If you want something more powerful, you might want to try the advanced search. Up near the search bar, there is a little gear/cog looking icon that will take you to the advanced search. I find this to be significantly more customizable and thorough. I personally like the ability to enter an author, or a specific date range, or even specify a few sub-sections. The other thing I find very useful is the ability to show results as posts instead of threads. It makes sifting through a number of posts for something specific a little faster for me. There are some limitations with the standard search function. Namely, I think there is a built in time frame that is not obvious or well stated so it only searches the previous few years, even though we have almost 14 years of posts. This is to make the searching faster, but comes at the expense of finding what you really are looking for. The advanced search does not do this, or at least tells you if it defaults to something I believe.
A few other things to note. There is a limit to the number search results. I believe it's 72 individual threads, or 200 posts. Searching for something generic like "black powder" will quickly hit the limit. For example individual threads it will cut off around April 2020, and for posts it will cut off about a year ago. You do need to be somewhat specific, or narrow down what you're looking for in subsequent searches in order to really find the good stuff. The advanced search will allow you to sift through everything if you have the patience to keep changing the date range.
This site is publicly viewable for the most part and indexed by search engines. A few sections, like the Agora, HE section, and Admin section are not but most of the information there really isn't really what most people are trying to find in order to learn. You can use Google, or your favorite search engine, to search APC which is incredibly powerful. Add this tag to your search term and you'll be all set "site:amateurpyro.com" Using the same black powder example as before, google gives 2,070 results.
It may not seem like the best, but compared to some other forums I've used, this software has an incredibly powerful search function. That speaks less to us, and more to how terrible some others are.