Originally posted by mdram
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SAAMI reamer has a .300" neck, .264 LBC-AR has a .295" neck. The only way you're going to see excessive pressure with a new chamber is if it is cut below SAAMI minimum chamber, assuming safe factory ammo. How I've managed to land on safe SAAMI chambers one after another all these years with several different manufacturers must just be a continually coincident occurrence I guess.
I have come across several chambers that were cut with some other reamer, and wouldn't allow chambering of a new factory cartridge. I finish-reamed them with a Manson SAAMI reamer that was checked with optical comparator and they were then fine.
If you are only using the rifle for accuracy and don't care about ever shooting steel case through it, then a genuine Les Baer .264 LBC-AR spec chamber is a great chamber, as long as it it held to Les's specs, which he controlled. The big thing here is how much freebore there is with the bullet you're using.
If you want a Bartlein pipe, I would also talk to Precision Firearms. The PF Bartlein Grendel barrels I've tested have been some of the most accurate AR15s I've ever laid hands on, right there with AA-chambered Satern cut rifled, Krieger, and bull profile Lilja with SAAMI chamber.
I've shot 5rd and even 9rd groups rapid with various 6.5 Grendels with top-end pipes on them, with group sizes measured in the .3s to .6s. With one particular PF Bart piped upper, I lost count of how many rounds kept going through the same hole at 100yds and had to look at my cartridge box. Several 5rd rows had been expended. I just say 9 rounds because people would have a hard time believing it. I offered to buy that upper from the customer who it was for, but they refused.
The fastest shipped 20" top-end barrel you will get is from Lilja, the AR740. They have a lighter medium profile under the handguard at .750", RLGS, immaculately-smooth bores. They usually ship the day you order from them. I've built at least 5 different uppers with that barrel, and they make for a surprisingly-lightweight rifle if you use a light handguard, maybe a bit light for a bench gun with a Luth-AR stock, but perfect for a hunter.
For a target gun, a .800" or heavier profile under the handguard on a 20" will tighten you up from a 1 MOA rifle to .75 MOA or less with good ammo in my experience. I think the most accurate ammo I've fired from them was PF 123gr SMK through PF Bartlein.
I've also seen and shot rapid-fired .3" to .6" 5rd groups through the Lilja bull barrels with flutes. I've seen 7rd 107gr/8208XBR groups from 20" AA/Satern with SAAMI chamber in the .2s.
If it were me, I would be leaning to the PF Bart pipe 20" with SAAMI chamber if you want to cut Xs and 10s at 600yds. Square your upper face and bed it, or have PF just do the whole upper for you. You might have a loose-fitting extension tunnel that won't do you any favors unless you shim and bed it.
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