I worked up a load for my rifle last winter. When I fired the same load this summer, it locked up my bolt. Why?

Temperature affects the burning rate of powder. This load was apparently near maximum for this rifle at the colder temperatures. As a result, when the same load was fired during the summer with much higher temperatures, the powder was easier to ignite, causing pressures to climb too high. For optimum performance, maximum loads may need to be adjusted slightly for temperature variations, especially if you are shooting in an area in which temperatures vary considerably during the course of a year.