. After all is clean, reassemble the upper to lower without the Accuwedge or equivalent. Check to make sure it goes together with only moderate pin pressure. You can rotate the upper to the lower in normal fashion but be sure you clean out any epoxy that might shave off. Now your receivers will fit tighter than any machined pair. Remember, any dirt will make the fit much tighter, if not IMPOSSIBLE. Often, some trim-outs will find their way to the corners of the lugs, making the fit too tight. Clean these areas if it it too tight. When done correctly, you will only notice a very thin line (0.001-0.002") between the receivers. A careful application of black marker will completely mask this.  Notice the grey JB weld all around the upper mating surface. If the upper is very clean, even these areas will adhere and the improved fit will keep dirt out better. There are areas where there is no contact immediately above the trigger group. Squeeze-out is trimmed in these areas as thick layers are more brittle. Trimming after 12 hours is recommended as the epoxy is still somewhat flexible and easier to cut without chipping.  
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Loose receiver's effect on accuracy requires match grade ammunition to detect. Its all detailed in Feamster's book. Yes, slight improvement in accuracy is possible with Accuwedge use. Even better is the epoxy bedding I detailed. On all of the uppers I have epoxied, the final fit was very tight. Pins go in just fine but no discernable movement between receivers is noted. I have yet to have one loosen. |
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thanks for the replies and the fix..... I'm far from a sharpshooter but I'm a bit obsessive over the overall condition of my weapons. When you pay extra for quality (RRA) I expect at least a perfect fit! I realize now that a little play is normal..... I just installed a large mag release button, charger handle latch and elongated rod for bolt lock/release from RBprecision.com As a lefty, these bits offer advantages. thanks again docgary |
That is the 9 ring. If you can keep it in a 4" circle at 200, then you are golden. Tight fitting receivers would be good for about .5" improvement at 200. Not much but every bit counts. |
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Has anyone tried one of the JP tension pins yet? They look like they would work really good to remove all play between the upper and lower. Only drawback would be a required allen wrench for breakdown. If anyone has tried this pin, I would be interested in their impressions. http://www.jprifles.com/photos/jptp.jpg |
They work but like you mentioned, it negates the toolless disassembly of the platform. I found Accuwedges to be more trouble than they are worth but they are useful for doing the bedding technique I detailed above. Tension on the upper is required to get a slight interference fit but this can be done with almost any bit of soft rubber placed under the rear lug. |
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O ring at the front take down pin, and a accuwedge solves all my problems on even the loosest receivers. Takes a whole minute to install, and don't need any special wrenchs to pull the unit down. Figure if your going to build a space gun, may as well start with a receiver that has the set screws to take up any slop. If the plan is to use a standard receiver, then just add the take up set screws to it from the start. Problem I see with glassing a AR receiver is that unlike a M-14 or bolt action, you have to open the two way to much for cleaning, and would require that they be re-bedded much to often. |
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