This is a necessary part when changing to a new handguard or rail for Marui M4 series GBBR.
The original Barrel Lock was easily damaged by even a weak impact. 
The GSI Outer Barrel Lock made of STEEL eliminates the damage issue.
It is made of steel, so it can be fixed firmly when changing the handguard
 
• EAN & SKU for Gold:  8809899600085, SKU: BN20-MM4G
•Product Dimension & Weight:1.13(L)x0.1.13(W)x0.39(H) inch 0.4 oz
•Package Weight & Dimension:  4.3(L)x 3.3(W) x 0.4(H), 0.5oz 
• Made in Korea,