puzzling chrono results

[Visionz45]

With hunting season approaching I worked up some 150 BT .30-06 loads for a friends Remington 710. Following my Lyman manual it stopped a grain short of their max with IMR 4064(52.0 grs was my load). The 710 sports a 22" bbl and yielded the following chrono data:
1: 2784
2: 2804
3: 2803
4: 2803
5: 2789
AV: 2797 fps
ES: 20 fps
(no sky screens and overcast, 50 degrees/damp)

Now today was my day off so I figured Id load some 150 bt's for my Montana 1999 with a 25 1/4" bbl. I worked up to the same load with ease and chrono'd the last two rounds I had in my box.
1: 3245
2: 3260

My chrono is a basic chrony F1 that has always giving accurate reading, I checked it today with some rounds that were confirmed by a friends Oehler. I'm surprised by this difference as I wouldn't have beleaved it and still dont beleave that 3.25" difference in barrels would create a 450 fps discrepancy.

 

[jerkface11]

It's now just a longer barrel it's a different barrel. Some just shoot faster than others. Or maybe the chronograph was closer to the gun this time?

 

[10mmreloader]

I have the same type of chrono and trust it as I often have been able to compare its results to other more expensive units. I suspect that your speed difference is WHOLLY due to the longer barrel. Those couple extra inches allow for much more energy release and constrain the burn. This produces that MUCH HIGHER fps readings.

 

[rcmodel]

It is very likely the Montana has a much closer to spec very fine bore compared to the Rem 710's cheap barrel.

Still in all, 3,250+ FPS with a 150 grain 30-06 using IMR-4064 is a pretty far stretch in any barrel. 3 1/2" more barrel would/could not account for an extra 450 FPS!

I would suspect a sky-screen was not set at the prescribed distance from the other one.

Or you were close enough to the screens to be measuring muzzle blast instead of bullets.

rc

 

[Visionz45]

I'll have to reload and do a retake, I totally agree that 3250 is a big stretch. I've pushed 150 accubonds to 3100 and that was at max and I reduced that load a bit.

 

[atblis]

Sometimes electronics need to warm up (applies to scales too). I have a chrono that has two distinct modes of operation. When it's cold, it reads high. It'll distinctly change to reading lower velocities very abruptly.

 

[Slamfire]

Instrumentation error.

Either your chrony is out of plumb, or you are shooting across the sensors at some wierd angle, or powder particles are going over the sensors confusing the heck out the the thing.

I put my chrony as far from the muzzle as readout and range limitations allow.

I plumb my Chrony.

I will shoot a "reference" load over the thing for a reality check. Then fiddle with.

 

[Visionz45]

I had moved the chrono and being out of plumb could be the culprit. I place it on a picnic table ahead of my bench that is level..now I'm going to look for tree pieces tomorrow and I suspect what Ill find. Maybe a tripod will be on the wish list.

 

[atblis]

"Out of plumb" could only make it read slower than it should.

 

[Ridgerunner665]

Its not so much the barrels...the chambers in those guns are quite different.

Remingtons have loooong throats, very long....I'm betting if you checked you'd find a short throat in that Kimber.

 

[USSR]

Now today was my day off so I figured Id load some 150 bt's for my Montana 1999 with a 25 1/4" bbl. I worked up to the same load with ease and chrono'd the last two rounds I had in my box.
1: 3245
2: 3260

Nice to know it was not overcast on your day off like it was the first time you ran your rounds over the chrony. What is going on is, you were getting direct sunlight on the chrony's sensors. The provided diffusers alone are not enough. What you need to do is attach the diffusers, then tape something to the two diffusers to completely cover the top and provide shade to the sensors.

Don

 

[cfullgraf]

I have two AR-15s with twenty inch barrels. With the same load, even the same lot of reloads, one chronographs 150-200 fps faster than than the other.

Different barrels with different chambers.

So, I would not be surprised at the difference in velocities from the two bolt rifles mentioned by the OP.

 

[Visionz45]

My barrel is a Douglas XX with a minimum chamber and the throat was reamed out for 180's/200's, I guess I would call it a long throat. The actions only was montana, the barrel/action are SS. I measure with an OAL tool and seat .01 off the lands. Today was overcast to the n'th degree, I assumed that no sky screens were the norm, at least my manual indicates that.
*Maybe my first cause some confusion, these loads were both worked up today.* Ill fill ya'll in tomorrow after I reshoot Thanks!

 

[DickM]

"Out of plumb" could only make it read slower than it should.

If by "out of plumb" we mean that one skyscreen is elevated relative to the other, I don't think that's true. In a perfect setup, a line between the two skyscreens would be perfectly horizontal and the bullet would travel a perfectly horizontal (parallel) path over them so that the distance of bullet travel between the screens would be exactly the same as the screen spacing.

As the line between the two screens departs from the horizontal, the horizontal distance between the two skyscreens decreases (by a factor equal to the cosine of the angle of departure), so the bullet (which we assume is still moving horizontally) passes over them more quickly. The processor, assuming that the distance between the screens hasn't changed, would interpret this to mean that the bullet is traveling faster.

That said, I don't think it's likely that anyone would set up the screens so poorly that "out of plumb" could account for the differences the OP is seeing.

 

[atblis]

As the line between the two screens departs from the horizontal, the horizontal distance between the two skyscreens decreases (by a factor equal to the cosine of the angle of departure), so the bullet (which we assume is still moving horizontally) passes over them more quickly. The processor, assuming that the distance between the screens hasn't changed, would interpret this to mean that the bullet is traveling faster.

Really? The thing that might change is the light source, and thus the way the sensors behave.

 

[DickM]

The thing that might change is the light source, and thus the way the sensors behave.

I don't think so. The sensors are just reacting to a momentary decrease in the amount of light reaching them and I don't believe the angle of the light source matters. I know that my own chronograph works fine at midday when the light source is close to overhead and also works fine either earlier or later in the day when the light is at a much more acute angle, as long as it remains bright enough (at which point it just stops working).

To expand a bit on my earlier post . . . If you start with the sensors mounted on a bar at distance d, imagine the bar being tipped up (or down) to a 30-degree angle - which is of course much more than anyone is likely to do inadvertently, but will illustrate what I'm saying. Then the original distance (d) between the sensors forms the hypotenuse of a right triangle, and the distance of the (horizontal) bullet path over the sensors is the length of the "adjacent" side, which is given by cos30*d, or about .87% of the original distance. The bullet now traverses this smaller horizontal distance in .87 of the time it did the original d, so the processor (assuming the distance to still be d) calculates a faster velocity by a factor equal to the reciprocal of .87, or about 15% greater.

 

[atblis]

Are you assuming that the sensors always detect vertically as defined by gravity rather than normal to the axis defined by the base?

 

[Visionz45]

I re-chrono'd those rounds with all of the considerations suggested to me, needless to say the actual number was 2950 which is exactly what I had predicted them to shoot at. I think the move to 15' probably corrected the readings as muzzle blast was likely the culprit.

 

[MCMXI]

To expand a bit on my earlier post . . . If you start with the sensors mounted on a bar at distance d, imagine the bar being tipped up (or down) to a 30-degree angle - which is of course much more than anyone is likely to do inadvertently, but will illustrate what I'm saying. Then the original distance (d) between the sensors forms the hypotenuse of a right triangle, and the distance of the (horizontal) bullet path over the sensors is the length of the "adjacent" side, which is given by cos30*d, or about .87% of the original distance. The bullet now traverses this smaller horizontal distance in .87 of the time it did the original d, so the processor (assuming the distance to still be d) calculates a faster velocity by a factor equal to the reciprocal of .87, or about 15% greater.

I'm with atblis on this one. His drawing does a good job of showing how the path length between the sensors INCREASES if the chronograph is tilted up or down. If the chronograph is tilted by 10°, the velocity would be approximately 15% slower than the actual velocity. The conceptual error that you're making is the assumption that the sensor planes (that the bullet passes through) remain vertical or perpendicular to the ground whereas the sensor planes rotate with the chronograph.

 

[Border Hopper]

An easy way to set your chronograph for the proper distance is to attach a small flexible wire or non-stretchy cord to the bench (or the chrono) and ALWAYS use it to maintain reliable distance from the muzzle. A measuring tape is dandy, but the cord is easier.

+1 to Slamfire1 for the level photo. Having a chronograph and using it well saves a lot of head scratching and re-redeveloping loads, saves powder, primers, bullets, etc.

 

[DickM]

I'm with atblis on this one. His drawing does a good job of showing how the path length between the sensors INCREASES if the chronograph is tilted up or down. If the chronograph is tilted by 10°, the velocity would be approximately 15% slower than the actual velocity. The conceptual error that you're making is the assumption that the sensor planes (that the bullet passes through) remain vertical or perpendicular to the ground whereas the sensor planes rotate with the chronograph.

OK, guys - got it, and we agree. You're correct about the error in my thought process, even though I was well aware that the sensor planes have to shift as the axis is tilted, just didn't think it through in the context of the problem at hand. Thanks.

 

[GooseGestapo]

I too have found that with the chrony, you have to set it further out for rifles than with handguns.

I was getting similar spurious readings when chronographing my .300RUM. I had to set the chrony out to 35' to get accurate readings.

The chrony is being affected by the muzzle blast and shock waves from the bullets causing "high" readings. I didn't think I was really getting 3,500-3,600fps from my 180gr loads.

By moving the chrony out to 35', the readings went to the nominal 3,250-3,300fps I was expecting.