Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Colt model no. | Military designation | Barrel Length | Barrel | Handguard type | Buttstock type | Pistol grip type | Lower receiver type | Upper receiver type | Rear sight type | Front sight type | Muzzle device | Forward assist? | Case deflector? | Bayonet lug? | Trigger pack |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 601 | AR-15 | 20 in (508mm) | A1 profile (1:14 twist) | Green or brown full-length triangular | Green or brown fixed A1 | A1 | A1 | A1 | A1 | A1 | Duckbill flash suppressor | No | No | Yes | Safe-Semi-Auto |
| 602 | AR-15 or XM16 | 20 in (508mm) | A1 profile (1:12 twist) | Full-length triangular | Fixed A1 | A1 | A1 | A1 | A1 | A1 | Duckbill or three-prong flash suppressor | No | No | Yes | Safe-Semi-Auto |
| 603 | XM16E1 | 20 in (508mm) | A1 profile (1:12 twist) | Full-length triangular | Fixed A1 | A1 | A1 | A1 | A1 | A1 | Three-prong or M16A1 birdcage flash suppressor | Yes | No | Yes | Safe-Semi-Auto |
| 603 | M16A1 | 20 in (508mm) | A1 profile (1:12 twist) | Full-length triangular | Fixed A1 | A1 | A1 | A1 | A1 | A1 | Three-prong or birdcage flash suppressor | Yes | No | Yes | Safe-Semi-Auto |
| 645 | M16A1E1/PIP | 20 in (508mm) | A2 profile (1:7 twist) | Full-length ribbed | Fixed A2 | A1 | A1 or A2 | A1 or A2 | A1 or A2 | A2 | M16A1 or M16A2-style birdcage flash suppressor | Yes | Yes or No | Yes | Safe-Semi-Auto or Safe-Semi-Burst |
| 645 | M16A2 | 20 in (508mm) | A2 profile (1:7 twist) | Full-length ribbed | Fixed A2 | A2 | A2 | A2 | A2 | A2 | M16A2-style birdcage flash suppressor | Yes | Yes | Yes | Safe-Semi-Burst |
| 645E | M16A2E1 | 20 in (508mm) | A2 profile (1:7 twist) | Full-length ribbed | Fixed A2 | A2 | A2 | Flattop with Colt Rail | Flip-up | Folding | M16A2-style birdcage flash suppressor | Yes | Yes | Yes | Safe-Semi-Burst |
| N/A | M16A2E2 | 20 in (508mm) | A2 profile (1:7 twist) | Full-length semi-beavertail w/ HEL guide | Retractable ACR | ACR | A2 | Flattop with Colt rail | None | A2 | ACR muzzle brake | Yes | Yes | Yes | Safe-Semi-Burst |
| 646 | M16A2E3/M16A3 | 20 in (508mm) | A2 profile (1:7 twist) | Full-length ribbed | Fixed A2 | A2 | A2 | A2 | A2 | A2 | M16A2-style birdcage flash suppressor | Yes | Yes | Yes | Safe-Semi-Auto |
| 655 | M16A1 Special High Profile | 20 in (508mm) | HBAR profile (1:12 twist) | Full-length triangular | Fixed A1 | A1 | A1 | A1 | A1 | A1 | M16A1-style birdcage flash suppressor | Yes | No | Yes | Safe-Semi-Auto |
| 656 | M16A1 Special Low Profile | 20 in (508mm) | HBAR profile (1:12 twist) | Full-length triangular | Fixed A1 | A1 | A1 | A1 with modified Weaver base | Low Profile A1 | Hooded A1 | M16A1-style birdcage flash suppressor | Yes | No | Yes | Safe-Semi-Auto |
| 945 | M16A2E4/M16A4 | 20 in (508mm) | A2 profile (1:7 twist) | Full-length ribbed or KAC M5 RAS | Fixed A2 | A2 | A2 | Flattop with MIL-STD-1913 rail | None | A4 | M16A2-style birdcage flash suppressor | Yes | Yes | Yes | Safe-Semi-Burst |
Question:
What is the rear sight type on the model with the acr muzzle brake device? 
Answer:
None