An essential tool for the precision rifle shooter, ballistic chronographs are used to measure the muzzle velocity of firearms. In addition to providing information about a given cartridge’s velocity, the ability to determine the standard deviation and extreme spread of muzzle velocities during the load development process is key to long range performance.
When I started reloading in the mid-90s, the Oehler 35P was the go to chronograph. I bought one and used it often (I actually owned two at one point). Years later, I tested the MagnetoSpeed barrel mounted ballistic chronograph and was hooked. Both of these units are praised in the shooting community. I’m often asked which one is better. Since I’ve never compared them directly, I’ve only been able to offer anecdotal advice. I decided to test both side by side and see how they stack up against each other.
The Oehler 35P chronograph uses traditional skyscreen technology. Common to most civilian chronographs, the skyscreens are mounted in front of the firearm and sense the shadow of a bullet passing through them. This is used to compute the speed. The Oehler is unique among chronographs using skyscreen technology because it records the velocity twice and compares them with what is known as a proof channel. If the chronograph feels the shots aren’t within a predetermined range (with the 2 foot skyscreen separation used here, 42 feet/second for 2,000 feet/second velocities and 63 feet/second for 3,000 feet/second velocities), the chronograph flags the shot. Data is recorded and summarized on a small printer attached to the control unit. A copy of the owners manual can be found here.
The MagnetoSpeed barrel mounted ballistic chronograph detects the magnetic field of the bullet passing over and uses this information to calculate muzzle velocity. Since it straps directly to the firearm’s barrel, setup is simpler than a traditional chronograph. Data is recorded in a small control unit and saved on a small memory card. The card can be inserted into a computer and the data can downloaded as a spreadsheet.
To compare chronographs, I shot the same 32 different ammunition and barrel length combinations over the MagnetoSpeed V3 and Oehler 35P and recorded the results. I felt the wide selection of velocities would provide a broad range to compare the two units (as opposed to testing the same load repeatedly). The MagnetoSpeed was attached to the barrel of the bolt action pistol (in a modified MDT LSS chassis) and the Oehler was placed 8 feet from the muzzle. Note: to read more about the test used to gather the data, see 223 Remington/5.56mm NATO Barrel length versus Velocity- Short Barrels- 6 to 14 inches
The test setup is shown in the image above. Note the MagnetoSpeed V3 attached to the barrel. The Oehler 35P is downrange from the pistol. This system has a two foot sky screen spacing and uses one tripod. Systems currently sold by Oehler use a four foot spacing and two tripods. Note: this is one of the original systems I purchased directly from Oehler years ago. The new system appears to be identical. I contacted Oehler on multiple occasions requesting a test and evaluation sample and was never contacted back.
Results:
The table above shows the average velocities in feet/second for each of the 32 series of shots recorded by both chronographs. Each series consists of five shots of UMC, Federal and Winchester loads, and three shots of the Black Hills for a total of 150 shots fired. The difference between the average velocities recorded by each chronograph for each shot series is shown in the table. This was calculated by subtracting the velocity recorded by the MagnetoSpeed from the velocity recorded by the Oehler.
The percent change between both chronographs is recorded in the table. This was calculate by dividing the velocity recorded by the Oehler from the velocity recorded by the MagnetoSpeed and dividing that value by the MagnetoSpeed’s velocity.
The MagnetoSpeed V3 registered and recorded all 150 of the shots summarized in the table above. The Oehler did not record one shot for the Black Hills 10″ barrel series, three shots for the Federal M193 7″ barrel, one shot for the UMC 6″ barrel , two shots for the Federal M193 6″ barrel, one shot for the Winchester M855 6″ barrel and 2 shots for the Black Hills 6″ barrel series. In total, the Oehler 35P recorded 140 out of 150, or 93% of the rounds fired. It should be noted that the Oehler managed to record all of the shots at higher velocities and only started to experience problems as the bullets began to slow down. The owners manual suggest problems with subsonic ammunition however, bullets traveling at the velocities encountered at these barrel lengths should have been picked up according to their documentation.
In 28 of the 32 shot series compared above, the Oehler recorded slower velocities than the MagnetoSpeed. This is intuitive since the Oehler is eight feet further downrange than the MagnetoSpeed. In one of the series the values were the same. In three of the series the values displayed by the MagnetoSpeed were lower than those recorded by the Oehler. The greatest difference between the values recorded was 1.1%, and in this case (7″ M193), the Oehler didn’t register three of the five shots recorded by the MagnetoSpeed. The Average percent difference was 7 feet/second or 0.3%. When removing the series the Oehler missed shots, the average difference was 4.4 feet/second or 0.2%. These findings are similar to those reported by Southwest Research Institute which can be found here.
Conclusions:
The velocity data generated by the MagnetoSpeed V3 and Oehler 35P are remarkably similar. It would be safe to say that both units generate information that is accurate enough to be used for precision rifle load development and testing. The real difference between the two units comes down to how they are used.
The advantage of the MagnetoSpeed is the ease of set up and it’s diminutive size. As a gun writer, I constantly gathering chronograph data. Simply strapping the devise to the barrel is a piece of cake. While I haven’t noticed an effect on accuracy with heavy barreled bolt guns or modern sporting rifles, when attaching the MagnetoSpeed to a sporter contoured barrel on a custom hunting rifle, things change. I have noticed the weight of the unit affects the performance of the rifle. This means you have to load twice as many rounds for load development if you are working with a light barrel, one set for velocity data, the other set for group size. I like the fact that the data is saved on a memory card, however, I always bring a notebook to write down the data. Adding a small printer to the unit would be a nice touch.
In my experience, the MagnetoSpeed works in bad weather and poor lighting. I’ve used all three generations of models and managed to shoot two of them off of the ends of my rifles (I haven’t shot a sky screen chronograph yet). MagnetoSpeed has since improved the mounting strap system, and I haven’t shot my V3 (yet). The MagnetoSpeed can record shots in rapid succession, something a traditional chronograph can’t. While this might not be of use for the precision rifle crowd, it will certainly be of interest to the Ar-15/M16 M4 shooter.
The Oehler 35P is a physically larger system that requires more time to set up and ideal lighting conditions. If the printer paper gets wet, you have a problem (I figured this out the hard way a few times over the years). The biggest advantage of the system is it doesn’t touch the barrel. This means you can fire lighter barreled rifles through your chronograph and not have to worry about it affecting accuracy.
So which would you recommend?
It really depends on your budget and needs. The MagnetoSpeed V3 sells for around $400 (they also make a sporter version for about half the price) and the Oehler is around $600. If you mostly shoot heavy barrel rifles, or modern sporting rifles, the MagnetoSpeed is hard to beat. It is accurate and easy to use. However, it does have some limitations, especially for use with light barrel rifles and short barreled handguns (you can’t strap it to most semi autos, however, they are making an adapter block for Glocks). The Oehler 35P is a great choice if you are mostly shooting lighter barreled hunting rifles and have plenty of range time to set it up and break it down. I have and use both, however, 95% of the time I head to the range with the MagnetoSpeed V3.
To learn more about the MagnetoSpeed V3, visit Brownells.
To learn more about the Oehler 35P, visit Oehler.
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