No one knows what percentage of the U.S. population actually owns guns. As with religion, the federal government does not keep official records or collect statistics on gun ownership. So we depend on surveys conducted by organizations like the Gallup Poll, Pew Research Center, NORC/General Social Survey, and others.
Those surveys often produce different estimates of gun ownership rates. Consider data from questions about whether respondents live in a household in which someone (not necessarily themselves) owns a gun (including the margin of error):
Pew Research Center (2017): 39-45% household gun ownership
Gallup Poll (2018): 39-47%
Monmouth University (2018): 43-49%
NORC/General Social Survey (2016): 29-35%