I would recommend .40 bbs for the barrel length. You can probably achieve 80-90yds pretty consistently. Your max range would be like 100yds-110yds but you might not achieve it consistently but it's Airsoft, your won't be making much past 80-90 yds without having to aim up. But action army parts are very high quality and will last you a very long time. If you have perfect air seal, you will get the maximum fps out of your rifle and very good fps consistency.