Does a loose connection between the upper and lower receivers of an AR-15 affect the rifle’s precision? Can tightening up the rifle decrease group sizes? These are controversial questions, and I am going to test the premise.
My Rock River Arms LAR-15 is no spring chicken. The once-tight connection between the upper and lower receivers now audibly rattles and wobbles. I can see light through the gap.
There are two schools of thought on this matter: one school says that consistent accuracy is reliant upon all elements being tightly fitted; the other says that there is no significant movement of the upper receiver before the bullet exits the muzzle. I suspect that the “tight” school is right. When I put together a long-range precision bolt-action rifle, I take great care to ensure that the action is perfectly mated to the furniture. I want no movement between any of the rifle’s components.
The X-Shim is a set of a four color-coded plates that fit over the lugs on an AR-15 or AR-10. They bridge the gap between the receivers, reducing or removing wobble. I will be using shim #4 at the back, and shims #3 and #4 at the front.
This cursory test compares two 5-round groups without the X-Shims and two 5-round groups with the shims. Without the shims in place, my group sizes were 1.700″ and 1.925″. The X-Shims reduced group sizes to 1.575″ and 1.250″. The average group size reduction was a whopping 0.400″. Not too shabby. This doesn’t PROVE the concept; these were only four groups fired by one guy. But it is some interesting evidence. I plan to continue testing the X-Shim to see how the rifle performs.
Here is a link to the product: http://varmintlight.com/x-shim-ar-15-m16/
A competing product is the Accu-Wedge. It performs a similar function, but only affects the rear lug.
– The Social Regressive