08-01-2020, 10:27 AM
drewthebrave Wrote:If you want to ensure a tight fit, you may want to consider using a BCM upper receiver. They intentionally size the receiver on the small side of the tolerances so that you have to heat it up to fit the barrel extension. I bought their blemished receivers at $60 and used them for my 11.5" 5.56 pistol and 18" 6.5 Grendel builds. For the first one, I held the threads over the stove, and on the second I used a heat-gun - both methods worked just fine.
However, you may have trouble truing the face, as the PTG facing tool that I have wouldn't fit in either of the BCM uppers I have. If you want a true face that's a thermal fit without buying extra tools, you might want to look at JP Enterprises or Precision Firearms billet receivers. They're pricey, but they have a good reputation.
This.
I was waiting for a while, unsuccessfully for BCM uppers to come back in stock. Then I found someone on arf.com who was unloading a 20" Lilja Grendel w/ GB, tube, bolt and he had an extra (used) BCM upper. I bought them. Before it shipped, he offered to lap the face of the receiver. I said I had the Brownells tool, he said he did too, but it doesn't fit, so I had him do it.
I froze the barrel overnight and used a heatgun on the upper for about 10 seconds, it slid in smooth on the first attempt.