The SAS - Shotguns

Remington 870

Calibre: 12 gauge, 3" Magnum Pump Shotgun 
Barrel: 26" smooth bore barrel with vented rib and interchangeable choke system 
Magazine: 5 Shell tube magazine 
Overall Length: 46.5" 
Average Weight: 7.25lbs 

MOD's 

• Timney All Steel Adjustable trigger assembly with safety 
• Remington 20" fully rifled, rear- sighted deer barrel. 
• Remington 18.5" Smooth bore barrel with recoil compensator system 
• Titanium Firing Pin 
• Steel Barrel Shroud 
• Choate 8 round magazine extension 
• Choate heavy duty magazine spring and shell follower 
• Choate polycarbonate pistol grip stock 
• Choate polycarbonate slide grip 

The shotgun is a very messy thing, and is liable to hit a hostage as well as the hostage taker when fired, instead the SAS use it as a door breacher, they use special door breaching cartridges (very powerful when concentrated on a small area such as a hinge or door lock) to blow open a door so the team can move in. 

http://world.guns.ru/shotgun/sh17-e.htm