The Mech-Tech System Carbine Conversion Unit (CCU) converts your Glock pistol into a semi auto carbine. Fast shooting and quick handling, fits the Glock pistol models Gen. 1-4. Easily installed or removed, This CCU allows you to shoot the appropriate ammunition from any of the Glock frames by simply using the appropriate Glock magazine. (9mm, .40 cal.,& .45 ACP) Add accessories to build your custom carbine. THIS IS DROP SHIPPED FROM THE FACTORY. GLOCK NOTES: The Glock pistols have three basic frame sizes - 'Full Size', 'Compact', and 'Sub Compact'. Mech-Tech CCU's are not available for sub compact Glocks. In the 'Full Size' series are the Models G20 (10mm), G21 (45ACP), G17 (9mm), G22, G34 (9mm Long Slide) and G35 (40 SW Long Slide). Within the full size series there are two frame widths as is the case for the compact series. Mech-Tech CCU's are not available for 10mm and 45 ACP compact series pistols. An interesting fact relates to Glock magazines. A frame separated from the slide and barrel will physically accept a magazine of a cartridge different from that for which the assembled pistol is designed (using magazines designed for the same series of frame sizes). What this means is that a Glock pistol & CCU combination can be set up to safely fire a cartridge different from that for which the assembled pistol is designed. Example: The frame (lower unit) of a Glock designed to fire the 45 ACP cartridge in the assembled pistol can be safely used with a CCU designed to fire the 10mm cartridge. This is accomplished by assembly of the 45 ACP Glock frame to the 10mm CCU and the use of a Glock 10mm magazine designed to be used with that series of frame size. This information is provided only to illustrate the flexibility of the Mech-Tech CCU. (Caution: the foregoing does not imply that a Glock pistol designed for a given cartridge can safely shoot a different cartridge by simply switching the magazine - this is dangerous - do not try it! Likewise a CCU can safely fire only the cartridge for which it is designed!) If further clarification is needed please contact Mech-Tech Systems Inc.