5 shots with an extreme spread of 265 fps is way, way outside of acceptable. Even if the accuracy is good, you have something majorly wrong here. I don't deal with mixed brand 5.56 and 223, so I don't know how much variation there is possible. I know Lake city 7.62 brass compared to commercial Federal or Winchester 308 brass varies, but I've not found the same thing with Lake City 5.56 brass. It's really easy to test now that you have cases fired in your chamber. It's easiest on an electric scale, put the case on it, tare the weight, fill the case to the lip with water, and that's your case capacity in grains of water. Small differences isn't going to cause this problem. According to quickloads you would need cases that vary from 27 to 30 gr h2o capacity to get such a crazy spread. That's not impossible, but that's not something I've seen.
My gut feeling is your load is simply not working well. With that powder charge I have to assume you are using a lighter bullet like a 50 grain, 25.5 gr looks over max or right at maximum for a 55gr depending on the load data. Sometimes those light bullets and slower ball powders do not play well together. If you happen to have magnum primers, I would definitely give them a try.