Odd chrono results

[grumman581]

Handgun: Taurus 617 -- 2" barrel, .357mag
Bullet: 147 gr LRN
Powder: Hodgdon Clays -- 2.7 gr
Velocity: 141 fps

Yeah, that's right -- 141 fps. Some even lower than that. I was looking to create a light round for killing armadillos and such around the house, but my estimate was for around 600-700 fps.

[BABore]

It sounds like it's time for new chrono batteries or you need to back off a bit more from the screens. A stubby's muzzle blast may be causing it. For most of the cannons I shoot, I start out with a 15 foot screen distance. Test the screen's with a 22 LR until you get things setup right.

[dubber123]

I think BA's got it. You should have seen me trying to Chrony my 50-70 Contender with a new Chrony. I was so mad, I drew a nice bead in the readout with my last round and ALMOST put it out of it's misery. Later, I read the instructions.... and backed up a bit more. It worked perfectly. Blast does funny things to them.

[Bill*]

Amen to that! I knew something was wrong when around 5 grains of bullseye was pushing 200 grain .45 boolits to 6500 fps. Backed up and all was well.

[grumman581]

I could understand it reading high if I was too close, but low?

I had tried it from the same distance with a 9mm and I was getting around 1000 fps which sounded about right given the load that I was developing for it...

[dubber123]

I had 63 grs. of H4895 in a .375 H&H read 800 fps. last week... Backed up, 2,400 fps.

[StarMetal]

Another thing to the stuff mentioned above, which is very dead on the money, is sometimes I accidentally switch my chrono to the metric reading which gives a lower entirely different number. Mine is from Competition Electronics so I don't know if all the other makes have that feature. Worth checking by reading the instructions.

[Rocky Raab]

Here's how you get a too-low reading:

The first gas out pushes past the bullet (it is supersonic) and trips the "Start" screen timer.

But, it loses density fast enough that it doesn't trip the "Stop" screen. Much later, the bullet comes along and trips the "Stop" -- and all the while the timer was running. Result: a drastically slow velocity readout.

Readings that are far too fast happen when the blast wave manages to trip both screens.

(It is fascinating to realize that the pressure wave can be seen - or at least cause a shadow - because it is of a much higher density that the ambient air. I first learned that while flying in close formation with another jet, both of us supersonic. The shadow of my sonic wave was clearly visible on the other plane as a crisp shadow line!)