Table:
text: AR-15 variants
Numbered Colt military models
Numbered Colt military models
Colt_model_no_ Name_ Stock Fire_control Rear_sight Forward_assist Case_deflector Barrel_length Barrel_profile Barrel_twist Hand_guards Bayonet_Lug Muzzle_device
614 M16 A1 S-1-F A1 No No 20 in. A1 1:12 Triangular Yes A1
615 M16 HBAR A1 S-1-F A1 No No 20 in. HBAR 1:12 Triangular Yes A1
616 M16 HBAR A1 S-1-F A1 No No 20 in. HBAR 1:12 Round Yes A1
619 XM177E1 2nd Generation S-1-F A1 Yes No 10 in. A1 1:12 Short Ribbed No 3.5 or 4.5 Moderator
620 CAR-15 2nd Generation S-1-F A1 No No 10 in. A1 1:12 Short Ribbed No 3.5 or 4.5 Moderator
621 M16A1 HBAR A1 S-1-F A1 Yes No 20 in. HBAR 1:12 Triangular Yes A1
629 XM177E2 2nd Generation S-1-F A1 Yes No 11.5 in. A1 1:12 Short Ribbed No 4.5 Moderator

Question: What is the nature of the muzzle devices on the round handguard variants among the AR 15 models numbered Colt Military Models?