Here are some of my thoughts on putting together an accurate AR upper, all based off building up and test firing many hundreds of accuracy based uppers. True consistent accuracy is the sum total of a lot of details. This post does not address accurate ammo, which is a whole separate item. I would say this encompasses most of the big areas, but there clearly are more beyond this, but other that writing a book on the subject this will get you most of the way there.
There are a lot of things that can be done to an AR to enhance consistent accuracy, and I use the words "consistent accuracy" because consistency is a key part of it (i.e. plenty of guns will give a couple great five shot groups here or there, or with a break in between, but won't do very good 10 or 20 shot groups, and some guns will shoot great one day and not so good on others).
Some items we think are important:
1. A premium barrel, well machined with a good crown and a match type chambering, true to the bore and well cut, with the extension threads also cut true to the bore, and everything true and in proper alignment.
2. A gas block that does not impose pointed stress on the barrel (i.e. clamp on types that grab all the way around the barrel are excellent). The blocks that are pinned on with tapered pins that wedge against the barrel or the slip on type of block with set screws that push up from underneath (or directly on the barrel) can deform the bore inside of the barrel and can wreck the accuracy of an otherwise great barrel.
3. A heavy walled rigid upper receiver. The typical AR upper receiver was made for a lightweight carry rifle and they stripped all the metal they could off it to make it light to carry around all day (i.e. advantageous in military setting). The net result are upper receivers that are so thin you can flex them with your bare hands - they're strong but flexible strong - but not ideal for accuracy which does better with a more rigid upper receiver.
4. Truing the face of the receiver. Some may argue this point but it is always best to keep everything related to the barrel and the bore in complete alignment with the bore (i.e. barrel extension, bolt, upper receiver, carrier, etc.).
5. Loctite or glue in the barrel extension into upper receiver. This holds it in place all the way front to back in the upper receiver, otherwise if there is any play (and there typically is) it just hangs on the face of the upper receiver completely dependent on the face of the upper receiver as the sole source of support for the barrel as opposed to being made more an integral part of the upper receiver by being glued in.
6. A rigid free float hand guard (and I emphasize the word rigid). There are many free float hand guards and a free float hand guard is in and of itself is a huge improvement over a non free float set up, but best is a rigid set up. Some of the ones on the market are small diameter, thin and/or flexible and if you are shooting off any type of rest, bipod, front bag, etc., a rigid fore end is best since AR's want to jump, bounce and twist when you let a shot go, as the carrier starts to begin it's cycle before the bullet exits the bore.
7. Some meat on the barrel. Between the upper receiver and the gas block don't go real thin with a barrel (we like 1" diameter if it's workable weight wise). When you touch off a round and the bullet passes the gas port the gas system immediately starts pressuring up with a gas impulse that provides vibrations and stress on the barrel, especially between the gas block back to the receiver, a heavier barrel here dampens that. Correspondingly staying a little heavier with barrel contour through the gas block area and out to the muzzle is good for the same reasons. AR's have a lot going on when you touch off a round and the gas system pressures up and the carrier starts moving (all before the bullet exits the bore) so the more things are made a bit heavier and rigid to counteract that the better - up to a point of reasonableness of course).
8. On a gas system, the DI system is the way to go for accuracy (i.e. piston system is no good for accuracy - not even arguable as it provides mechanical moving parts that are moving and departing direct impulses and stresses on the barrel before the bullet exits the bore - not good).
9. A gas tube that runs freely through the barrel nut and through the front of the upper receiver and through the gas key in the carrier is not impinged by any of them so that it does not load the carrier in a stressed orientation, nor is it bound up so that when the gas tube pressures up it immediately wants to transmit more force and impulse to the barrel than would normally occur. We spend a lot of time sometimes taking the gas block with gas tube on and off "new build" uppers and "tweaking" gas tubes to get that best (and most gas tubes need a little "tweaking" to get them right - factory tubes go in and they work but they are typically not optimum without hand fitting).
10. Not over porting the gas port. Being over gassed makes the gas system pressure up earlier and more aggressively leading to more impulse, forces and vibration to the top end and barrel. Do what you need to function properly and adequately but no more.
11. Front/Back bolt play (keep it .003" but no more than .005"). If accuracy is the game, don't leave a lot of front back bolt play. We've seen factory rifles run .012" to .015", which is o.k. if you need to leave room for dirt, grunge, etc. like in a military setting out in the field, but not ideal for an accuracy build up. A lot of front/back bolt play makes it so rounds can be hammered into the chamber and actually re-sized and re-formed in a non-consistent way as they are loaded into the chamber.
12. Use good parts from a reputable source and watch out with "gun show" specials. All parts are not the same, some are good and some are not so good, and some after market parts are not only not good, they are bad. Don't be afraid to use a mil spec type carrier either, by and large they are excellent for an acccuracy build up, and just because a carrier says "national match" or something else on it does not necessarily mean it's any better. Be careful of chrome plated parts as the chrome plating can change the parts dimensionally and can also make it hard to do hand fitting and necessary stoning for fit and function.
13. A good upper/lower fit is helpful. For quick and dirty, an Accuwedge in the rear helps a lot. The ultimate is to bed the upper to a specific lower (with bedding stuck to the upper but release agent on the lower) so that the upper and lower, when together are more like one integral unit. For the upper receivers we have made for us we try to get the specs as close as we can, but still fit the mix of lowers out in the market place.
14. Don't screw up the muzzle (literally). Leave as much metal on the barrel at the muzzle as you can (i.e. stay away from 1/2" x 28 threads for an accuracy build and 5/8" x 24 threads are right on the edge, we like 3/4" threads for such attachments). People like to thread the muzzle for a flash suppressor, suppressor, muzzle brake, or some other attachment, but if you really want accuracy, leave as much metal as you can there and if you have something that screws on, set it up so that it can be put on and have it stay there without putting a lot of torque and stress on it right where the bullet exits the bore. If you are going to thread the end of the barrel, make it concentric with the bore and make sure what you screw on there is as well, and that the through holes where the bullet passes through are dead true to the bore (a lot of after market screw on things are not so good that way). Anything that vents gas should vent in a bilateral manner (i.e. if it vents left, it should vent equally right, and the for same up and down). Uneven venting of gas can wreck accuracy.
There are some more items on things like ammo, some tweaks for the lower, chamber design, etc., but time is short for me now and this post encompasses most of the big ones.
Robert Whitley
www.6mmAR.com
