This is because the relationship between the barrel and scope doesn't change with each shot. However, todays modern break barrels are typically so well made that the differences are very small. If you have a break barrel gun, you can definitly use it for field target and do quite well. Any shooting position is allowed in a field target match. As long as you can see the hit zone and have the gun supported only by your body, you can use any position that you are comfortable with.
Most shooters use a sitting position since that offers the best stability.
However, some lanes may be set up to force you to use one position over another, ie. There are various holds that field target shooters use. Most shooters rest their right elbow on their right knee. The gun is then either rested on the left knee or on the left arm which is on the left knee . These are the most stable holds.