Your argument confuses stocks and flows. The flow of guns went up by about 50% over 2020. The stock of guns went up much less. Wikipedia says there are about 400 million guns in the US. That means that in 2019, when people bought about 14 million guns, the total number of guns was going up about 3.5% (and murder was low). In 2020, when people instead bought about 22 million guns, the total number of guns went up 5.5%, so about 2 percentage points more than in a normal year.

So this theory requires us to believe that number of guns increasing 3.5% every year from 2015 - 2020 had no effect on the murder rate, but that guns going up 5.5% in 2020 had a very strong effect on the murder rate. Specifically, an extra two percent increase in guns must lead to a 30% increase in murder rates. Why would we believe that?

One reason might be if the people buying guns in 2020 were very different from the people buying guns in previous years. For example, if previous gun buyers were collectors who had 100 guns each, but 2020 gun owners were new buyers getting their first gun, then the share of people with at least one gun would go up by more than 2% over an average year.

Miller, Zhang, and Azrael (2021) explores this question and find the opposite:

The people buying new guns are mostly (~80%) people who have guns already. This varies a bit by time period but other periods (the beginning of the pandemic and the 1/6 insurrection) were more disproportionately new gun owners than the June period when homicides started to spike.

This also shows that the largest month-over-month increases in gun purchases, both new and total, were March 2020 and January 2021. There was no sudden homicide spike associated with either of these months, only May/June 2020.

Finally, guns are usually more correlated with suicide deaths than with homicide deaths, but there was no spike in suicides at the same time as the murder spike. This is what you’d expect given that the number of guns only increased by 2% over trend - a completely invisible effect on suicide. Unrelatedly, homicides rose by 30%.

So the gun hypothesis requires that:

Crime tracks the flow, rather than the stock, of guns. …so a 2% increase in guns can cause a 30% increase in homicides. …even when only a fifth of those guns are going to people who don’t have guns already. For some reason, an earlier larger spike in gun purchases (March 2020) and a later larger spike in gun purchases (January 2021) failed to have any detectable effect on homicide rates, but the comparatively small spike in June 2020 immediately (ie within less than a week) caused homicide rates to rise 30%. A secular 20-year tripling of yearly gun purchases also failed to affect the homicide rate, or was disguised by an exactly equal counter-trend. Although usually rising gun ownership increases suicide somewhat more than homicide, this time and this time alone it increased only homicide.

None of this seems very plausible to me.