That's not true. The Biden admin has tried several times to get a new ban on assault weapons.

June 2022 https://www.washingtonpost.com/politics/2022/06/04/biden-assault-weapons-ban/

November 2022 https://apnews.com/article/biden-gun-violence-colorado-shootings-f85b4b9a59e3852868950d797f79a351

February 2023 https://www.congress.gov/bill/118th-congress/house-bill/698

March 2023 https://www.politico.com/news/2023/03/27/biden-assault-weapons-ban-nashville-shooting-00089050

June 2024 https://www.pbs.org/newshour/politics/watch-live-biden-speaks-on-gun-control-at-gun-sense-university-conference-in-atlanta

July 2024 https://www.usatoday.com/story/news/politics/2024/07/17/biden-ban-ar-15-trump-assassination/74416454007/

There's plenty more instances of Biden backing and trying to get through gun legislation. The Obama admin tried too.