Tubb offers weights for bolts. In the instruction manaul there is some interesting information regarding the buffer length and the spacing required between the bolt and the bolt stop.
The following is some information I also found on the web (http://yarchive.net/gun/rifle/ar15.html):
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 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 0.5 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 also 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.