Shim AR behind front pin?

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gregj

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I was punching some holes in paper this am with my RRA AR-15, and was getting a little frustrated with the grouping. I then recalled reading someone had used a small piece of rubber behind the front pin to tighten the connection between the upper and lower. I fashioned a shim out of a small piece of paper and tried it. It seemed to tighted my groups up quite a bit.

Anyone heard of or tried this? The one really odd thing is it dropped my zero about 4" :confused:

My 5 shot groups before the shim were about .6", after about .5" @100 yds
RRA 20" Predator Pursuit/223Rem 1:8
RRA two stage NM trigger
Shooting from a bench with bags front and rear
Millet 4-16x50 optic
69gr SMK HPBT-Match, 2.260 OAL, 22.5g of H335
 
Those were little rubberized "L" shaped bits that went behind and underneath the rear pin. It's always arguable as to whether they really improve the accuracy of the firearm as the entirety of the sighting and post-feed round control is isolated in the upper receiver. It does have the potential to increase the firer's control of the upper and consequently increase the resulting shooting but of course that's only a "potential" issue. If you're shooting better by tightening the receiver interface - well, then it works for you.

I wouldn't tighten at the front though as the holes are quite specific whereas in the rear you'll see the elongated hole in the upper receiver to accommodate variations in uppers and lowers.
 
I had shot several groups prior, and 2 groups after, and I was also shooting at the dot target from last winter's centerfire match, and had my best results on that target. I realize I need to do more testing. Guess I've got another excuse to head back to the range. :D
 
ApacheCoTodd, good advice on the rear pin, Thanks!

Any ideas why it would cause my zero would drop 4" ?
 
Shooting style. Necessarily, at least one of your hands is always on the half of the rifle that can be wigglin' about in relation to the upper. Some folk shoot in such a way as to not allow the jiggle, some folk shoot in such a way as to let it dance and yet not mattering and some shooters will allow the movement and need it to be eliminated.

'Course, there's also myriad positions to throw into the mix. It didn't really drop 4" unless everything was equal. You might just plain feel better about the rifle all taughted up and are setting up a slightly different sight alignment.
 
I just wanted to mention that the trigger group is contained in the lower and anything that changes the way a primer is stricken can change POI.
 
I'll be ordering or picking up an accu-wedge soon. I'll also try some groups with and without it, just to see the difference. Thanks
 
Just be certain to buy an authentic accu-wedge as most of the knock-offs are too hard and require too much pressure to install in some receiver combinations. Remember that too much tension in closing will result in the aluminum at the receiver holes suffering from the steel of the receiver pins. if you buy an accu-wedge, always get 2 as it's a good idea when the fit is too tight, to trim the wedge a bit to not over stress things. The second one is if you get a little nuts in the trimming of the first.
 
I have just put an oring around the entire rear lug to tighten down my uppers to my lowers, if I really needed it.
 
The bolt, firing pin, cartridge, chamber, front & rear sights are all in the upper receiver. You could hit the primer with a hammer and nail and it will not affect the accuracy.
 
I drilled a hole in the lower receiver under the take down pin, tapped it and put a neoprene set screw against the take down pin with a little pressure. Then I locked the set screw with another behind it like a jamb nut. Kinda' copied it from an Adax lower I read about. It took me about a half hour and two bucks. Works great!
 
Stack said:
I drilled a hole in the lower receiver under the take down pin, tapped it and put a neoprene set screw against the take down pin with a little pressure. Then I locked the set screw with another behind it like a jamb nut. Kinda' copied it from an Adax lower I read about. It took me about a half hour and two bucks. Works great!
Click to expand...
This is a very worth while mod, you can make any upper fit as tight as you like.
 
Can someone provide a link for this mod? I havent been able to find anything as of yet. TIA
 
A lot of the AR guys say to just stick a foam earplug in there instead of spending money on an accu-wedge. I do know that a couple of little strips of masking tape about 1 1/2" long on top of the lower will stop the wobble.
 
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