Heat might not or might not have been the issue. It's hard to tell from your post whether you're comparing the first and last shots of the day with many more in between, or the first two and last three shots of a single 5-shot string. Heating the rounds can definitely speed them up. How many shots did you fire before the chrono string and how long did you wait between shots during the chrono string? If the barrel was already hot to begin with, I can't imagine it heated up that much more in five shots. So that shouldn't have caused a 70 fps velocity excursion. If the barrel was cold to start with, 5 shots, even done rapidly, still isn't usually enough in a .223 (small case) to heat it above barely warm to the touch. So in that case I still wouldn't think heat was the likely culprit. Alternatively, if you're comparing velocity of the first two shots and last three of the day with more in between, heat could definitely be the cause.
If you clean your rifle between firings, it's also not at all uncommon to get lower velocity on the first couple shots, sometimes more, depending on how long the barrel takes to remove any residual cleaning components and properly foul. On most of mine, the velocity usually stabilizes after the first couple shots. However, I usually fire at least 5 foulers/sighters before trying to do any chrono work, just to be sure. The first one or two are always slow, although not usually 70 fps slow. In any event, now that you are aware of the issue, you should be able to determine whether heat or cleaning (or something else entirely) was the cause by shooting rounds through a cold barrel at a slow enough cadence not to heat up the barrel, while also measuring velocity. If you go through about 10 shots no faster than one per minute (or two), the barrel shouldn't heat up too badly. You will then see whether you experience the same velocity excursion after the barrel fouled, but under conditions where it wasn't hot.
Another possibility could be the brass. If it's from mixed lots, you might have a big difference in the volume of some of the cases. Measuring internal (water) case volume would reveal if that might be the cause.
