ZM weapons: Complete guns, parts and kits for the high quality high priced ZM systems based on the AR15 designs but heavily modified. The ZM weapons use a gas pistom system similar to that of the FAL and the AK47. The folding stock is an Alan Zitta original and is probably one of the best designs ever made. The downside is that these guns and parts kits cost as much as three times the price of a standard AR. On the upside, a ZM derivative needs little or no improvement out of the box, as accessories like the Knight rail system, fore grips, metal folding stocks and low profile removable sites are costly options on standard AR15s. Go here for ZM website http://www.zmweapons.com/index.htm
Bushmaster: Considered by many to be the best parts and parts kits available outside of government contractors. Bushmaster has held government contracts for the manufacture of M4 carbines from time to time and is known for high quality and good service. Their prices are at the upper end of the scale for the more common parts dealers that deal directly with the public and their wholesale pricing is not much better. Their quality is generally good, but quality control has lagged at times, especially when there has been panic buying based on impending legislation and their production facilities were running faster than normal. They are also known to restrict the sale of parts in favor of the sale of complete guns or completed upper receivers (as opposed to selling individual component parts).