Metro Washington's hugh federal-military retirement colony that pumps more than $100 million a month into the local economy is due another inflation-caused (not inflation causing) raise this September. The annuity hike will be worth at least 2.5 percent to the 100,000 plus retirees or their survivors who live in this area, and to ex-bureaucrats and retired military personnel living in every state and more than 30 countries around the world.

The COL boost will be more if living costs, as measured by the Consumer Price Index, go up for the three remaining months -- April, May and June -- in the countdown. Under the present system, which the White House and congressional budget cutters want to change, government retirees get COL catchup raises every six months, in March and September. Because of the high inflation rate last year, federal-military retirees got a 7.7 percent COL raise in September, and a 4.4 percent increase last month.

Despite his pre-election promise not to change the COL system, President Reagan has asked Congress to limit U.S. retirees to the same one-a-year COL system used for people under Social Security. Both the Senate and House Budget committees have tentatively okayed the idea.

The one-per-year COL system would save nearly $1 billion a year, according to congressional estimates. But elected officials, especially House members who must run again in 1982, may find the political price tag very high. Retirees dominate the voter rolls in many Sun Belt communities of Texas, Florida, Arizona and California.

There are 56,928 federal retirees or their survivors in the District of Columbia, 72,389 throughout Maryland and 94,113 throughout Virginia. Those figures do not include the big military retiree communities. The average annuity of federal civilian retirees nationwide is about $950 per month -- but many of the oldest retirees, and the surviving spouses of most former federal employes get much, much less.

Federal and postal unions, and especially the National Associate of Retired Federal Employees (largest of all government groups) have shown an amazing ability to raise money, get out the vote and change politicians' minds when the economy ax is raised at retirees. They hope to do it again by convincing 35-year-old members of Congress, who think tampering with federal retirement benefits is harmless, that it could result in their becoming 36-year-old ex-members of Congress.