Dating acronym gu
The term is applied to those varieties of coal which do not give off tarry or other hydrocarbon vapors when heated below their point of ignition.
AKs are known for their robustness and were made in huge numbers, so that they are ubiquitous in much of Asia and the Third World.
The best of the Kalashnikov variants are the Valmets that were made in Finland, the Galils that were made in Israel, and the R4s that are made in South Africa.
They boost efficiency as long as they are used intelligently.
The advantages of brevity should be weighed against the possibilities of crypticness (making the communication harder for others to understand) and ambiguity (having more than one possible interpretation).