Sandy Hook Promise is an organization dedicated to preventing gun violence (on humans), named after the school shooting at Sandy Hook, Connecticut (State in America).

Disclaimer:

This article is subject to change and is not up to my usual standards.

The answer is more research about their tractability/how much funding is a bottleneck, so I can’t say what it is just yet, so stay tuned. (Update: it's now February $8^{th}$, 2024, and they still haven't responded - I might try calling them this weekend.)

For now, though, I will state that it makes the most sense to donate to them only when someone is going to [3x or more] match your donation, If you ever donate to them (which I don’t recommend until I have more info), which happens quite often. 

In the meantime(I.e. Until I get more info), I’d recommend donating to GWWC’s recommendations.

For more info on whether you should donate to Sandy Hook Promise, some of the most relevant(but potentially not reliable or valuable)^[1] pieces of information I could find so far would be their Annual Impact Report 2022-2023 (p. 39) for info on their spending and funding and  Annual Impact Report 2022-2023 (p. 12), and their Our Impact page for information on their impact (e.g., Lives saved, School shootings prevented, etc.).

1. ^{^}

   Regardless, it might be useful to know what the sites say for other reasons.