308 muzzle velocity variation questions

[ripcurlksm]

Two questions shooting Federal Ascent 308 175gr on my chrono this month--

1) At 200 yd distance starting top to bottom, here is my cold bore shot (2660fps at muzzle), 2nd shot (2642fps), 3rd shot (2626). Could a 16-18fps difference between each 3 shots cause this much drop at 200 yards?


2) Here are two range trips with chrono data, my battery died in Nov so only got three shots recorded. Same rifle, same ammo (Federal Ascent 308 175gr), same gun range. The only difference is on 11/12 the conditions were 54*F @ 30.06 inHg and on 12/9 was 28*F @ 30.39 inHg. Could this 26 degree temperature and .33 pressure delta cause such a difference in muzzle velocity?

 

[Bluefish]

Two questions shooting Federal Ascent 308 175gr on my chrono this month--

1) At 200 yd distance starting top to bottom, here is my cold bore shot (2660fps at muzzle), 2nd shot (2642fps), 3rd shot (2626). Could a 16-18fps difference between each 3 shots cause this much drop at 200 yards?
View attachment 639723

2) Here are two range trips with chrono data, my battery died in Dec so only got three shots recorded. Same rifle, same ammo (Federal Ascent 308 175gr), same gun range. The only difference is on 11/12 the conditions were 54*F @ 30.06 inHg and on 12/9 was 28*F @ 30.39 inHg. Could this 26 degree temperature and .33 pressure delta cause such a difference in muzzle velocity?
View attachment 639722

Depending on powder, temperature can cause big swings in velocity. Varget is .5 fps/deg (low end) and a more unstable powder may be 2 fps/deg f. You had a roughly 25 deg difference, so the temp would swing speed 12-50 fps depending on powder.

 

[sierracharlie338]

IMO yes those variations are possible especially in factory ammo. As a side note I wouldn’t dive too far into the weeds with that chrono. Get you a solid average on velocity and go shoot because you can’t change anything with how it’s loaded.

 

[Vern400]

1. Your vertical stringing is not the result of velocity if your chronograph data is correct. You have a 34-fps ES. That's just your group size or perhaps your barrel is grounding out touching something. I have one rifle little shoot four MOA high if I prop it on a fence post without my hand under it.

2. Yes temperature can easily result in 100 fps change. And if you want to see a REALLY big drop in velocity, store poorly, or marginally sealed ammunition or powder in a humid location for a week or two. I just experienced over 100 FPS drop because I got stupid and left my powder in the chargemaster for 3 weeks. Forgot to put it back in the original container and the load went to crap. Now I got to dry it out and normalize it. Pain in the rear.

 

[EdP]

In regard to question #1, IMO, absolutely not. One way to check instead of just taking my or anyone else's opinion is to plug the numbers in a ballistic program and look at the results. Plug in your bullet BC and the 2660 MV and look at the drop at 200 for a 100yd zero. Then plug in lower MVs until you get an additional 2" of drop. For a 165gr GK it would take about a 400 fps delta to give an additional 2" of drop at 200 yds.

 

[ripcurlksm]

1. Your vertical stringing is not the result of velocity if your chronograph data is correct. You have a 34-fps ES. That's just your group size or perhaps your barrel is grounding out touching something. I have one rifle little shoot four MOA high if I prop it on a fence post without my hand under it.

2. Yes temperature can easily result in 100 fps change. And if you want to see a REALLY big drop in velocity, store poorly, or marginally sealed ammunition or powder in a humid location for a week or two. I just experienced over 100 FPS drop because I got stupid and left my powder in the chargemaster for 3 weeks. Forgot to put it back in the original container and the load went to crap. Now I got to dry it out and normalize it. Pain in the rear.

Extreme Spread (ES) is the same as range, correct? If so my ES/Range yesterday on 12/9 was 92 FPS. Here is my data sheet, with values added to the chart

 

[deadwolf]

I mean no offense here whatsoever, perhaps the vertical stringing is due to the shooter. What setup did you have when shooting the three shots on the target? Bipod, lead sled, rear bag, etc.

The good news is your windage is perfect : )


Sent from my iPhone using Tapatalk

 

[EdP]

We are looking at a 1 MOA group of 3 shots at 200 yds. I would not draw ANY conclusions from the shape of the group. Need to see at least 7 more shots to see if the vertical stringing is real and if the 1 MOA group holds.

 

[deadwolf]

In the words of shooting instructor Marc Taylor, 5 is a group, 3 is a ménage a trois.


Sent from my iPhone using Tapatalk

 

[ripcurlksm]

I mean no offense here whatsoever, perhaps the vertical stringing is due to the shooter. What setup did you have when shooting the three shots on the target? Bipod, lead sled, rear bag, etc.

The good news is your windage is perfect : )


Sent from my iPhone using Tapatalk

bipod on a free floated barrel and a rear bag

We are looking at a 1 MOA group of 3 shots at 200 yds. I would not draw ANY conclusions from the shape of the group. Need to see at least 7 more shots to see if the vertical stringing is real and if the 1 MOA group holds.

agreed on the group size, its a pencil barrel and was my last three shots of the day, trying to see how it groups with my first cold bore shot and two follow ups