A newspaper article indicates he was buried in the Lone Mountain Cemetery in San Francisco. Now you have an actual place of burial.
Unfortunately, the cemetery is now defunct and there are no tombstones. Either the markers were used to build part of the city or they were sent in mass to Colma.

Charles W. Piercy (1833-1861), California Democrat politician, and Assemblyman, that as a Douglas Democrat politician was killed in a duel with a Southern Democrat Assemblyman Daniel Showalter in the last political duel in California.[1]
Charles Wesley Piercy, was born, in Decatur County, Iowa, on June 11, 1833 to Nathan Piercy and Elizabeth Scott Piercy. He came across the continent to California in 1852, in a small party one of whom was Daniel Showalter of Pennsylvania, but they when they arrived in California went their separate ways. [2] Piercy settled in El Monte and in 1858 joined with others there in purchasing and reselling the lands of Mormons returning to Utah at the time of the Utah War. Allied to those interests he was elected Sheriff of San Bernardino County in 1859. Piercy and resigned in Oct. 1860 to run for a seat in the State Assembly.
In the bitterly contested campaign of 1860, Charles W. Piercy was nominated for member of the 1st District of the California General Assembly by one party, and W. A. Conn the incumbent, by the other. Piercy was elected, but there was a claim of fraud. The accusation was that polls at Temescal, maintained by a resident named James Greenwade, kept open shop for three weeks and that whenever candidate Piercy was in need of more votes, they were furnished from this precinct. The case was taken to court, where the two opposing lawyers, H. M. Willis and Bethel Coopwood, had a fight in court wherein Coopwood sustained a slight wound, but won the case. Piercy won his seat as a member of the 1861-62 California State Assembly, representing California's 1st State Assembly District.[3][4][5]
Piercy and Daniel Showalter, an Assemblyman who represented the Breckinridge Democrats, disagreed over the U.S. Senate election to a point that provoked a duel. On May 25, 1861 their duel with rifles, was held in Fairfax, California at the home of Democrat politician Charles S. Fairfax. After attempts to prevent the duel were made by friends, the first shots were exchanged and both parties missed, Showalter asked for a second firing, and managed to shoot Piercy, who was killed. It was the last duel between political figures in California.[6]
Charles W. Piercy was buried in Lone Mountain Cemetery, in San Francisco.[7] When San Francisco passed an ordinance in 1912 evicting all existing cemeteries from city limits the remains of Piercy were sent to Colma.
References[edit]
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^ JoinCalifornia: Election History for the State of California
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^ "The antagonists, Daniel Showalter of Kentucky and Charles W. Piercy of Iowa, had crossed the continent in the same small company years before but on arriving in California had gone different ways." Mildred B. Hoover, Douglas E. Kyle, Historic Spots in California, Stanford University Press, Stanford, 2002, p. 187
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^ Western States Jewish History, MEMOIRS OF MARCUS KATZ — San Bernardino Pioneer, Volume 1, Issue 1, October, 1969, PART II
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^ Editors, John Brown, James Boyd, History of San Bernardino and Riverside counties with selected biography of actors and witnesses of the period of growth and achievement, The Western Historical Association, Madison, Wis., 1922, pp.148-149
5. Political Graveyard, 1859-60 California State Assembly
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^ Theodore Henry Hittell, History of California, Volume 4, N.J. Stone, San Francisco, 1897, p.279
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^ Funeral of Mr. Piercy, Marin County Journal, June 1, 1861

There is also a burial card on him from the coroner which verifies he was interred in Lone Mountain.