How to add DuckDuckGo bangs to Google (if you're using Chrome)
Chrome has a feature for defining your own search engines for the omni bar. It's intended for adding search boxes present on other websites, but it accepts arbitrary urls, including modified google search URLs. The URL for a google search query is just google.com/search?q=your+query+with+spaces+replaced+with+plus+signs, which means if you bind a "search engine" like google.com/search?q=wikipedia+%s to "w" you will automatically have the word "wikipedia" added to your search query if you search "w something". You won't even have to type the "!". Here's an example
Go to Settings → Manage Search Engines → Other search engines → ADD and then
Search engine: wikipedia
Keyword: w
URL with %s in place of query: https://google.com/search?&q=site%3Awikipedia.org+%s
Now if you search "w russels paradox" it will open https://google.com/search?&q=site%3Awikipedia.org+russels+paradox which is search results only from wikipedia.
The big drawback is that you can't use google search's autocompletion when you use a prefix. You still get it when searching without a prefix though.
I've included a list of all the search engine prefixes I've defined, roughly sorted by how often I use them, with related searches grouped.
I don't think you can do this in Firefox.