Magnetospeed Error Correction

[Mabia]

Shoot over the Magneto Speed and confirm with ballistic software at different ranges. We shoot over the MS and confirm at 1200 and 1000.

 

[MudRunner2005]

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

That's some cool stuff man, I wish you had slow-mo videos of all that.

 

[Trickymissfit]

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.

computer says differently that you guys. It says that the bullet is still accelerating after it leaves the barrel. Just as soon as it passes thru the muzzle you will see a jump in speed, and then a steady acceleration for a few feet afterwards. Where it peaks I can't say, but it appears to be about 15 feet and maybe even further. Remember the bullet passing thru the barrel has a compressive stress on it from the bore diameter, plus another stress from the twist in the rifling. Once it's free of these stresses, there is only the resistance of air pressure and gravity. From what I see the Magneto Speed reads a little past the muzzle; which of course is better.
gary

 

[Trickymissfit]

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

incorrect; the chamber itself grows under pressure. The barrel actually whips and stretches. The receiver moves in the bolt lug area rearwards. (depending on the recoil lug location.) This is from a transfer of energy.
gary

 

[Trickymissfit]

That's some cool stuff man, I wish you had slow-mo videos of all that.

my boss and I did it (he was a 600 yard bench rest shooter) just to see how well the system worked. I'd had it installed for a year or so, and went to school on it a couple times, but just never used it. This system was so good that it could predict failures long before they happened! In something like an internal combustion engine, it could give you an accurate temp on every bearing, and even a valve seat. Some metals were more predictable than others. Titanium being about the best. You ought to see what a typical loading die does with some of the big diameter cases!
gary