In Defense of Magnetospeed - A Compendium to Set the Story Straight

[snakepit]

I beleive the POI change is gas reflecting of the bayo not harmonics, I've seen groups rotate around with the direction of the bayo, if rotated up the groups went low, rotated right groups go left.

I am not sure if the gas reflecting off the bayonet is deflecting the bullet since the gas is behind the bullet.

i think it might be the bow shockwave caused by the bullet traveling at supersonic speed through the air. The bow shockwave of the bullet is caused by air being greatly compressed at the tip of the bullet as it slices through the air. As the bullet moves forward a conical wave of compressed air moves out from the bullet tip and I think this shockwave over the bayonet may cause a slight deflection of the bullet. With everything being equal I would think this deflection would be consistent with each firing. Just a hypothesis.

 

[chkunz]

I am not sure if the gas reflecting off the bayonet is deflecting the bullet since the gas is behind the bullet.

i think it might be the bow shockwave caused by the bullet traveling at supersonic speed through the air. The bow shockwave of the bullet is caused by air being greatly compressed at the tip of the bullet as it slices through the air. As the bullet moves forward a conical wave of compressed air moves out from the bullet tip and I think this shockwave over the bayonet may cause a slight deflection of the bullet. With everything being equal I would think this deflection would be consistent with each firing. Just a hypothesis.

"I think this shockwave over the bayonet may cause a slight deflection of the bullet." Since the bullet is supersonic, is it possible for there to be interaction of this gas and the bullet?

 

[snakepit]

"I think this shockwave over the bayonet may cause a slight deflection of the bullet." Since the bullet is supersonic, is it possible for there to be interaction of this gas and the bullet?

I don't know for sure but on the Magnetospeed website there is a high speed photo of the bullet traveling over the bayonet and the gas exiting the barrel is visible and behind the bullet.

 

[Doug Beach]

There is a column of air in the barrel that is forced out ahead of the bullet, and undoubtedly encounters the m-speed. It would be interesting to know how the resulting turbulence interacts with the projectile.

 

[Nude nut]

When I made my bipod mount for my magneto speed the muzzle blast woul push the bato downwards and mess with the readings.
So I put two bolts in to stop this from happening but wha I found was even mounted off the barrel it would still push the pullet of POA so I mounted it further away from barrel an messed around with the sensitivity and it now is not affected by muzzle blast.
The furthest distance I have compared impact on target with it on and off is 900yrds and it did not have any affect on target both ten shot rounds held the same elevation on target only shots away from groups where wind related.
So is the magneto affected by muzzleblast YEP sure is no doubt about it I have no things bad to say about the magneto speed I just set it up to suit what I wanted from it im happy.
It never goes on the barrel any more no need the straps broken anyway.

Cheers Trev.

 

[chkunz]

There is a column of air in the barrel that is forced out ahead of the bullet, and undoubtedly encounters the m-speed. It would be interesting to know how the resulting turbulence interacts with the projectile.

Aerodynamically I can see that having an effect on the bullet. You may have hit on the answer.

 

[bigngreen]

If you look at high speed or shadow graphs the bullet has to clear a couple shock waves that are a head of it for a very short time, the small blast that comes from the gasses ahead of the bullet then the main muzzle blast envelops it for a short time before the bullet leaves it.
The interesting thing I've see with my rifles is that I mainly shoot braked rifles and never saw hardly any if any movement of my poi but my heavy varmint barreled 308 with no brake moves up a good bit, much more than any of the lighter contour braked rifles, so far the only thing that makes sense is the gas deflection.

 

[snakepit]

Bullet Shockwave Gas Column From Barrel

 

[chkunz]

The question is does the MS interact with the rifle or bullet in a way that would cause a change in group size or in the POI? Aerodynamically it appears the answer is no from the information presented so far. Mechanically it may but that gets very complicated also. I think the empirical answer is the most reliable in this case but it would take a careful, thorough experiment with enough data to reliably predict. This would take considerable effort. I think there are a lot of us that would like to shoot groups with the MS on the gun and have confidence we are getting good data for groups and velocity. A change in POI is not as important to most folks. Or we would like to know that we can not have both and just use the MS for velocity.

 

[gotcha]

The question is does the MS interact with the rifle or bullet in a way that would cause a change in group size or in the POI? Aerodynamically it appears the answer is no from the information presented so far. Mechanically it may but that gets very complicated also. I think the empirical answer is the most reliable in this case but it would take a careful, thorough experiment with enough data to reliably predict. This would take considerable effort. I think there are a lot of us that would like to shoot groups with the MS on the gun and have confidence we are getting good data for groups and velocity. A change in POI is not as important to most folks. Or we would like to know that we can not have both and just use the MS for velocity.

Hmmmm, Sure would be interesting to see group results with a heavy barreled rail gun. Seems as though this would eliminate some of the possible variables and prove group displacement is caused by gas deflection not harmonics Anyone?

 

[br1111]

Hmmmm, Sure would be interesting to see group results with a heavy barreled rail gun. Seems as though this would eliminate some of the possible variables and prove group displacement is caused by gas deflection not harmonics Anyone?

a chronograph is to read velocity. that's all I need it to do, not read velocity and hold groups. whats wrong with just running some test shots for velocity and then test the accuracy after that? for me the magnetospeed is a great piece of equipment. and by the way ,have some fun doing it.

 

[ericbc7]

There is nothing "wrong" with doing the steps separately, but
Chronographing every shot during load development is a good Way to conserve components and barrel life since you can evaluate a given load's consistancy when choosing between nodes.

 

[pressedRat]

i have one and use it. it effects some rifles differently from others. the fact that barrel tuners can be used to improve groups by simply moving a weight some small distance along the barrel or on the end is enough to demonstrate that any object that is attached to the barrel has the potential (and probably will) change how a rifle shoots.

i shoot series of load and then review. once i have a 100 (or sometimes 200) yard indication of a good load, i take it to 300 meters. Once a load is verified at that range, i put the ms on the rifle and do 10 shot groups to get muzzle velocity to use in calculating scope adjustments.

i see this is being one of the main advantages of the labradar. it is about as easy to set up and is completely detached from the firearm. but for me for now the ms is good enough.

 

[normmatzen]

I think you will find that the point of impact and the change in group size or tune of the barrel are only slightly connected but from two separate effects.
First, the Magnetospeed has two printed inductors mounted around a core made up of a powerful magnet, both mounted a measured space apart. If a small DC current is passed through the inductor, The size of the magnetic field around the magnet increases slightly. When your bullet passes through the field, a tiny current is generated in the metal bullet due to inductive coupling. This current subtracts from the inductor current and the resultant voltage spike can be read electronically. The first inductor passed causes a "START" signal and the next one a "STOP" signal which gates a counter to determine transit time which is converted to velocity.
Second, the magnetospeed weighs about 5.5 oz or so. Also, the way it is mounted sticking out in front of the end of the barrel makes it look like a weight on an extension of the barrel. This weight will tune (or de-tune) your barrel roughly equivalent to a 5.5 oz "tuner" weight a bit beyond the end of the barrel.

Regardless, it still indicates accurate muzzle velocity! But, I wouldn't use it to do any statistical load ladder style tuning, for that borrow your friend's tripod mounted light activated chroney!

 

[ericbc7]

Regarding load development using the Magnetospeed - the only issue I can see affecting group size (not poi) is the somewhat flexible mounting system. The fabric belt can allow some shifting of the bayonet's location on the barrel from shot to shot, which may affect repeatable group size.

 

[sbhooper]

Why is everybody so bent on leaving the chrono attached to do accuracy testing? Attach the chrono and figure out the velocity AFTER finding the accurate load.

 

[Nude nut]

As we read these posts then general conclusion is that a force of some discription is bouncing of the bayo and pushing bullet off intended POA then the conclusion that the bayo is acting like a tuner due to extra weight on the end of the barrel are all good conclusions but in my opinion it is more whatever forces are bouncing off the bayo is it.
As mounted off the barrel on my mount It still changed point of inpact UNTIL I moved the V block atleast 1 1/2 to 2cm from the barrel and adjusted sensitivty.
When I use to use in original form strapped on the barrel I had varied results all barrel dependant to profile and length as did many others as we all know. My thoughts are weight will effect the group size slightly and as mentioned what ever force is bouncing of the bayo effects POI now Im no egg spurt just noting what I have seen by using the bipod mount its not hard to make on then you could see the results first hand.And Im sure many of you would test it out much more than I have as I dont have accsess to a range to use willy nilly as much as I would like to.

Cheers Trev.

 

[Nude nut]

Why is everybody so bent on leaving the chrono attached to do accuracy testing? Attach the chrono and figure out the velocity AFTER finding the accurate load.

WEll one must ask themselves your comment is justified I did it for sometime with great results until the buckle broke and to be honest thats the only reason I made the mount didnt want to wait for replacement but reaped the benifits in doing so.
If I just want to do a quick velocity check I undo two bolts go to the range when I can and duct tape it to the barrel job done.

Cheers Trev.

 

[Ned Ludd]

I have fired thousands, if not tens of thousands of rounds over my Magneto. I was fortunate enough to receive one of the very first batch made/sold. I can tell you without question that it can and will affect both POI and group size/shape. Further, the specific observed effects are in fact, caliber, load, and rifle-specific; which is to say that sometimes groups shrink with the bayonet attached, sometimes they get larger; sometime POI increases by over 1 MOA at 100 yd, sometimes it is only marginally affected. I have not yet observed any specific trend, meaning the behavior can't be reliably predicted. If that doesn't bother you with regard to load development, that's fine, I certainly wouldn't try to tell anyone else how they should be using it. However, it matters a lot to me during load development, so I choose to load double the number of rounds and determine velocity data separately from group dispersion. YMMV. For what it's worth, I love my MagnetoSpeed. It's easily the simplest and most reliable chrono I've ever used, I just don't shoots groups for critical evaluation during load development with it attached.

 

[Ned Ludd]

Where is the bullet when the gas is being deflected off the bayo?

A couple years ago, there was a lot of discussion regarding the cause of the shift in POI. I and several others did extensive testing, some of which was reported here and some elsewhere, SH, I believe. The thought was that the magnet had something to do with the POI shift, which turns out to be very unlikely. The best conclusion that was reached from the various tests was that muzzle blast off of the sensor deck was the most likely culprit, although that may still be subject to question. The two most critical tests that point to this are that the change in POI goes in whichever direction the sensor deck us pointed as bigngreen mentioned above. The second is that the effect on POI seemed to be dramatically reduced when using th Magnetospeed with a muzzle brake, which diverts a significant amount of the muzzle blast off to the sides. I can't tell you exactly where the bullet is located when the muzzle blast crosses over the sensor deck. Most likely at the leading edge or slightly in front of the blast. However, the bullet wouldn't need to be fully enveloped in the blast to cause such an effect; a very small influence on the boattail/bearing surface would be enough. Regardless, the behavior of the POI shift is most consistent with the muzzle blast being the major cause.