Today I decided to make the venture into building a precision AR platform in 556. Larue is/was having a fantastic sale on their precision barrels, so I picked up a 16" stealth barrel for only $199, stripped (these retail at $400!).
Ive built some basic ARs before, but not one geared toward precision. I think im going to pick up at Vltor MUR upper, unless anyone has a different suggestion. My question is, besides ammo selection, is there anything else I need to be concerned about when building a precision rig? What, if any, affect does bolt and carrier selection have on accuracy? What do you prefer to torque your barrels too, and are there any special steps you take? I remember reading somewhere that people sometimes coat the barrel extension with loctite (or maybe i’m dreaming/mis-remembering).
I read a pretty good article in a magazine awhile back about “piv”, or " pre-ignition vibration, from the swing of the hammer and how the rotating mass being at the top of the hammer effects accuracy. Might be something to consider.
Molon has some good advice in his timely post on general set up for precison.
For torque on the barrel nut the general consensus around here is that lower torque on the barrel nut is better than too much, so you probably want too keep it in the 30-40 ft-lbs range for best accuracy. If you are using a muzzle device, I would also go with a minimal torque on that as well to keep the stress on the crown to a minimum. 8-10 ft-lbs should be plenty to hold it on without stressing the muzzle too much. I’ve seen others recommend going just past hand or wrench tight. Less is likely more in these two cases.
Loctite, bedding, or gluing the barrel to the receiver? I’m not a fan of it, and really it comes down to the barrel extension to receiver fit anyway. For a nice tight fit, ideally the receiver will require a little heat to expand its’ ID on the threaded end so that the barrel extension will just fit in and be tight once cooled down. On my own builds as long as the barrel ext. isn’t like a hotdog in a hallway in the receiver, then I am fine with it. Usually with my Vltor MUR uppers the barrel is a pretty tight fit that takes a bit of pressure to go in after I lightly oil both pieces. The newer BCM upper receivers are supposed to be a little bit tigher than normal in this area and may require the heat method and result in a nice tight barrel to upper fit.
Now for the bits that will cost you, and may or may not yield noticeable results for you.
BCG selection? The carrier shouldn’t make much of a difference as long as it is of good quality and isn’t binding with the gas tube. As for the bolt? Well your barrel is pre-chambered, so there is no option to have it chambered/headspaced to a specific bolt. You may want to check the headspace with a set of go/no-go guages to make sure that you are in acceptable headspace range. Usually if it is a new bolt form a quality mfg. then is should headspace just fine.
An adjustable gas block may help your accuracy. Adjusting the block to a minimum should slow down the operation cycle and delay bolt unlocking which is generally viewed as beneficial to accuracy. Another benefit to the adj. gas block is that you can shut the gas all the way off and fire it single shot. Shots fired in this way should give you brass that is consistently fire formed to your chamber and another way to check your headspace IF you reload your ammunition.
The Vltor A5 buffer system may help you for several reasons that all boil down to delayed bolt unlocking.
Receiver face truing. Some folks swear by this and I believe Brownells sells the tool/fixture for it. I think most folks around here do not feel it makes much difference if starting with a high quality upper receiver, but I thought I’d mention it for the sake of completeness.
As far as lapping the receiver face. It can’t hurt. I usually lap those that I have trouble timing the gas tube on. By lapping the receiver face you can control your barrel nut torque very easily.
I would only lap/true/square the receiver face if you really want it dead nuts square. There is no sense in doing it otherwise. If you are having trouble timing a barrel nut to fit the gas tube, then there are shims available for the barrel nut that are much cheaper and faster to use. Better yet would be to use a freefloat handguard that does not require any timing, just torque and go.
Regarding free float barrel, ammo, trigger, optics: that is more or less what Molon said in his linked post. Build it to spec? Who’s spec? Kinda meaningless without more context. He already has a barrel and is looking for tips on assembly.
Thanks for your advice. What do you think about running an LMT enhanced carrier. I thought I remembered hearing that also delays bolt unlocking?
Thanks, but I already bought a barrel - Larue Stealth. Barrel, optics, trigger, ammo… these are things I know. I’m looking to see if there are other variables I’m not aware of, and, how people build them.
For someone looking for a great setup out of the box, that BCM upper is a great option. I want to do this myself.
Then I’d get a rail that doesn’t require “timing” it to the gas tube so you can use the minimum torque, 30 ft-lbs like any Geissele or BCM rail and shoot it and not worry about the rest of it.
Thanks - this is something I had not considered. I had planned on running a Centurion CMR, not sure if that requires timing or not. I had also considered the BCM KMR, but I get a little nervous with these super light rails, as I am a big guy and rough on equipment. I could see myself applying a lot of force to a bipod during certain unconventional shooting positions.
If you are using a good adjustable gas block then the LMT Enhanced-carrier is totally unnecessary. If you are not using an adj. gas block then it may or may not benefit you. I believe it was originally intended for 14.5" carbine gassed barrels to do as you say, delay the unlocking. on a 16" mid barrel, maybe not much help. I’m no expert on it, but forum member here ‘BufordTJustice’ and other have a bit of experience with the E-carrier.
No timing required with the CMR. Keep in mind that the accessory rails for the CMR are plastic unless you hunt down another mfg. aluminum rails with the same hole spacing. The CMR rail also uses a mix of steel helicoil inserts and plain threaded aluminum to attach any rail accessory. Only a few low pro gas blocks fit inside of it without modification. Keep that in mind when weighing it against the KMR.
Thanks. Though I am running a mix of suppressed and unsuppressed, I have no desire to run an adjustable gas block at this time. I’ll likely try an enhanced carrier, and if I see little benefit I’ll move it over to my Mk18.
Well, I dont like the sounds of that very much. Thanks for pointing this out. Do you think the KMR is durable enough for hard use with a bipod?
Suppressed? All the MORE reason to use an adjustable gas block. I’d go either SLR Rifleworks, or Syrac Ordnance. Once you figure out your settings, it’s easy to go from your suppressed to non-suppressed setting with the SLR Sentry block (haven’t tried the Syrac), and you will have one of the smoothest suppressed builds possible. SLR has actually upgraded their adjustment screw since I received my Sentry, and should be even easier to use.
Which bipod, and how are you planning on mounting it? If it simply clamps to a picatinny rail like an Atlas bipod, then simply add a BCM, or better yet, a V7 Weapon Systems keymod picatinny rail section to your KMR. If it is a standard Harris style mount, there are a couple direct keymod mounts out there (IWC, and one other), and a thread in the General, or Techinal forums about mounting that way. Either way if you follow the mfg. instructions you should be good to go.
I dont disagree an adjustable gas block makes sense for most people. This rifle may see use in a duty role, and im a big, big proponent of simplicity rules. I like to be able to deploy a weapon with the least amount of steps possible, and an adjustable gas block is one more variable I dont want.
I’ll likely go with an Atlas. I’d read about people deforming their keymod rails with hard use, but maybe that was with improper mounting. I think this is why I was a little hesitant with the KMR route.
While I think the KMR is a superb “gaming & range” handguard, hanging equipment off of it and especially loading a bipod with it, there’s no way I would declare it “Duty” or “Hard Use”. The military turned down keymod FWIW. Not very confidence inspiring when you see the KMR Keymod Slots getting deformed/gouged/crushed from simply torquing the fasteners on things like QD Sling Mounts, Rail Sections, VFGs etc. Nor did I like the fact that bolt a Pic. Rail Section, to be used ICW an Atlas Bipod, was right under the BCM Gas Block, actually making contact with it and pressing it upward… had to wind up moving the Rail Section forward of the gas block so it wouldn’t touch.
Not trying to really ruffle any feathers, but this is my honest experience. I’d stay Picatinny or go M-LOK.
While I think it’s great as a standalone tubular (ergonomic & lightweight) handguard, I don’t think it’s much else. Just been my experience with 3 different KMRs so far.
I was like you, at first, with the Adjustable Gas Block thing… in fact, I actually was super disapointed on my first range outing, with an SLR AGB when the Metering Screw broke on the very first range trip… of course the screw broke on setting 1, so my Gas was essentially choked off, making it a one-shot wonder.
SLR took phenomenal care of me and actually rushed out a new updated Gas Block w/ the much improved and stronger Metering Screws… and since then I’ve put roughly 1,800 Flawless rounds thru her.
Adjustables Gas Blocks are by no means a “must”, but there sure are nice to have. I get what you’re saying though… nothing wrong with a Pinned, non-adjustable Gas Block.
But anywho, the new latest SLR Gas Blocks are super durable… I am no longer hesitant like I was in the beginning.
Thanks for your honest assessment. This was sort of my gut feeling as well, though i’ve had not experience with a KMR other than handling it.
I think the only adjustable GB i’d consider is the Noveske Switchblock, as I can physically and visually check the gas setting in low light/no light and with no tools. Unfortunately, they seem to be impossible to find by themselves, and I cant justify the cost of a complete switchblock upper.