Chrony not matching Book Data

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Slapshot

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I was out with my new Shooting Chrony this afternoon (that Santa is a wonderful gal:) ) and had some of my reloads that shoud be in exess of 3200fps clocked in at 2850. Anybody have any ideas about this?

load is:

257 roberts
75gr Hornady HP
46 grains of IMR 4350 (0.2 grains under book max)

This rifle has an extremly long throat, If I tried to seat these short bullets 30 thou off the lands, they wouldnt be in the case. Would that affect velocity?
 
Book data is only valid for the test barrel setup its measured with, and can only be a guide to your own experience. The chrony gives you benchmarks to make your adjustments against, and within the limits of the accuracy of the chrony (read the fine print about that) provides you with feedback as to how you're doing...
 
This is very normal and is the reason why you should take the velocities listed in the manuals with a grain of salt.There are far too many variations in chamber,throat and barrel dimensions as well as in lots of components to expect a given load to produce velocities in your rifle that match the velocities printed in reloading manuals.
 
very normal

if you have a 7mm Mag, try running that over the screens, but bring something to throttle at the range when you see the speeds :D
​
 
Personally, I have found Nosler manuals to be about 200fps too optomistic.
what book are you quoting?

PS Todbartell, nice avatar!
 
I could see 100fps difference, but this is 450. That is like 12-13% off. That is quite a bit. The case show no pressure signs, I might try to push it a little harder.
The data is from the second from newest Hornady manual.
 
Usually the loading data in the book also shows what barrel and length the test rifle was.... Were you using the very same components - substituting nothing?

Every barrel can be different and should be looked at as an individual.

Chronys are the cheapest on the market and not noted for exceptional accuracy.

The long throat reduces pressure and that results in lower velocity.

Taking all that into consideration, there is no clear answer to this.
 
guntech said:
Chronys are the cheapest on the market and not noted for exceptional accuracy.

.
Click to expand...

Gotta disagree with you on that, Guntech. I have shot the same ammo over a $75 refurbished Shooting Chrony and TWO Oehelers that cost $650 or so, and there was virtually no difference.

All the readings where within 50fps of each other.:)
 
The books correct the fps back to the muzzle,from a given length barrel often married to a universal receiver...Now heres the humdinger..Usually the average fps of THE FASTEST 3 shots,over several hundred....And MOST of all,according to the head tech at speer,"all this and more is done to make damn sure our arses are covered"
 
My Speer #13 Manual lists a max 49.0 gr. of IMR 4350 using a 75 gr. Speer Flat-SP bullet as 3265 fps from a 24" barrel. 45.0 gr. shows 2895 fps, which is close to your chrono results.

And you are using IMR 4350 not Accurate Arms 4350, right? Which does the Hornady manual list? Close to the same powders but not quite.


Fudd
 
Another factor to consider is temperature.

What was the temp at when they gathered the info? What was the temp you were shooting at?

Also, what is the altitude difference?

A lot of companies use a pressure barrel with a tight bore.

To many variables.

RePete.
 
Curious? - a difference of 450 fps is 15 percent in velocity, and 30 percent in energy - thats alot! Accounting for "safety margins" in load data, barrel length and friction, chrony error, etc - its hard to envision such as loss.
Here's another guess - you're using short bullets and a very long throat. Bullet exits case before engaging rifling, causing blowby around the bullet, with resultant loss of energy. Alternatively, the throat is extremely worn, allowing excessive blowby.
Do you get the same error with a longer bullet?
 
Slapshot said:
I was out with my new Shooting Chrony this afternoon (that Santa is a wonderful gal:) ) and had some of my reloads that shoud be in exess of 3200fps clocked in at 2850. Anybody have any ideas about this?

load is:

257 roberts
75gr Hornady HP
46 grains of IMR 4350 (0.2 grains under book max)

This rifle has an extremly long throat, If I tried to seat these short bullets 30 thou off the lands, they wouldnt be in the case. Would that affect velocity?
Click to expand...


The Shooting Chronys - and most other makes for that matter - are very particular about the position of the light overhead. I have found that trying to get a correct reading very early in the morning or late in the afternoon can really screw up your reading especially of you are not using the sky screens. Try shooting the same loads at mid-day using the sky screens.

Your temperatures in NE Saskatchewan at Christmas are also probably a LOT lower than the temperatures at the time Hornady did their testing. This will slow down your velocity somewhat too! ;)

The long throat in your rifle will not slow down the velocities. If anything it should allow you to boost velocities without developing excess pressures. As you are still a fair ways off of Hornady's max load I would try creeping it up a bit further unless the load you are shooting now is very accurate. In that case I would leave well-enough alone as accuracy is more important than a few hundred fps of velocity. LoL :D
 
Gatehouse said:
Gotta disagree with you on that, Guntech. I have shot the same ammo over a $75 refurbished Shooting Chrony and TWO Oehelers that cost $650 or so, and there was virtually no difference.

All the readings where within 50fps of each other.:)
Click to expand...

I don't know of a Chrony that uses three screens and measures the velocity of each shot twice as does the Oehler 35P. The 35P then compares those two velocities and if there is a descrepancy, marks it as an unreliable velocity or if the two velocity readings are within the acceptable standard, it marks the velocity as accepted. That eliminates some high and some low readings that actually are an error, while accepting high and low readings that are actually correct.

A chrony simply records each shot with no knowledge it was actually a correct reading.

Some Chonys are more accurate than others, you just never know which one is right or when it is.
 
[QUOTE='Boo]
The long throat in your rifle will not slow down the velocities. If anything it should allow you to boost velocities without developing excess pressures. [/QUOTE]


I disagree. If you increase the length of the throat and do nothing else, you have lowered the pressure and lowered the velocity. Pressure and velocity go hand in hand. You can not increase the velocity if you decrease the pressure.

If you take two identical barrels chambered identically and test a given load in both and the velocities measure approximately the same, the pressures would also measure approximaately the same. Now if you throat one of the barrels longer and then retest using the same test load, the shorter throat barrel will not have changed velocities or pressure, but the longer throat barrel will now have a lower velocity and an actual lower pressure. To get the same velocity out of this longer throated barrel, the load would have to be increased until the pressure is brought back up to equal the previous pressure it had with the shorter throat.

Old Weatherby rifles used to have very long throats and the ammo had to be loaded with a load that in shorter throated barrels would lock up a bolt. When Weatherby discontinued the long throats they had to sell all the old "hot" ammo before the newer rifles were on the market. Warnings went to the dealers advising them to make sure the new rifles used the new ammo and if any owners had old ammo to shoot it in the older rfiles only.

The new ammo in the new rifles had the same velocity as the old ammo in the old rifles....... because the pressure levels were the same even though the old shells were loaded with more powder.

If a throat is cut just long enough so a bullet is not seated past the shoulder area, there is no advantage to lengthen it. In lengthening it more you are simply shortening your barrel life and you will have to load more powder to get the same velocity you had before overthroating.

You can not have an ideal throat length for short and long bullets. It is a compromise if you use each end of the bullet weights available.
 
Here I disagree with you Dennis.

While chamber pressure and velocity are closely linked they are not directly comparable. Also total pressure is not the end-all and be-all of internal ballistics as most powder and bullet manufacturers have found out. The pressure curve, and how it builds, is actually just as - if not more so - important than total chamber pressure.

The longer throat allows the bullet to get a running start as it were before the chamber is hit with peak pressure. The most pressure a cartridge delivers is just as the bullet hits the lead. Extend that pressure curve a small fraction of a second longer and you can boast velocities by getting away with a larger powder charge. This is something that Roy Weatherby knew and that is why all the early rifles had their chambers cut with a long throat. The factory Weatherby loads were right on the edge - in not over - when it came to maximum pressures. All other things being equal that same load in a rifle with a short throat would not develop more velocity but it sure would boost pressures.
 
guntech said:
I don't know of a Chrony that uses three screens and measures the velocity of each shot twice as does the Oehler 35P. The 35P then compares those two velocities and if there is a descrepancy, marks it as an unreliable velocity or if the two velocity readings are within the acceptable standard, it marks the velocity as accepted. That eliminates some high and some low readings that actually are an error, while accepting high and low readings that are actually correct.

A chrony simply records each shot with no knowledge it was actually a correct reading.

Some Chonys are more accurate than others, you just never know which one is right or when it is.
Click to expand...
There is little question that the more expensive units are more consistent and accurate to a fine degree, however, for the average shooter, a chrony, properly set up, is accurate enough, and usually within a few fps of the expensive units. That's been the case with every side by side test I've read.
I suspect, being as it's his first go with the chrony, he forgot to calculate ACTUAL muzzle velocity, as there is a drop to the chrony, and he may not have the chrony set up just right either.
Read the manual.
Others have made the rest of the points, BBl length differences. Universal receiver vs sporter, temp, alt, is your load EXACTLY the same as theirs? Primer, case, powder and bullet? Case capacity, primer temp, even a crimp can sometimes make a difference.
 
With a long throat on this rifle a lee factory crimp may help. It'll get the pressure up a little quicker. Other than that add a grain of powder, just watch for pressure signs.
 
[QUOTE='Boo]Here I disagree with you Dennis.

While chamber pressure and velocity are closely linked they are not directly comparable. Also total pressure is not the end-all and be-all of internal ballistics as most powder and bullet manufacturers have found out. The pressure curve, and how it builds, is actually just as - if not more so - important than total chamber pressure.

The longer throat allows the bullet to get a running start as it were before the chamber is hit with peak pressure. The most pressure a cartridge delivers is just as the bullet hits the lead. Extend that pressure curve a small fraction of a second longer and you can boast velocities by getting away with a larger powder charge. This is something that Roy Weatherby knew and that is why all the early rifles had their chambers cut with a long throat. The factory Weatherby loads were right on the edge - in not over - when it came to maximum pressures. All other things being equal that same load in a rifle with a short throat would not develop more velocity but it sure would boost pressures.[/QUOTE]

I guess we will disagree.... pressure and velocity are closely linked and in the case of using the correct powder charge with a "normal" throat and then lengthening the throat, pressure and velocity are directly comparable... it is also demonstrated when you switch to moly coated bullets, you need to load more powder to get the velocity you had with bare bullets - this is due to the loss of pressure with moly bullets because they slide with less friction. Less pressure = less velocity.

Chamber a rifle with a normal throat so the bullet being used does not seat into the shoulder of the case and the bullet is just short of hitting the rifling, and work up a maximum load where the bolt handle is easy to lift, and record the velocity.

Now lengthen the throat one half of an inch and then fire and measure the velocity of the same loads as above.

I guarantee you will record a lot less velocity - why? - Because the pressure is lower .... and to regain the velocity you lost, you are going to have to load more powder to get the pressure back up.

I think Weatherby throated the original rifles the way he did to advertise no other cartridge could load that much powder.

The old Weatherbys with the old ammo are not any faster than the new Weatherbys with the new ammo.
 
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If you read some Denton posting on Chronys,he says the Chrony's data is within 5%,he backs that up with data.;) But I have had odd readings with my el-cheapest,from evening light, not seeing the bullet(felt-tip the bullet black),low battery,and rain drops.If in doubt, shoot a 22 over it.
 
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