M249 SAW Question

[IntelToad]

I have a buddy that is an engineering professor at Northwestern. The last two summers he's being doing these Dept. of Defense trips to different bases and different services to see equipment, talk to the people that use them etc.

On his last trip he test fired an M16 and M249 at an indoor range. He said the bullet holes the SAW made were rectangular with slight rounding at one end, and the rectangles were pointed at different angles (in comparison with the round rifle holes.)

My guess was the difference in the barrel rifling ? Anyone else know why ?

[Silverback]

I have a buddy that is an engineering professor at Northwestern. The last two summers he's being doing these Dept. of Defense trips to different bases and different services to see equipment, talk to the people that use them etc.

On his last trip he test fired an M16 and M249 at an indoor range. He said the bullet holes the SAW made were rectangular with slight rounding at one end, and the rectangles were pointed at different angles (in comparison with the round rifle holes.)

My guess was the difference in the barrel rifling ? Anyone else know why ?

Rate of fire would also effect the trajectory of the round

[Flesh Thorn]

On his last trip he test fired an M16 and M249 at an indoor range. He said the bullet holes the SAW made were rectangular with slight rounding at one end, and the rectangles were pointed at different angles (in comparison with the round rifle holes.)

My guess was the difference in the barrel rifling ? Anyone else know why ?

Sounds like key holeing which is normally from firing a bullet too heavy for the barrel's rate of twist. Not sure why a M249 would do this with M855 ammunition unless the barrel was shot out. My $0.02......

[IntelToad]

Thanks everyone. Makes sense.

[The Surgeon]

[Black 6]

Just a wild guess-
if the indoor range was close distance, bullets may have not been stabilized, which in turn would produce a keyhole, i.e. bullets tumbling. Same thing happens at 1K+ distances when bullets go subsonic

Flesh's theory sounds better though

I don't remember the exact number, but a 5.56 round leaves the barrel doing between 30 and 35,000 rpm. if the barrel is within guaging. That being said, a round spinning that fast has one hell of a gyro effect and would be stable coming out... unless the barrel was shot out which would allow the round to "bounce" around the interior before exit. Another thing that can cause it is a messed up crown. This causes extra pressure on one side of the bullet at exit causing an erratic trajectory the same way that a miss aligned suppressor can cause the same issue.

My money is on the shot out barrel though.

[Black 6]

The speed doesn't have much to do with the effects I was referring to, but rater the action of tipping the round as it leaves due to an imperfection as it leaves the barrel or not getting the proper spin and stabilization while in the barrel due to being shot out.