First time using a cronograph

[ripcurlksm]

I have some basic questions and I think this may be the best place to ask. Took out my new Rem 700 5R with a 24" barrel and some 175 Federal Match .308.

Its my understanding that the ballistic info (2600fps) on the back of the 175 match ammo box was done using a 24" barrel, yet:

1) My chronograph and rifle measured differently averaging around 2550 fps, or 50fps short. Is this normal?

2) Using the match ammo, I found a variance of 100 fps where I would have some loads close to 2500fps and 2600fps. Is this typical for match ammo and is 100fps acceptable?

3) In a perfect scenario where I had 2 rounds at 2550fps that grouped on top of each other and third round at 2600fps, would that round be considered a flier or is 50fps or 100fps not a large discrepancy to effect POI?

 

[vinsonr]

Re: First time using a cronograph

I'm going to assume you typoed your velocities and meant 2500/2600 etc. An extreme spread of 100 fps is pretty high and if it was accurate data then a batch of ammo with that amount of variance will result in differing POI. At 100 yards with a 100 yard zero it won't be so noticeable (the slower rounds will impact higher), but out at 300+ those slower rounds will drop more and impact lower.

My ballistics app calculates a 6.2" difference at 500 yards for two of my loads that are 125 fps different but the same bullet, brass and powder.

 

[ripcurlksm]

Re: First time using a cronograph

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Vinson</div><div class="ubbcode-body">I'm going to assume you typoed your velocities and meant 2500/2600 etc. An extreme spread of 100 fps is pretty high and if it was accurate data then a batch of ammo with that amount of variance will result in differing POI. At 100 yards with a 100 yard zero it won't be so noticeable (the slower rounds will impact higher), but out at 300+ those slower rounds will drop more and impact lower.

My ballistics app calculates a 6.2" difference at 500 yards for two of my loads that are 125 fps different but the same bullet, brass and powder. </div></div>

Yes that was a typo meant 2600 not 1600... /coffee

Thanks for that info.

Since I'm not reloading, would it make any sense to open a few boxes of the match ammo and weigh them all on a digital scale and group them into similar weights?

 

[ripcurlksm]

wrong data

thanks lost. i actually am home now and have my data to post. i made a mistake it was my m14 that was shooting +/- 100fps. Here is a 5 shot string for three types of ammo i had on hand.

Shooting <span style="font-weight: bold">Federal Match 175gr 7.62x51</span> out of Rem 700 5R 24" bbl:
2561
2554
2529 (27.75 fps slower than 4-shot average)
2558
2554

Shooting <span style="font-weight: bold">Federal Gold Match 175gr .308</span> out of Rem 700 5R 24" bbl:
2514
2510
2530 (16 fps faster than 3-shot average)
2518

 

[ripcurlksm]

Re: wrong data

One last followup. Given both of these (very) limited test shots for velocity, for the reloaders out there: How typical is it to get a 16-27fps variance in your loads?

 

[K_4c]

Re: wrong data

Depends on your choice of powder.. With varget out of my gas guns I've seen an ES of 25+....however I made the switch to CFE223 and my highest ES was 12 fps.

CFE223 is a very fine powder and meters well... Varget is a little harder to dish out consistently (I run a charge master with a modified McDonalds straw....works well.)

There's alot of variables and rolling your own takes out most of them with ammunition issues...

Best of luck

 

[Deadshot2]

Re: wrong data

Since this was your first time using a chronograph a couple of questions. Was your light source steady or was it changing such sa clouds moving across sky or changing shadows?

Was your chronograph on a sturdy mount. I had one that gave me weird readings until I realized that the cheap camera tripod was moving about in the muzzle blast.

Was your chronograph Level. If on an angle, sloped up or down, this changes the effective distance between the sensors.

What brand of chronograph? Some are more accurate than others in sensing the bullet. Any variation here, on the start/stop signals can give you a variation in reading that isn't present in the actual bullet speed. Some shooters will mark up a bullet with a "Sharpie" turning it completely black, so it "triggers" the sensor more uniformly. At least that's what they say.

Chronographs can be great and they can be frustrating. I've had mine set up, shooting at a 100 yard target, and making bug-holes with every string. The chronograph has often shown me that those nice accurate loads should have been lousy according to the ES and SD readings. I now just use mine to work up a load and when the "Paper" shows me the accuracy, I then use the chronograph to measure the speed of about 25 rounds or more, just for the average speed. That's what I use for ballistic's calcs, not the SD, MAD, ES.