.12gr - best used in shotguns or for weak handguns - they have very low accuracy at ranges beyond 30ft and will deviate wildy outside. Generally these are not used for our airsoft sport.
.20gr - the standard round used for bench-testing and normal indoor skirmishing - they are decent rounds for use outdoors but they are not too accurate beyond 30yds - best used in indoor guns/handguns or high volume of fire types of guns; this is the standard round people discuss when talking about muzzle velocity readings
.23gr - designed to be a compromise between the velocity loss from using .25gm rounds and the low accuracy of .2gm rounds. I have tested some and they performed very well- better than .2's for sure. Best used in any gun.
.25gm - Standard round for outdoor skirmishing - They maintain flight path accuracy better than .2's especially in the presense of a cross-wind. Best used in any gun to gain accuracy.
.28gr - Heavier round for enhanced stable flight and accuracy compared to .25g. Tend to be a bit more expensive than .25g.
.30gr - Good round weight to use in upgraded gun for an excellent level of accuracy on the field. They have a much better ability to punch through vegetation than lighter rounds.
.36gr - Round typically used in sniper applications. They carry a good part of their momentum for a better downrange impact force and they maintain stability even in the presence of a good wind. The Straight series of teflon bbs are very good and help preserve the life of you sniper barrel. Typical guns this round will be used in: PSG-1, APS, M40, M24... guns typically shooting over 450fps with .2gm bbs.
.39gr - Again another round that would be used almost exclusively in sniper rifles. A further step up in accuracy and stability but the higher weight requires a strong setup and good hopup to get good results. Expect to get consistent shots and wind should only play a part when engaging targets beyond 80 yards. I would not recommend this round for a gun shooting slower than 450fps. Again the round will be teflon coated for better barrel life.
.43gr - The highest weight round currently avalible. It is an extremely heavy round in comparison to normal skirmishing rounds. Expect the .43 round to maintain flight stability well beyond 100yards. I would only be concerned with wind if you are engagin targets at 100yards+ because in all honesty this round flys exceptionally true. Again this is a teflon coated round. Only use in rifles with a base velocity over 500fps with.2g.
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