--- title: What is a Mk4? description: The Mk4 line of upper and lower receivers are based on the traditional mil-spec AR15 design. Mk4 upper receivers are made from billet 6061 T6 aluminum. Mk4 lower receivers are made from forged 7075 T6 --- [Skip to content](https://support.cmmg.com/what-is-a-mk4#main-content) [Contact Us](https://support.cmmg.com/kb-tickets/new?hsLang=en) [![CMMG\_LOGO\_HORIZ\_BLACK](https://support.cmmg.com/hs-fs/hubfs/CMMG_LOGO_HORIZ_BLACK.png?width=160&height=47&name=CMMG_LOGO_HORIZ_BLACK.png)](https://support.cmmg.com/?hsLang=en) Open main navigation Close main navigation - [Contact Us](https://support.cmmg.com/kb-tickets/new) - [Go to CMMG.com](https://cmmg.com/) [Go to CMMG.com](https://cmmg.com/) Hey there! What can we help you find the answer to? - There are no suggestions because the search field is empty. 1. [Knowledge Base](https://support.cmmg.com/?hsLang=en) 2. [FAQs](https://support.cmmg.com/faqs?hsLang=en) # What is a Mk4? The Mk4 line of upper and lower receivers are based on the traditional mil-spec AR15 design. Mk4 upper receivers are made from billet 6061 T6 aluminum. Mk4 lower receivers are made from forged 7075 T6 Aluminum and are designed to accept standard AR15 style magazines. - [Common Product Questions](https://support.cmmg.com/common-product-questions?hsLang=en) - [FAQs](https://support.cmmg.com/faqs?hsLang=en) - [DEFCAN](https://support.cmmg.com/defcan?hsLang=en) - [DISSENT](https://support.cmmg.com/dissent?hsLang=en) - [BANSHEE](https://support.cmmg.com/banshee?hsLang=en) - [RESOLUTE](https://support.cmmg.com/resolute?hsLang=en) - [ENDEAVOR](https://support.cmmg.com/endeavor?hsLang=en) - [Lead Times](https://support.cmmg.com/lead-times?hsLang=en) - [Dealer/OEM](https://support.cmmg.com/dealer-oem?hsLang=en) [![Chill listening crop-3](https://support.cmmg.com/hs-fs/hubfs/Email/cmm-logo.png?width=85&height=24&name=cmm-logo.png "Chill listening crop-3")](https://support.cmmg.com?hsLang=en) support.cmmg.com Knowledge Base Copyright © 2026, CMMG Inc.