All barrels will shift point of impact as they heat up. The thinner the barrel and how close your shots are will determine how much they shift POI. Knowing what barrel you have and how close together your shots are and how hot the day is would help us determine what may be your problem. Bedding will have an affect on your accuracy but wont help with the heating problem. I assume you are talking about bedding the action. If the barrel is touching the stock as it heats up or before it heats up that will affect the POI as well. Loose mtg screws also will cause a problem. The most common cause of shifting POI is loose mounting screws on your scope base and rings. Checking the free floating of the barrel and the scope mounting screws and reciever mounting screws is something you can check yourslelf.

El Lobo