Here are some hints on things I ran into building AR-15s that are not covered in the FAQ. Jamming problems: There are certain things to look out for here. Many people prefer to buy only Colt bolts, or GI issue parts and are afraid of other aftermarket parts such as offered by Nesard and others. I believe that if you carefully inspect some critical parts you can build a parts gun that is as reliable as any. Jamming can occur from short cycling, poor extraction or ejection. Binding bolt carrier: First check for binding in the bolt carrier. With the charging handle in place and the upper detached, slide the bolt carrier in and out without chambering any rounds. The carrier should slide in and out very freely without requiring much force. Bad carrier keys are common and have been known to cause binding that will cause a lot of wear on the upper and lead to jams. Make sure the buffer, spring and tube are properly lubed. The buffer tube should be smooth with no lathe turning ridges. With the weapon assembled, pull the charging handle back and note some force is required to cock the hammer. Then let the handle back and repeat. There will be some dragging of the carrier on the hammer, but it should not require much force. Try again with keeping the trigger pulled. This time the disconnector will keep the hammer pulled back even farther, so the dragging will be very light but still there. There may be a problem with the lower/hammer if too much drag exists. Ejector tension: Bad ejector tension is a major stumbling block that can be very frustrating and has lead people to sell their guns or by Colt bolts, or pay gunsmiths a lot of money. Aftermarket bolts are frequently assembled using the same springs for the ejector as is used as a detent spring elsewhere on the gun. The problem is that the ejectors themselves vary in length and possibly the holes in the bolts do too, so when these springs are used, they usually create excessive force on the ejector. The gun may appear to function properly but after a rapid fire string in the summer, cases will start jamming up into the gas tube groove in the upper receiver. At a DCM match this then leads to a number of helpful people poking at your gun with screwdrivers trying to get the case out. What has been observed is that a strong ejector tension will kick the brass out at about 5 o'clock 10-12 feet back. The gun will mostly function OK until the spring heats up and loses tension. The brass will not kick out as far and then jams occur. Proper ejector tension can be achieved by cutting or grinding the spring so there is practically no tension when the ejector pin is installed. The brass will then typically kick out about 2-3 ft at 2-3 o'clock, and there will be no more jam problem. Apparently when the ejector tension is very high the gun will operate, or low it will operate properly, but in between it will jam. Short cycling: If rounds are sometimes not chambered it is due to the bolt not coming back all the way. This also causes jams since the case may not be ejected. The problem is either something is binding or the gas system is leaking or plugged. The gas tubes are supposed to be self cleaning and seldom plug up unless perhaps you are using wimpy loads that produce a lot of carbon. It is important to know your gas system and where the possible leakage points are: near the front sight, at the carrier key, at the gas rings, and where the bolt extends through the carrier by the firing pin. There will typically be some leakage at the front sight as seen by some black deposits, but if the tube fits tight it is probably OK. The bolt post to carrier hole clearance is typically very tight - less than .5 mil and I have not had a problem there. If the bolt does not fall out of the carrier when you invert it, the gas rings are probably OK. Gas rings will rotate when you fire the gun and are not like car rings that seal into position. Sometimes people experience problems when all the ring gaps line up, but I have not had that problem. It is a good idea to separate the gaps each time you install the bolt anyway. The biggest source of gas leak trouble I have found is the carrier key to gas tube contact. Carrier key: There are a lot of bad carrier keys out there. This is the one item that should be looked at very carefully. Not only are they a source of potential bolt/carrier binding, but they can cause excessive gas leakage if the fit to the gas tube is bad. Gas tubes have a little ridge on them where they fit into the carrier key that determines the degree of gas leak sealing. All the new gas tubes I have seen have a quite uniform diameter ridge and have not been a problem except when they get worn down. The ridge can wear down a couple mils which then can cause excessive leakage. I have had that happen after 1000 rounds or so, but the quality of your carrier key can definitely affect this. My first carrier key caused binding as described above. The next one had a bit of a ridge inside increasing the inside diameter after a point. This caused the worn gas tube and excessive gas leak problem. The next key I selected to make sure it was uniform, symmetric, and had a nice smooth inside surface that made a good fit to the gas tube, and this one has lasted a long time and is still giving good service. Even supposedly GI issue carrier keys I have seen can have fairly rough inside surfaces, so pick them carefully. Headspace: The FAQ makes a lot of hoopla on headspace. There are a few things that should be pointed out on this. If you look in the Forster catalog for example, you will see the following for .223 Rem: Go No Go Field Abs Max 1.464 1.467 1.470 1.474 If we call 1.464 0 headspace, then the others are at +3, +6, and +10 respectively. Originally M-16 armorers had no-go and field gages, but recently they went to one gage which I am almost certain is the Abs Max +10 gage. So I would not be overly concerned if your rifle closes on a Forster No-Go gage. Typically the headspace on a rifle will be around +3 to +5-6 or so and they will be fine. Another point is that the armorer gages are machined down on the edges of the base so no contact is made with the ejector so no tension will be applied due to this when the bolt is closed. The proper way to operate the gage is to load it in the chamber and let the bolt/carrier drop slowly into position under its own weight and not to push it down under pressure. If the bolt does not close all the way, then you know the headspace is below that of the gage. You can easily be fooled by several mils if you force the bolt closed. This is more of a problem with normal Forster type headspace gages that have flat bottoms that would put the ejector under tension. Also if you use a Go gage (minimum) and add paper or other spacers to simulate No-go gages you have to be careful not to put too much force on closing the bolt or you will be fooled like I was once about your headspace. Another way to measure headspace is to get a RCBS Precision case mike. After you get it you probably will have to send it back to get it adjusted properly for 0 at 1.464 in. from recent experiences, but then you can resize a series of cases and measure the headspace of them and select a set that range from 0 to +10 mils and you then have a set of headspace gages.
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