The UK's Finest Independent Forum for Shooters and Gun Owners. Est 2010.

AR15 wrote:All the high speed video we have shows the bullet exiting the barrel before any rearward bolt carrier movement. High speed video is misleading though and movement is relative, as the carrier is still getting loaded with energy to get its mass going and slow movement is very very very slow in high speed video. The bolt carrier HAS to move in order for the bolt itself to rotate as its movement is driven by the cam pin track in the carrier, although there is play in the linkage before the bolt starts to rotate. Dwell time (the distance between the gas port and the end of the muzzle) is key to function as the bolt has to have pressure in order to work, as soon as the bullet exits, the barrel will return to normal pressure. If you capture gas closer to the chamber, say with a pistol length gas system then you are going to get a larger pressure spike and longer dwell time through the gas parts to the carrier, this will effect the bolts behavior, lock time and carrier velocity amongst other things. If you capture the gas from further down the barrel then the gas pressure is much flatter and dwell time is shorter.
Over gas a gun and it will tear case rims and beat itself apart trying to function. Increasing lock time is a most desirable thing on a semi as it gives the case time to contract a bit more, that why folk run Full Auto carriers (they weigh a bit more) and heavy buffers.

In some very closeup video we could actually see micro rearward movement of the rifle due to the bolt lugs loading into the extension as the gun was fired.