Magnetospeed Error Correction

[Engineering101]

You guys with the new Magnetospeed chronos. How does it correct for the recoil of the rilfe? A 9 lb 30-06 will be moving backward (and taking the Magnetospeed with it) as the bullet is leaving the barrel. The velocity of the rifle at that point in time (just before the velocity of the bullet is measured) will be about 9 feet per second and the rifle will have moved backward about 0.060 of an inch (basically in free recoil since it hasn't compressed the recoild pad or the flesh of your shoulder). This velocity will get added to the velocity that the Magnetospeed is reading. Does the unit allow you to tell it the velocity of the rifle so it can be automatically corrected?

 

[MontanaRifleman]

You guys with the new Magnetospeed chronos. How does it correct for the recoil of the rilfe? A 9 lb 30-06 will be moving backward (and taking the Magnetospeed with it) as the bullet is leaving the barrel. The velocity of the rifle at that point in time (just before the velocity of the bullet is measured) will be about 9 feet per second and the rifle will have moved backward about 0.060 of an inch (basically in free recoil since it hasn't compressed the recoild pad or the flesh of your shoulder). This velocity will get added to the velocity that the Magnetospeed is reading. Does the unit allow you to tell it the velocity of the rifle so it can be automatically corrected?

In a test this past summer shooting a Magneto against an Oehler 35, the Magneto was within a few FPS of the Oehler with 4 different rifles. Based on that, I would assume recoil has little to no effect.

 

[ksubuck]

Bullet moving 3000fps in one direction, rifle 9fps in the other. Works out to an error less than a third of a percent. Big whoop.

 

[MudRunner2005]

You guys with the new Magnetospeed chronos. How does it correct for the recoil of the rilfe? A 9 lb 30-06 will be moving backward (and taking the Magnetospeed with it) as the bullet is leaving the barrel. The velocity of the rifle at that point in time (just before the velocity of the bullet is measured) will be about 9 feet per second and the rifle will have moved backward about 0.060 of an inch (basically in free recoil since it hasn't compressed the recoild pad or the flesh of your shoulder). This velocity will get added to the velocity that the Magnetospeed is reading. Does the unit allow you to tell it the velocity of the rifle so it can be automatically corrected?

As an engineer, I would figure you would be aware of velocity vs disance traveled. You might ask how that plays into this scenario? Here we go...

The bullet actually leaves the barrel WAY before you even feel the first hint of recoil. If a bullet is traveling 3,000 fps, and your barrel is only 26" long (2.17 feet), then it takes your bullet approximately 0.0007233 seconds to travel the full length of your bore and out of the muzzle, after ignition. Felt recoil will not happen until after the bullet has left the muzzle expelling all of it's gasses and energy into the atmosphere causing a rearward force (recoil).

I've waved my MSv2 bayonet around (while plugged in) to see if it would pickup anything, and I think they've got it set to where it has to have something solid traveling VERY close to it, for it to read anything, so I'd assume you'll be ok. Mine reads perfectly, and is very close to book load velocities on several of my loads, so I would guess it is pretty accurate.

 

[deadidarren]

if you watch a shot in slow motion you will see the bullet is gone then the recoil happens .

 

[rick523]

Bullet moving 3000fps in one direction, rifle 9fps in the other. Works out to an error less than a third of a percent. Big whoop.

What he said.

 

[Engineering101]

MontanaRifleman

I assume you are saying that the numbers on the displays of the two chronos closely matched. That is very interesting. I would expect that they would not match because there is velocity loss from the end of the barrel to a conventional chrono of about 7 fps (depending on bullet BC and how far down range you put the sky screens) so a conventional chrono should always read slower than a magetospeed which is right on the barrel. And for the reasons I stated above, the magneotospeed should read fast by about the same amount (depending on rifle recoil). Sooooo… when everything is working right the magneto readout should be about 15 fps faster than the conventional chrono. Did you take this correction into account when you say the chronos agreed with each other?



MudRunner2005

What gave me away? I knew I should have picked a different login right after I pushed the button. Yes I was a engineer – and now I'm just an old and senile gun nut.

You are of course correct when you say the bullet leaves the barrel way before you feel any recoil. In fact the rifle has moved about 0.060" backwards at that point, which on a human scale is essentially zero. It does however have a velocity of about 9 feet per second due to the equal and opposite reaction thing. I was just wondering how the magneospeed dealt with that.

 

[morcey2]

Until the bullet has actually left the barrel, everything is operating from a fixed point of reference, a.k.a., the rifle itself. If the recoil is somehow slowing the bullet a measurable amount, it's also accelerating the magnetospeed device in the opposite direction and the measured velocity should be the same. What you're asking is actually an issue (theoretically) with normal chronographs, but not with barrel-mounted ones.

But, as everyone else has said, the recoil forces haven't overcome the inertia of the rifle until well after the bullet has left the barrel. And, even if that weren't the case, the rifle is moving so slowly relative to the speed of the bullet that it isn't an issue.

Matt

 

[MudRunner2005]

MudRunner2005

What gave me away? I knew I should have picked a different login right after I pushed the button. Yes I was a engineer – and now I'm just an old and senile gun nut.

You are of course correct when you say the bullet leaves the barrel way before you feel any recoil. In fact the rifle has moved about 0.060" backwards at that point, which on a human scale is essentially zero. It does however have a velocity of about 9 feet per second due to the equal and opposite reaction thing. I was just wondering how the magneospeed dealt with that.

It's all good man...No sweat. Yeah, I don't think that it necessarily ready wind movement, just metallic movement traveling over the bayonet, so I'd assume it would not read the recoil reaction, and just read the bullet as it travels over the bayonet.

But I could be wrong. Wouldn't be the first time.

 

[MudRunner2005]

Until the bullet has actually left the barrel, everything is operating from a fixed point of reference, a.k.a., the rifle itself. If the recoil is somehow slowing the bullet a measurable amount, it's also accelerating the magnetospeed device in the opposite direction and the measured velocity should be the same. What you're asking is actually an issue (theoretically) with normal chronographs, but not with barrel-mounted ones.

But, as everyone else has said, the recoil forces haven't overcome the inertia of the rifle until well after the bullet has left the barrel. And, even if that weren't the case, the rifle is moving so slowly relative to the speed of the bullet that it isn't an issue.

Matt

That's exactly what I was trying to convey in my ramblings... LOL

 

[MontanaRifleman]

MontanaRifleman

I assume you are saying that the numbers on the displays of the two chronos closely matched. That is very interesting. I would expect that they would not match because there is velocity loss from the end of the barrel to a conventional chrono of about 7 fps (depending on bullet BC and how far down range you put the sky screens) so a conventional chrono should always read slower than a magetospeed which is right on the barrel. And for the reasons I stated above, the magneotospeed should read fast by about the same amount (depending on rifle recoil). Sooooo… when everything is working right the magneto readout should be about 15 fps faster than the conventional chrono. Did you take this correction into account when you say the chronos agreed with each other?

I did correct the Oehler's velocity readout for distance downrange which I believe was about 15'. I added 9 fps. Here's the data, with corrected Oehler velocities.

223

Oehler....... Magneto .............. Super ...............O>M ........ O>S
3309..............error........ ...........3240 …………………………...….. -69
3322..............3335......... ..........3230 …………….. +13…………. -92
3303..............3315......... ..........3256 …………….. +13…………. -47
3325..............3337......... ..........3259 …………….. +12…………. -67
23……….......…..22……........………….29

6 BR

2830..............2829......... ...........2859 …………….. -1………….+29
2833..............2832......... ...........2859 …………….. -1………….+26
2822..............2820......... ...........2839 …………….. -2………….+17
2814..............2810......... ...........2836 …………….. -4………….+22
2824..............2823......... ...........2849 …………….. -1………….+25
19......…………..22…………..........…….23

284 Win

2843...............2837......... ...........2862…………….. -6………….+19
2851...............2855......... ...........2852 …………….. +4…………+1
2866...............2860......... ...........2878…………….. -6………….+11
2834...............2831......... ...........2845…………….. -3………….+11
2837...............2836......... ...........2865…………….. -1………….+28
32…….......……..29……….........……….33

338 RUM

3093...............3099......... ............3013…………….. +6………….-80
3109...............3117......... ............3036…………….. +8………….-73
3081...............3092......... ............3013…………….. +11…………-68
3097...............3105......... ............3009…………….. +8…………-88
3103...............3097......... ............3036…………….. -6………….-67
28……….......…..25……..........………….27

 

[Trickymissfit]

Bullet moving 3000fps in one direction, rifle 9fps in the other. Works out to an error less than a third of a percent. Big whoop.

must mean something as many of you trashed one chronograph for a .0025% difference from the Ohler #35, and yet no one could truly this which one was right. Now I'm also one that thinks 9fps is nothing to get alarmed about, but also realize that the 9fps is only the start of the equation. It's probably going to be a minimum of three times that and maybe as much as 4.5 times that with a high recoil round.

Not knocking any system, but can see an issue here as well.
gary

 

[morcey2]

That's exactly what I was trying to convey in my ramblings... LOL

It's the cold medicine talking. I still don't understand what I wrote.

 

[Trickymissfit]

As an engineer, I would figure you would be aware of velocity vs disance traveled. You might ask how that plays into this scenario? Here we go...

The bullet actually leaves the barrel WAY before you even feel the first hint of recoil. If a bullet is traveling 3,000 fps, and your barrel is only 26" long (2.17 feet), then it takes your bullet approximately 0.0007233 seconds to travel the full length of your bore and out of the muzzle, after ignition. Felt recoil will not happen until after the bullet has left the muzzle expelling all of it's gasses and energy into the atmosphere causing a rearward force (recoil).

I've waved my MSv2 bayonet around (while plugged in) to see if it would pickup anything, and I think they've got it set to where it has to have something solid traveling VERY close to it, for it to read anything, so I'd assume you'll be ok. Mine reads perfectly, and is very close to book load velocities on several of my loads, so I would guess it is pretty accurate.

actually there's a lot more to it than that. I used to have access to a mechanical CAD system that would read out in 1/10,000 of a second or less (think it went down to much less than that). It's mostly used to find stresses and reactive movement in a mechanical device. I drew up several different rifles with it to see what was going on when you pulled the trigger. I kind of used 60K psi as a standard pressure, as I didn't want to take the time to plug in several others. Not only did I see every action bend and torque, but saw barrels doing the same thing. Not only does the barrel try to unwind as the bullet is traveling, but it also stretches a little bit. Then it comes back because the steel was not taxed beyond it's yield point. The chamber also grows in every direction (more than most would think) I think there were nine vectors of force (counting reactive forces), and that didn't add in things like recoil lugs and movement of the stock. Short heavy barrels moved far less than the typical varmint profile (some moved as much as .10" at the muzzle). Fast twist barrels really are fun to watch a bullet go thru. They're like a coiled spring unwinding.
gary