I recently purchased a the Magnetospeed Sporter V2 to determine the muzzle velocity of the Hornady 168 grain A-Max Match product number #8096. According to the box, this round has a muzzle velocity of 2700 FPS. Hornady's web page documentation states that their muzzle velocities are calculated using a 24" test barrel. The gun I'm using is a M&P10.

Using the Magnetospeed, I received three sets of averages of 2326, 2318, and 2425 FPS.  Averaging these three, I get 2356 FPS. Looks like I'm losing 344 FPS over what's stated on the box. If the test barrel is 24" and the barrel of the M&P 10 is 18" than I should lose 6" of muzzle pressure, correct? To calculate the muzzle velocity per inch, I divide 344 FPS by 6". If my calculations are correct, I’m losing 57 FPS lose per inch. Am I correct? This seem like too much muzzle velocity lost per inch to me.

I would like to know what FPS you guys are getting with this ammo from an 18" barrel. I'm  not confident that the Magnetospeed is accurately reading the muzzle velocity because the Magnetospeed kept coming loose and displayed various error codes while trying to use it. I had to keep switching to different sensor sensitivities after every 4 or 5 shots.  I wasted about 60 rounds just trying to get the three averages (7 shots per average)!

I can’t recommend the Magnetospeed Sporter V2, due to the difficulty of maintaining a tight connection between the barrel and mount on an AR type weapon.

Going off topic here, but I’m a left-handed shooter, and every fourth shell casing would hit me in the face. Is this normal for the M&P 10?  The brass defector is already showing green primer, after only shooting 200 rounds of the Hornady 168 grain A-Max Match. I never had this problem shooting any of the PMC ammo? On the bright side, I'm shooting 3/4" MOA with this ammo.

Sounds about right to me.  I would suggest doing a drop test.  Sight at 100 dead on, shoot at 200 and compare the drop to app entered velocity until the drop matches.  There is your velocity check.

A few things...

1. Are you sure you're using the attachment device properly? Loosen the bottom clamp/nut all the way, the strap goes around the barrel, through the cam buckle, back around the barrel and through the keeper snugged down, then you tighten the bottom clamp to tighten things up more. MagnetoSpeed Chronographs are HIGHLY regarded in most circles for being easy to use and accurate. I've got a V3, not the sporter, but I've had no issues with mine.

2. The velocity numbers you are seeing line up with what i'd expect. You are failing to consider that the test velocities likely came from a bolt action rifle, with 100% of the gasses following the round, where your semi-auto is diverting a portion of that gas to cycle the bolt, hence lowering the velocity.

Thanks for the confirmation, so I'll test them at the range.

I did attach the MagnetoSpeed correctly, but it would come loose after each shot, so I improvised and used a 2.75 automobile hose clamp which worked for several shot before coming loose. Are the V2 and V3 sensors the same dimensions because I had to mount the V2 sensors halfway on the flash suppressor of my M&P 10 to get a sensor reading. I had the same problem trying to use it on my AR-15. It seems to me that the V2 sensor need to be farther from the mounting block for it to work on AR type rifles.

For me, the box has never been right, and the magnetospeed has been pretty much spot on.  I plug my magnetospeed results into my Kestrel AB and get hits to 1000 yards regularly.


The box is a test barrel.  Barrels change.  You will probably find after a few hundred rounds the speed will change.

I've tested the magneto speed to my Labrador and they were only ever 3fps different at most. Trust your magneto speed. They are accurate.

Sounds like he is having mounting problems so that may give a faulty number if the bayo is moving.

But yes, I do agree the the Magnetospeed is accurate, I have compared mine against my Labradar with a few different rifles, all within a couple fps.

Velocity and barrel length isn't a linear relationship.  Your results sound about right.