10-12-2019, 01:03 PM
Tm1293 Wrote:Lapping is a good start to improve accuracy as bore alignment to the bolt face is more crucial than most realize.
HOWEVER, what most dont realize it that no matter how much you tighten the barrel nut to a perfectly square receiver face, if the barrel extension isnt a perfect (down to as much as .001in) fit to the receiver's barrel port, there WILL be movement and degradation of accuracy. No barrel extension mass produced is the exact size and fitment to any receivers barrel port. Check it with a micrometer next time.
In the many builds I have done for match and varmint, the best and easiest method of accuracy gain (while building the upper) comes by shimming the barrel extension into the receiver. Brownells, Midway, etc sell sheets of shim metal ranging from .001 to .05 thickness.
Start with a sliver about 2mm wide and the length of the extension from the lip (where it fits the receiver face) to the edge where it is just past the inside of the receiver. Insert the barrel with the shim attached to the extension (preferably at the top side away from feeding ramps) and take a small bit off the width of the shim (2mm and slowly work down to 1mm wide) until it fits. When it is a proper fit, you will have to tap the barrel in with a deadblow (thread a junk A2 on the end to protect the crown) and have a decently hard time removing the barrel. I have commonly used green loctite (press fit, works like a tacky glue) on the bearing surface of the extension to reduce any chance of movement further. Careful to not get it inside the action or lug area. Then just tighten barrel nut to factory torque specs.
I have made m16-a2 Vietnam era rifles shoot sub-moa with this trick.
I use the shim stock myself. But I cover the whole extention and then do a freeze and heat method. So when I install the barrel with shim it's a very tight compression fit.