https://www.americanrifleman.org/articles/2020/7/12/us-army-patents-new-m4a1-rifle-barrel/
U.S. Army Patents New M4A1 Rifle Barrel
by Guy J. Sagi - Sunday, July 12, 2020
U.S. Army Patents New M4A1 Rifle Barrel
by Guy J. Sagi - Sunday, July 12, 2020