Chrony, parallax, trajectory

[Guest Lester Weevils]

Finally got a chrony. It is smaller and lighter than I expected from the pictures in the catalog. Which is fine because it will be less trouble to tote to the range.

But until having the device in hand, hadn't given much thought how to avoid shooting it to pieces. Instructions say the bullets need to pass about 6 inches over the center of the chrony.

Ferinstance if a rifle is zero'd for 50 yards, and the chrony is set up 6 or 10 feet away, then the bullet is gonna be some amount lower than the point of aim when it passes the chrony.

Or if a pistol hits point of aim at 10 yards, and the chrony is 6 or 10 feet away, same deal, bullet will be some amount lower than point of aim.

Any tips on guestimating how high to aim, to compensate for parallax/trajectory, and avoid shooting the chrony?

Was thinking worst case, could set up a "real estate sign" type of cardboard portable target holder a couple of feet behind the chrony, and test the point of aim versus point of impact before moving the chrony into position?

I tend to over-think everything, but OTOH it would be embarrassing to shoot the chrony!

[Westwindmike]

I look down the side of the barrel and make sure it is not pointed lower than I want.

Oh, and I had a friend weld up a steel bullet deflector that my Chrony sits on in case I "miss".

Edited August 28, 2010 by Westwindmike
Pics added

[Guest Lester Weevils]

Thanks Mike

That bullet deflector is a fabulous idea!

[Westwindmike]

Thanks, Lester.

I drilled and tapped the deflector to screw right on to the tripod I use. Then put another screw up through it to secure the Chrony to the deflector. Works great.

[mikegideon]

Which chrony do you have? Mine has a lot of room under the screens. You need to use a good rest on a stable bench. It's also a real good time to get a ballistic program. I use this one, but there are a bunch out there.

Dexadine Ballistics Software - ballistic data for shooting and reloading

It's also sold by Oehler Research. You can work the velocity curve, and simply bump the muzzle velocity in the program until it matches the measured velocity at 10 ft. No need to even use a target when you're clocking boolets. The stories I've read about folks shooting their crony's usually involved something goofy. Westwind's shield idea isn't bad if you have another way to see the display.

[Westwindmike]

. Westwind's shield idea isn't bad if you have another way to see the display.

My Chrony has a remote display that sits on the bench with me. And a remote control.

[mikegideon]

My Chrony has a remote display that sits on the bench with me. And a remote control.

In that case, it's probably a great idea. I just have to avoid shooting mine

[Guest Lester Weevils]

Hi Mikegideon. Mine is an F1 Master chrony. It looks like Westwindmike's, but I get the impression that all the chrony chassis look about like that. It requires an optional box or printer to output shot stats, Mean, SD, etc. The little included remote display box displays velocity only of each latest shot, but I don't currently expect to use the gadget very often, because I don't experiment much with loads once I find a conservative recipe that works good enough.

For a starting experiment, I have a couple of different factory self-defense 9mm loads that use XTP bullets. One brand is significantly more expensive than the other. And have been reloading the same XTP bullets lately. Am curious how they compare. Also have some Rem Golden Saber and Corbon 9mm's to compare.

[mikegideon]

I bought mine for a different reason. Most of my rifles have shorter barrels than the ones used to specify the muzzle velocity of factory ammo. Muzzle velocity is a biggie in calculating trajectory.

I don't reload yet. I'm guessing that you're going to find real close muzzle velocities in bullets of of the same weight with the same amount of powder. There are folks on here that can tell you off the top of their heads. Trajectories may be different because of different ballistic coefficients, but it may not matter with a pistol. I'm not a good enough pistol shot to worry about it. I'm pretty much minute of thug at 25 yards.

[Westwindmike]

Lester,

Here are the plans to make a remote switch to plug into your Chrony remote display. It will then show you all kinds of good info. Avg velocity, high velocity, low velocity, standard deviation, etc. Easy and cheap to make one. I used an old stereo headphone to get the cord and plug. Bought a momentary switch at Radio Shack for a couple of bucks and housed it in an old 35mm film can(free). Works great!

nopLabs, Shooting Chrony Remote Control

[mikegideon]

Lester,

Here are the plans to make a remote switch to plug into your Chrony remote display. It will then show you all kinds of good info. Avg velocity, high velocity, low velocity, standard deviation, etc. Easy and cheap to make one. I used an old stereo headphone to get the cord and plug. Bought a momentary switch at Radio Shack for a couple of bucks and housed it in an old 35mm film can(free). Works great!

nopLabs, Shooting Chrony Remote Control

I think Lester said once that he's an old audio guy, so... Lester, that's a Switchcraft male XLR housing with regular mic cable. All guitar center stuff (duh, except the switch), or a good Rat Shack.

Edited August 28, 2010 by mikegideon

[Dolomite_supafly]

If the gun is sighted in for longer than the chrony distance I always aim just below the sky sceens, like 1/2 inch or so. I have never had a strike.

You can also tape a piece of paper on the front fork and place a aiming mark just below the sky screens.

Dolomite

Edited August 28, 2010 by Dolomite_supafly

[Guest Lester Weevils]

Thanks All for the great advice.

Thanks Westwindmike for the pushbutton schematic. Looks great and I have all the parts out in the shop. It would have come in handy today, copying off just mean and SD into the Palm 700, rather than typing in the individual velocities of 10 shot strings.

Managed to avoid shooting the thang so far, using Dolomite's method. Aimed slightly lower than the sky screens.

Held off getting a chrony for a long time because I expected it to be a hassle to tote and use, but it wasn't much trouble at all. I like to carry everything except rifles in a giant Cheaper Than Dirt Israeli duffel bag, and didn't expect a chrony and tripod to fit in there. But the thing is small and light. I have some heavy-duty telescope tripods that would be a hassle to tote and set up, but bought a $30 Walmart Targus tripod that fits in the range bag fine and is plenty steady enough to hold the Chrony.

Interesting results first time out. Lots of instruments require skill to get proper measurements. The Chrony may require more skill than it first appears, but it captured every shot except one, and it appears to be a Go/NoGo kinda thang. It will either display a measurement or not. The measurements seem consistent and reasonable, but MAYBE it can be more or less accurate depending on how accurately you pass a bullet over the sensors. Dunno.

[Guest Lester Weevils]

If a pistol is basically a decent quality machine, would it be likely that slop in the pistol would significantly contribute to standard deviation in velocity?

Measured some .22 ammo with a Beretta 92 and the Beretta .22 conversion slide with 5.1" barrel.

CCI MiniMag, 36gr HP-- Mean=1167.8, SD=30.7, FtLbs=109, RelativeSD=2.6%

Win 22T Target, 40gr lead-- Mean=1063.7, SD=26, FtLbs=100.5, RelativeSD=2.4%

Fed BulkPak 36gr HP HighVelocity-- Mean=1062, SD=16.3, FtLbs=90.2, RelativeSD=1.5%

If the pistol can affect StdDev, then perhaps some other pistol would have lower overall SD's.

Seems to answer why CCI Minimags seem to operate my semiautos more reliably than most ammo-- More FtLbs to operate the mechanism!

But it is also interesting that the cheap Federal 550 round bulk ammo had the lowest Relative Standard Deviation. Unless weaker power ammo interacts with this pistol mechanism to result in lower Relative SD. The Relative SD allows one to directly compare the 'tightness' of different-mean samples. RelativeSD = (SD * 100) / Mean

[Guest Lester Weevils]

Tested some 9mm ammo in a Beretta 92

Fed Champion 115gr FMJ (from walmart)-- Mean=1156.8, SD=7.0, FtLbs=341.8, RelativeSD= 0.61%

Win White Box 115gr FMJ (from walmart)-- Mean 1212, SD=11.6, FtLbs=375.2, RelativeSD = 0.96%

Corbon 115gr +P JHP-- Mean=1405, SD=7.1, FtLbs=504.2, RelativeSd = 0.51%

=====

Hornady Custom XTP HP 124gr-- Mean=1139.8, SD=12.7, FtLbs=357.8, RelativeSD=1.1%

Fiocchi Extrema XTP HP 124gr-- Mean = 1082, SD=9.9, FtLbs=322.4, RelativeSD=0.91%

My 124gr XTP HP Reloads, 5.4gr Silhouette powder, 1.06 OAL--Mean=1114.2, SD=6.7, FtLbs=341.9, RelativeSD=0.6%

The Corbon was advertised on the box as 1350FPS, 466FtLbs. So the Corbon measured higher than advertised, and it doesn't feel especially stiff in a full size pistol

I don't see any velocity advertisement on the Rem Golden Saber box.

The Hornady Custom was advertised on the box as 1110FPS, so the measurement is close to what is advertised.

I don't see any velocity advertisement on the Fiocchi box.

Am not aware if it is settled that higher velocity or energy definitely correlates to better stopping power. The Corbon, Golden Saber, Hornady Custom, and Fiocchi all appear well made of good components. But the Corbon +P is head-and-shoulders hotter fer sure, and the Corbon has the lowest Relative Standard Deviation. Appears to be pretty good stuff.

My XTP reloads are almost as hot as the Hornady Custom. Probably another 0.1gr would match it. Same OAL as the Hornady Custom. Relative SD of mine is very close to the Corbon, but the Federal Champion FMJ also has a pretty low RelativeSD.