12-20-2013, 10:51 PM
For heavier barrels with bull profiles, it makes a lot of sense to use a billet upper with more beef to it. The VLTOR MUR is a great upper, as are the ones mentioned like Seekins, JP, Mega, and Precision Firearms.
If you are about to order your Grendel bolt from Precision, seriously consider using one of his billet receiver sets. They are actually Mark's own design, with some really nice touches in terms of funneling to the magwell, mate-up, and aesthetics. He also is a big supporter of this cartridge with both rifles and ammunition, and makes extremely accurate guns as those of you who own them can attest.
If you have the opportunity to buy an AA upper receiver, they are already trued and anodized, with a tight extension tunnel. There's a 16" AA carbine for sale here on the Forum as we speak.
Blue Loc-tite is one common method of bedding the extension to a somewhat loose tunnel, after you have trued the receiver face. Just keep the Loc-tite off the threads after you apply it to the OD of the extension, and clean up well. Heat from a heat gun on low is all you need to remove it at a later date.
Another thing I do with uppers that tend to be loose is Cerakote the inside, then lap the carrier raceways so the carrier doesn't gall with the Cerakote during cycling. The Nickel Boron BCG's work really well in this regard. Some guns with over-sized "match" bolt carriers can and have been known to run sluggishly in the past, so you do have to be mindful of what it takes to make an AR15 run well when you start talking about changing the ID with coatings.
If you are about to order your Grendel bolt from Precision, seriously consider using one of his billet receiver sets. They are actually Mark's own design, with some really nice touches in terms of funneling to the magwell, mate-up, and aesthetics. He also is a big supporter of this cartridge with both rifles and ammunition, and makes extremely accurate guns as those of you who own them can attest.
If you have the opportunity to buy an AA upper receiver, they are already trued and anodized, with a tight extension tunnel. There's a 16" AA carbine for sale here on the Forum as we speak.
Blue Loc-tite is one common method of bedding the extension to a somewhat loose tunnel, after you have trued the receiver face. Just keep the Loc-tite off the threads after you apply it to the OD of the extension, and clean up well. Heat from a heat gun on low is all you need to remove it at a later date.
Another thing I do with uppers that tend to be loose is Cerakote the inside, then lap the carrier raceways so the carrier doesn't gall with the Cerakote during cycling. The Nickel Boron BCG's work really well in this regard. Some guns with over-sized "match" bolt carriers can and have been known to run sluggishly in the past, so you do have to be mindful of what it takes to make an AR15 run well when you start talking about changing the ID with coatings.