The description on headspace for the AR-15 is written by Chris J. Pikus (firstname.lastname@example.org)
The AR-15 is the semi-auto civilian version of the military M16 battle rifle. Since its introduction in the nineteen-sixties, this rifle has been issued to several armies and is still the standard issue weapon of the U.S. Army and the U.S. Marines. It fires a round known as 5.56 mm NATO or Winchester .223. This rifle was originally designed by Eugene Stoner when he worked for the Armalite Corporation in Southern California. The rifle went in to production at Colt's facility. The rifle and parts for it have been produced by many companies. During the Viet Nam era, it was even produced by the Hydrostatic division of General Motors. As this is a U.S. basically a U.S. military device, parts and documentation are freely available on the surplus market. The easy availability of parts and the simplicity of the gun make it a cheap and effective rifle. The gun, in all it's various configurations can be purchased ready made, or assembled at home with simple tools. People assemble their own rifles for several reasons. Most do it to save money. Some do it learn the mechanics of the rifle. Many do it because they have accumulated parts at gun shows and realise they are just a few parts short of a second rifle. Some build their own rifle to be able assemble a custom unit that has the kind of barrel and stock that they want. What follows is a list of documents and parts supplier that will enable anyone who has some mechanical skills to assemble, repair or replace parts on an AR-15. Legal Note: It is illegal for a civilian to own a full auto or select fire M16. The AR-15 is a semi-automatic only version. The difference between the two rifles is in the bolt, sear and safety mechanism. As a general rule the parts sold to civilians are the semi-auto bolt and safety. But tales have been told of M16 bolts ending up on the surplus market. Documents and Guides If you are planning to do any work on an AR-15, the best help is the Video available from Quality Parts. It costs $20.00 and shows techniques and tools. There are also manuals and guides available. Some are U.S. military documents and some civilian books. Below is a listing and a list of suppliers: From the Government there is the "Technical Manual". This covers repair, adjustment and use of special tools. Lots of people sell this one. Of course the Marines and the Army have different versions of the same book. Also there are M16 A1 versions out there as well as A2. Here are the government numbers for this one: U.S. Army TM 9-1005-319-23&P U.S. Air Force TO 11W3-5-5-42 U.S. Marine Corps TM 05538C-23&P/2 "Technical Manual Unit and Direct Support Maintenance Manual (Including Repair Parts and Special Tools List" This is available from Sierra Supply, Quality Parts, Tapco, Lone Star Ordinance, L.L. Baston and no doubt others. Note the numbers above may vary a digit or two depending on which edition you buy. The title will remain the same though. There is another book, which looks like it was a cut and paste job from a government manual. AR-15, M16 Assault Rifle Handbook. Edited by J. David MacFarland. Firepower Publications El Dorado AR 71730 Another Book is "The AR-15/M16 - A Practical Guide" by Duncan Long. Paladin Press, Boulder, Colorado. Below is a list of companies that should have what you need. Lone Star (800) 482-3733 (512) 681-9280 L.L. Baston (800) 643-1564 (501) 863-5659 Sherwood International (818) 349-7600 (800) 423-5237 Quality Parts (207) 892-2005 (800) 998-7928 TAPCO (800) 359-6195 Sierra Supply (303) 259-1822 Sierra Supply has the best selection of books. For history etc, read: "The Black Rifle", Jane's Infantry Weapons, and Small Arms of the World. What To Build There are several variations of the AR-15. The two greatest variations are the A1 and A2 types. The A1 has a triangular handguard and no elevation adjustment on the rear sight. These are the guns you will see pictures of in books about Viet Nam. The A2 version has a round handguard and an elevation wheel on the rear sight. There are other differences between the two guns such as stock length. But these are the differences which can be seen from a distance. The pistol grip is also different between the A1 and A2 versions. Reading the books mentioned above will give more detail on the differences. The barrels are available in several lengths and twists. There is also the HBAR - This is a "Heavy Barrel", which means it is less prone to bending. The twists available are 1:7, 1:9 and 1:12. The 1:7 is the current military standard and is designed for the heavier bullet. The 1:12 is the original twist and was designed for the 55 grain bullet. The 1:9 twist is a compromise between the two. The other barrel option is chrome plating. Accuracy buffs will say that a plated barrel is not as accurate as a regular steel barrel as plating is never uniform. The advantage of chrome plating is a more rugged barrel that is resistant to rust and wear. Barrel lengths available are: 10, 11.5, 14, 16, 20, 24 and 26 Inches. The legal minimum for a rifle barrel is 16 Ins, so if you buy a barrel shorter than 16 Ins, it needs to have a long flash suppressor brazed on to it so it meet the 16 Ins minimum required by U.S. law. All of these options are explained in the various catalogs of the companies supplying parts. The standard stock is the nylon rifle stock with a trap door in the butt. The other stock is the Telescoping butt stock. This stock with a 16 Ins barrel makes a nice carbine often referred to as a "shorty" or CAR-15. There is also an option on the pistol grip. Lone Star Ordinance sells a pistol grip with a trap door. This grip is available in A1 or A2 styles. Looking over the catalogues (see end of this article for a list) will give you an idea of what you may want and what it will cost. Then you may order the parts. The Lower Receiver The part that has the serial number on it is the Lower Receiver. This is the only part that can not be bought without paperwork via mail or at gun shows. To purchase a lower receiver, you need to be a Licensed firearms dealer (FFL) or carry out your transaction via an FFL. There are some FFLs on the net. Also Shotgun News will send you a list of FFLs if you send them a dollar and an S.A.S.E. You can make as many barrel/upper receiver assemblies as you like and swap the lower receiver between them. This can give you several rifles. The top and bottom parts of all AR-15s will break apart and are interchangeable. Special Tools You can assemble an AR-15 with a well stocked tool box and some patience. But to do the job well with the minimum of frustration, you should use the special tools. You can buy your own tools or you can borrow tools from someone who has already assembled a rifle. Remember, if you borrow tools, you have to return them immediately. Having someone nearby who has assembled a rifle is helpful. Here is a list of tools: Drift Punches Barrel Wrench (Handy for all AR-15 Owners) Vice Jaw Blocks (Aluminium) (You can make out wood) Telestock Wrench (Handy for assembling the Telestock) Sight Adjustment Tool (For adjusting the sights) Headspace Gauges (For checking Headspace) Sleigh Punches (No one I know sells these - make your own) You can buy kits from Quality Parts, Lone Star Ordinance and others. At a minimum, you need to buy the Headspace gauges and the barrel wrench. Everything else a well stocked workshop will have or will be able to make. The sleigh punches are shown on the Quality Parts video. They can be made out of brass rod with the aid of a drill press. They are used to push in the roll pins. The AR-15 is pretty much held together with roll pins. You can put the roll pins in with a pair of needle nose vice grips and a small ball peen hammer. After you have put two in the hard way, you will take a break and make the sleigh punches. Besides the special sleigh punches, you can use nipple punches, they will work, but are still not as good as the sleigh punches. Getting the front detent pin in the lower receiver can be a trial. This is a pin and a spring that holds the front push pin in place. Brownells sell a special tool for this. You can use a knife blade to do this. It is the most tricky part of the whole assembly. Putting together an AR-15 pretty much consists of pushing in roll pins and inserting compressed coil springs. These are fiddly parts that can fly across the room, only to be found when they get eaten by pets or jam the vacuum cleaner. It is a good plan to place a white bedsheet under and around the workbench when assembling. You should wear safety glasses when building the rifle so a spring will not head damage your eyes. If you loose a thirty cent spring, you will have to put the whole assembly on hold while you wait for the part to arrive. Note also that all the "fiddly" parts are not marked, numbered or otherwise identified. You will have to identify them yourself during assembly. It is a good idea to tip the parts onto a white dinner plate or into a white plastic egg tray. The egg tray will allow you to separate the parts into springs, pins etc. Quality Checks on a Completed Rifle When you have finally assembled the rifle, the next step is to check your work. Remember you are assembling a piece of machinery that could be dangerous if it malfunctions. Go over the rifle and make sure that all parts are correct and tight. Check assemblies with the drawing supplied with the Technical Manuals. Before you fire the rifle, check the headspace using a set of headspace gauges. The correct use of headspace gauges is explained in the Quality Parts video. If your rifle is not headspaced correctly, contact the company that supplied your barrel and bolt, or take it to competent gun smith who knows what to do. If you know how to adjust the headspace, you don't need to read this document. But should you be doing so, Quality Parts sells reamers for the AR-15. Note that the headspace check is mostly a safety check, chances are everything will be OK. At the end of this description is a description the purpose and process of headspace checking. Military barrels are chrome plated, it is believed that reaming a these barrels is not possible because of the risk of flaking chrome. If you have any doubts or questions, call the supplier of your barrel and bolt. The rifle should be cleaned before firing. The barrel is liable to have a coating of heavy grease inside it. This can be removed with solvents such as kersosine or mineral spirits (Turpentine substitute - paint thinner) and a proprietary gun cleaner such as Hoppes. Pass patches through the gun until they come out clean. If all the visual tests pass, then it is a good idea to load a couple of dummy rounds (Snap Caps) into a magazine and manually feed them through the rifle using the charging handle to feed them. Check that the safety works - it will only engage when the rifle is cocked - and that the rounds are ejected when the bolt travels back. The final test is at a range. Pointing the gun at a solid backstop, a target is not needed yet as you are not adjusting the sights, load one round in a magazine. Insert the magazine into the rifle and feed it into the chamber. Pull the trigger and make sure that the rifle fires and ejects the shell. With only one round in the chamber, the bolt should be held back by the follower in the magazine. Now load two rounds and repeat. This test ensures that the rifle is feeding and firing properly. Finally insert 5 rounds. Take careful note that each time the trigger is pulled only one round fires. With a new rifle, there may be some ejection problems. These can be caused by a rough and sticky chamber or bad lips on the bolt face. Check by carefully hand cycling dummy rounds. If the return spring is not strong enough, the rifle will cycle by hand, but will tend to fail to feed in semi-auto use. Try another return spring. To sight in the rifle and learn how to clean and care for it, you should read the manual: U.S. Marine Corps TM 05538C-10/1A U.S. Army TM9-1005-319-10 Rifle, 5.56.mm M16A2 W/E (1005-01-128-9936) "Operator's Manual W/Components List" This is available from Sierra Supply. Parts Suppliers Below is a list of companies that sell parts for AR-15s. Some of these companies also sell whole guns. Don't forget also that gun shows will also have parts. In no particular order, here is a list of suppliers. I have called all of them. Amherst Arms (301) 829-9544 DPMS (800) 578-3767 Eagle Arms Inc (309) 799-5619 Essential Arms Co (318) 566-2230 Gun Parts (Numrich Arms) (914) 679-2417 L&G Weaponry (714) 840-3772 L.L. Baston (800) 643-1564 (501) 863-5659 Lone Star Ordinance (800) 482-3733 (512) 681-9280 Nesard (708) 381-7629 Olympic Arms (206) 459-7940 Pac West Arms (206) 438-3983 Quality Parts (207) 892-2005 (800) 998-7928 Rock Island (309) 944-5739 Sarco Inc (908) 647-3800 Sherwood International (818) 349-7600 (800) 423-5237 Brownells also sell special tools for AR-15s (515) 623-5401 Headspacing The description on headspace for the AR-15 is written by email@example.com (Chris J. Pikus) Headspace is that measurement describing the size of the chamber on a barrel. In the case of rimless rifle cartridges, it is the distance from some arbitrary point on the case neck taper back to the bolt face. Here is a crude ascii drawing. _______________________ | \ | \_____ | |) | _____ | / |_______________________/ | <--------------------> | headspace In the more general case (e.g. pistol, rimfire, rimmed cases, and belted magnums), you can imagine headspace as measured from whatever the cartridge rests against on the front of the chamber all the way back to the bolt face. These numbers are specified for each cartridge by the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) and it's important to have this measurement fall within SAAMI specified tolerances. Too small, and you may not be able to close the bolt on some ammunition that's on the large side of its tolerance range. Worse yet, it will close and let you fire. Firing ammunition in a chamber that's too tight leads to dangerously high pressures. Too large a chamber, and there's lots of room for the cartridge to rattle around in there -- well not really. What happens is that the brass casing can rupture and 55,000 psi hot gases start rushing out in every which direction, including at your precious body parts. Think of the high pressure gases that force a bullet down the barrel finding other avenues of escape. This is what some people mean by "the gun blew up on me". One sign that this is going to happen soon is that the backs of your brass casings start looking like someone took a sledge hammer to them and flattened out the primers and lettering. This is because the casing is literally being hammered against the bolt face. So that's headspace, and why it's important. But what are all these gauges about? The look like little steel cartridges without the bullets. You stick them into the chamber and try to close the bolt. Whether the bolt closes or not -- and whether that's good or bad -- depends upon which gauge you're trying to use. The Go gauge measures whether it's too small. It's made just a teeny bit smaller than the minimum requires chamber size. If you cannot get the bolt to close on the "go" gauge, then the chamber is too small. Thus, if you can close the bolt, it's a "GO" as far as chamber size is concerned. The No-Go measures whether the chamber is too large. It's made just a bit larger than the factory tolerance chamber size. You're not supposed to be able to close the bolt on these; if you can, then the chamber is too large. Thus, if you can close the bolt, it's a "No-Go" on the barrel/ bolt combination. So much for the GO and NO-GO gauges: what about the "Field" gauge? Well, the GO/NO-GO measure the SAAMI specs for *factory* tolerances for newly chambered rifles. The field gauge checks the end-of-life tolerance for a chamber. If you're familiar with manufacturing machinery, you may be aware of two sets of tolerance specs: the factory tolerance for when something is new, and a more generous end-of-life spec for when to throw something away. So is the case here. Fortunately, since the wear mechanism for a firearm chamber is to stretch, we only have to worry about the dimension beyond the NO-GO mea- surement; which is our "field" gauge. The field gauge is sized to be the largest acceptable headspace possible. If you can close the bolt on the field gauge, then the barrel is worn out and it's time to replace it. To continue shooting it is dangerous. In theory, the bolt of a well used rifle will close on a NO-GO gauge too. BUT ABSOLUTELY, UNDER NO CIRCUMSTANCES SHOULD A BOLT CLOSE ON A "FIELD" GAUGE. That's when you take the gun back for rebarrelling. Personally, when I've bought really old guns, they closed on the NO-GO gauge, but not the field gauge. That's acceptable for me. I know not to shoot really hot ammo through it, and that the accuracy suffers because of the large chamber. I keep them for the historical value, but I could rebarrel them back to factory size if I wanted to. Since you've got a new gun (with less than 10,000 rounds through it), I would suggest you buy both a GO and a NO-GO gauge. Play it conservative, check the headspace every few thousand rounds and don't go beyond the NO-GO. As you know, most people don't even bother with that but I like to play it real safe. Basic rules.... For a new gun: a. Closes on the go gauge b. doesn't close on the no-go gauge For an old rifle that's been rode hard and put up wet: a. closes on the go gauge b. probably closes on the no-go gauge c. definitely does NOT close on the field gauge It's cheap insurance to have a set around for every rifle you have, especially if you shoot a lot. Plus if you change around bolts and/or barrels like you plan to you have to check each combination because each setup could vary a few thousandths between them. (fortunately you only have to do it when you first introduce them to each other. And every few thousand rounds if you wish.) You can get your very own set by calling Brownells at 515-623-5401 and ordering at least the GO (part #319-223-464), and the NO-GO (#319-223-467) for $16.00 each. The field gauge you can buy later if you wish (#319-223-470). Don't forget to ask for their catalog; it's an education all by itself. Headspace gauges are also sold by Quality Parts.