Citizens carrying guns reduces crime

The government has good reason to lie about this topic to it's citizens. It doesn't want people to own guns because it threatens its power. Therefor it would have good incentive to under report the times when guns are used for defending life and stopping crime. In a new study done by CPRC, they prove that the FBI has been under reporting incidents of crimes being stopped by civilians due to their overly narrow definitions. "Massive errors in FBI’s Active Shooting Reports from 2014-2022 regarding cases where civilians stop attacks: Instead of 4.6%, the correct number is at least 35.7%. In 2022, it is at least 41.3%." You can't blame the media either. Evidence shows that the sources relied upon by the media were only successful in reporting ten percent or less of the incidents, with the media reporting even fewer of those.

Here is the study.

While you are right that the FBI underreported as per your source, a look at the numbers reveals that there were "only" 302 FBI active shooter events in total (over the course of 9 (!) years). So the difference here is 14  vs. 108 such shootings being stopped by armed civilians. These numbers more than pale in comparison to firearm deaths in general.

A study (1) done in 2010 compared homicide death rates between the US and 22 other high income OECD countries, which have much stricter gun regulation laws than the US. They found there were more than 86 firearm-related deaths per day in the US alone vs. approximately 18 per day in all the 22 other OECD countries combined. It is important to note here that the latter have a combined total population of about 660 million, which is twice that of the US.

These findings are hardly surprising. If you don't own a gun it proves quite difficult to commit a crime with it. And while it is true that stricter regulations won't stop organized criminals from getting their hands on one, there is also domestic violence to be taken into account.  For example, a 2003 study (2) found the odds of abuse ending in femicide was up to 5 times higher when the abuser had access to a gun. 

Sources:

1.: https://www.sciencedirect.com/science/article/abs/pii/S000293431501030X?via%3Dihub

2.: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1447915/

Since the only argument my debate partner offered was that armed citizens can, in some cases, stop attacks, I would like to touch upon the also often brought up argument that owning guns as a civilian is crucial as a matter of self-defence. 

A 2013 report from the U.S. Department of Justice (1) found that 'less than 1% of victims in all nonfatal violent crimes reported using a firearm to defend themselves during the incident' from 2007-11.

Furthermore, if one feels unsafe walking about in a neighbourhood that has a rather disconcerting reputation, carrying a gun might make you feel safe, but it won't reduce the likelihood that you will get mugged. And with a less than 1% chance that you are actually going to use that gun to defend yourself in that case, a much more effective way of protecting yourself would be to avoid the neighbourhood altogether, talk to someone on the phone or get a taxi. Arming yourself in the hopes that it will make you untouchable will probably not work.

Sources:

1.: https://bjs.ojp.gov/content/pub/pdf/fv9311.pdf