I'm sure aslong as the paintball itself stays on your property you are fine. But, really you should only fire your marker in your back garden if it is 100% neccessary. If not, just wait until you are at a paintball site.
Also, always wear a mask, and use a barrel sock, and as mentioned above, if possible, aim the marker at the floor, or a brick wall. Where the paint is guarenteed to break and not bounce into someones garden etc.
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Sorry for any spelling mistakes, I fail at spelling.