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Discussion Starter · #1 ·
Early this week I bought a Ruger American .17 HMR compact model. I have a Savage 93 .17 HMR with a bull barrel but wanted something a little lighter for carrying in the field. I have always taken the ammunition manufacturers word for the velocity of this ammo product. So today I decided to run some rounds through the chrony to see if there was any loss in velocity with the compact models 18 inch barrel. With ten shots I recorded a high of 2687 fps, a low of 2629 fps, and a spread of 58 fps. Average was 2657 fps, well above the advertised speed of 2550 fps. I don't think there is anything wrong with my chrony as it records at or slightly below center fire ammo advertizements and is also very close to reload data.

Has anyone else with a 17 HMR ever checked to see just what kind of velocity you are really getting from your rifle?
 

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When the barrel heats up, it changes the muzzle velocity. I'm not sure it would account for that much though. I've never seen numbers.
 

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It is possible the HMR burns all its powder in the 18" tube and the longer barrels are just friction slowing it down. I would think the factory used a 22 or 24" for testing velocity.
The 28" barrel on the Ultra Lux CZ rifles are slower than the shorter ones.
 

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Discussion Starter · #4 ·
When the barrel heats up, it changes the muzzle velocity.
A hot barrel will affect the burn rate of some powders, some more and some less. This can account for a possible increase in velocity. However, even taking that into account it does not explain the higher velocity the first 3 or 4 shots from a cold barrel.

I would think the factory used a 22 or 24" for testing velocity.
Running the rounds through the Savage with a 22 inch bull barrel also shows speeds far above the factory speeds.

Found an article by Chuck Hawks on testing different brands of 17 HMR ammo and his results pretty much parallel my findings. Kind of dispells the rumor also that all 17 HMR ammo is loaded by the same company but only branded differently.

  • CCI - high 2613 fps; low 2517 fps; extreme spread 96 fps; average = 2559 fps.
  • FEDERAL - high 2631 fps; low 2539 fps; extreme spread 92 fps; average = 2595 fps.
  • REMINGTON - high 2626 fps; low 2591 fps; extreme spread 35 fps; average = 2609 fps.
  • HORNADY - high 2698 fps; low 2621 fps; extreme spread 77 fps; average = 2658 fps.
There is one fly in the ointment though....the rounds I was shooting were culls that had visible split necks. Out of 300 rounds I found 55 of these and separated them from the others. I guess it is possible these rounds may have increased chamber pressure but I don't know how. No problem with extraction. These split neck rounds don't seem to affect accuracy though. I've only had a chance to shoot this gun at 50 yards while setting up the scope and 3 shot groups averaged .242 inches on three targets. I'm sure this gun is going to be a shooter.

 

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Lets do some quick math. We have to eliminate sample variation as a cause. We can use the range of values to approximate the standard deviation. From that, I can construct a confidence interval for the mean muzzle velocity.

If the sample size were small and the standard deviation was large. Sample variation might account for your discrepancies.

Can you tell me how many rounds you used to generate your numbers? The raw data would be better.
 

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Discussion Starter · #7 · (Edited)
I've already done that for you by shooting the industrial standard ten shots. As stated in post #1, "With ten shots I recorded a high of 2687 fps, a low of 2629 fps, and a spread of 58 fps. Average was 2657 fps"

A google search showed me that a higher than advertised speed is normal for the 17 HMR. Some shooters found the speed to be as advertised, some found them higher as I did but none found the speeds lower than advertised. I was simply surprised to discover this but now I know......
 

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Did you happen to calculate the standard deviation? The range or "spread" is less useful for measuring variation. I'll take your word that 10 shots is the industry standard. You would know better than me. I have a PhD in math, so I only know numbers.

If we approximate the Standard deviation using the range we get 14.5 fps.

Using a 95% level of confidence, we can compute the margin of error on your sample mean. That is plus or minus 9 fps. That is, your measured muzzle velocity average is probably no more that 9 fps more than the actual average muzzle velocity.

That completely supports your conclusion that the advertised muzzle velocity is incorrect.

I fully suspect that the calculated margin of error would be much less using a more accurate number for the standard deviation. This only makes your conclusion stronger.
 

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Discussion Starter · #9 ·
I wouldn't go so far as to say the advertised muzzle velocity as advertised is incorrect. Not knowing what test barrels were used, test equipment, and lab conditions, their velocities probable are correct for the methods they used. But, for the average shooter with the equipment and rifles available to them, the results are certainly different. The surprise here is generally when one tests muzzle velocity of factory ammo the result is normally right at or just below advertised speed. The only reason I ever test ammo with the chrony is to be able to calculate the rise and fall of the bullet from line of sight which aids accuracy in distant shooting.
 

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The current issue of "Rifle Sporting Firearms Journal" has a nice article on the 17 HMR caliber. In the add it states just what you're saying in that the actual velocity of the rounds exceed what the mfr. claims across the board. They said that deviation ran an average of 20-50fps and up to 100. So you're finding out the same thing that they found. They don't mention testing with different barrel lengths though.
 

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Your results are not surprising at all. Here's an excellent article by Bullberry, the folks that make custom barrels for Contenders and Encores. Their tests showed a loss of only 132 fps between an 11 inch barrel and a 22" barrel.
 
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