Bonas Wrote:It's not about the cost of shims. It's about the fit between the barrel extension flange and the receiver. I wrap extensions with stainless stock to achieve a press fit between the extension and the receiver tube. From tearing down uppers and reassembling them, I find this improves accuracy by about .25 moa. I don't care what the barrel nut looks like under the handgaurd. Based on my experience, I would not want a shim between the receiver face and the extension flange, if I could avoid it. I would rather cut off a tooth. In fact, I prefer handguards with nuts that don't have to be timed for this reason. For what it's worth, I have rifles that have shims between the nut and the receiver, but I prefer otherwise. YMMV.
The shim does not go between the upper's receiver face and barrel flange, it goes between the barrel flange and barrel nut..This is why it is important to grease the front of the barrel flange and shims..
Putting the shim between the upper receiver and barrel flange can change the lock up timing/indexing, gas tube depth into the gas key and create other problems..
I assumed this was common knowledge. This is what I get for assuming things..