Larry P. (Jack Bumps) Hallman, 28, pleaded guilty yesterday to one count of armed kidnaping and one count of armed robbery, for each of which he could get a maximum sentence of 15 years to life in prison.

It was understood that Hallman "pleaded out," in the language of courthouse regulars, in order not to make his situation any worse than it is.

He already is serving two consecutive life terms for two murders in which he took part plus three to nine years for arson. Under D.C. law, this means he must serve at least 43 years in prison before he is eligible for consideration for parole.

Hallman's guilty pleas came as he and a codefendant, Frank L. Henderson, 23, went on trial for numerous counts of kidnaping, armed robbery, burglary, rape and related crimes charged in a 46-count indictment. Henderson remains on trial. Hallman's plea means that the rest of the counts against him will be dropped when he is sentenced.

Judge Nicholas S. Nunzio set sentencing for March 24. In accepting the guilty pleas, Nunzio said he would consider ordering that whatever additional sentence he metes out run concurrently with the minimum of 43 years that Hallman already must serve.

Hallman was one of four persons charged in the stabbing deaths on Oct. 22, 1975, of Curtis Arrington, 23, and Lois Ann Davis, 16, in an apartment at 1200 Delaware Ave. SW. The apartment was set on fire. The other three defendants in those cases already have been sentenced.

Henderson was not charged in that case, but faces trial in incidents in which Hallman also was implicated.