• *The discussion of the creation, fabrication, or modification of airgun moderators is prohibited. The discussion of any "adapters" used to convert an airgun moderator to a firearm silencer will result in immediate termination of the account.*
Airgun moderators do not (should not) make contact with the projectile, so the projectile speed is not reduced by the moderator.

If you heard that moderators reduce the speed of the projectile, that idea may have come from fire arms, mounted to moderators that use rubber "wipes". These wipes are perforated by the projectile and that takes energy. Airgun projectiles generally don't have much energy to spare, so wipes are not used. Any contact with the projectile also tends to disturb the direction of the projectile, so wipes are only used over short ranges. Light projectiles used in airguns are more easily deflected, so another reason why contact baffles are not used with airguns.

Adding a moderator may increase a "gun's" overall length, often to the point of compromising its handiness. So, moderators are often used with shorter barrel guns to keep the overall length reasonable; or the barrels are cut shorter for that reason. A shorter barrel would tend to reduce the projectile velocity. In that instance, moderators are associated with reduced velocity.

So, if your airgun moderator reduces projectile velocity, the barrel and moderator are not coaxial and the contact is slowing the projectile. Or, you are shooting a .22 pellet through a moderator made for .177 ... :) . To prevent unwanted internal projectile contact, a moderator made for .25 may be mounted to a .22 caliber airgun as extra insurance against misalignment.

A slight loss in projectile velocity is not the main consideration. It is unpredictable projectile deflection on (or off) target that is the main concern. The second concern is damage to the moderator.
 
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