Velocity seems a bit high

[Hook'N'Bullet]

To start off, I'm shooting a 20" .308win using 150grn Sako Super Hammerheads (I love them)

I was curious as to what speed my bullets were scooting along at. Without access to a chrony, I decided to do the old, shoot a group at 100 (zeroed range), then shoot a group at 200 and fiddle the speed in the ballistic calculator to match the drop. Unfortunately, my max range I could get in the area was 186 meters. It will have to do. First group at 100 was bang on the money. Couldn't have been happier. I actually surprised myself. Next group at 186 meters was good too. Drop between the two groups was 3 inches.

I went into my Sako Balistics App, input my barrel length, cartridge info, scope height etc and fiddled the velocity to match the drop. To mimic a 3 inch drop between 100 and 186, the velocity would have to be running at around 2900fps.
Now to me that seems way too fast. The advertised velocity on the box is 2805fps which I assume is from a 24" barrel. I was expecting a number more towards 2600 - 2700.

Anybody have an ideas? Have I missed something?

 

[nun_hunter]

I don't think an extra 86m is really long enough or enough of a difference to be that accurate to calculate velocity with 100% certainty.

Assuming all the other data is 100% accurate like scope height, range is 100% confirmed, BC is accurate for the expected velocity then I'd put it down to not being far enough away to really be sure. Either that or you have a gun that shoots much faster than expected!

 

[Hook'N'Bullet]

I don't think an extra 86m is really long enough or enough of a difference to be that accurate to calculate velocity with 100% certainty.

Assuming all the other data is 100% accurate like scope height, range is 100% confirmed, BC is accurate for the expected velocity then I'd put it down to not being far enough away to really be sure. Either that or you have a gun that shoots much faster than expected!

Yeah I realize 186 wasn't ideal. I worked with what I had at the time. The fact I shot two decent groups, one at 100 and one at 186 I though would have given an accurate enough reading.

1. Ranges where calculated with a rangefinder so I'm happy that is accurate enough.
2. BC is taken from Sakos website at BC G1 @ .273
3. Scope height, here's what I did and resulted in 5cm/1.9inch height above bore
   1. I measured the diameter of the bell, divide by 2
   2. Measure the barrel diameter just under of the scope's bell, also divide by 2.
   3. Measure the gap between the bottom of the scope bell and your barrel.
   4. Add all 3 resulting numbers together to get your sight height.

 

[Hook'N'Bullet]

I don't think an extra 86m is really long enough or enough of a difference to be that accurate to calculate velocity with 100% certainty.

Assuming all the other data is 100% accurate like scope height, range is 100% confirmed, BC is accurate for the expected velocity then I'd put it down to not being far enough away to really be sure. Either that or you have a gun that shoots much faster than expected!

although, I just read a post on here of a person shooting a .308 130grn bullet at 3000fps so maybe my 150grn at 2900 isn't too far off, he might be making roll your owns though

 

[DVS1]

Truing your data in the process you are using is a method I use. I will zero at 100m then fire a group at 200m 250m and 300m to true my data. Never fired my 223 over a chrono and happy with my data. Granted I only have access to 400m but from the data I had gathered I had easy central hits on steel at the 400m

As you say and others you need a longer range to get better data

 

[Muir]

Anybody have an ideas? Have I missed something?

A chonograph? ~Muir

 

[Hook'N'Bullet]

A chonograph? ~Muir

Do you have one going cheap? Ship to Ireland, cash waiting

 

[Namman]

To start off, I'm shooting a 20" .308win using 150grn Sako Super Hammerheads (I love them)

I was curious as to what speed my bullets were scooting along at. Without access to a chrony, I decided to do the old, shoot a group at 100 (zeroed range), then shoot a group at 200 and fiddle the speed in the ballistic calculator to match the drop. Unfortunately, my max range I could get in the area was 186 meters. It will have to do. First group at 100 was bang on the money. Couldn't have been happier. I actually surprised myself. Next group at 186 meters was good too. Drop between the two groups was 3 inches.

I went into my Sako Balistics App, input my barrel length, cartridge info, scope height etc and fiddled the velocity to match the drop. To mimic a 3 inch drop between 100 and 186, the velocity would have to be running at around 2900fps.
Now to me that seems way too fast. The advertised velocity on the box is 2805fps which I assume is from a 24" barrel. I was expecting a number more towards 2600 - 2700.

Anybody have an ideas? Have I missed something?

This is not scientific or accurate at thesr ranges.
I trust your groups were .308 diameter?
Invest in a chronograph!

 

[GWP]

A chonograph? ~Muir

Does it really matter at stalking ranges

 

[Hook'N'Bullet]

Yeah

This is not scientific or accurate at thesr ranges.
I trust your groups were .308 diameter?
Invest in a chronograph!

Yeah I think I will, found a caldwell one on amazon for 150 so will pull the trigger so to speak and take out any guess work

 

[ofbiro]

I repeated the calculations with Quickload/Quicktarget. The result is 2890fps i.e. 889.9 m/s. Some measurement could be wrong, either the 3 ins drop or the 186 m or both.
A chronograph may be useful.

 

[Hook'N'Bullet]

I repeated the calculations with Quickload/Quicktarget. The result is 2890fps i.e. 889.9 m/s. Some measurement could be wrong, either the 3 ins drop or the 186 m or both.
A chronograph may be useful.

Thanks for your help, I've replicated it in strelock too and get 2890/2900 to mimic the drop give or take a centimeter. The 3 inch drop is correct, I was out with the tape measure and everything . I'm using a vortex rangefinder ranging the target and surroundings a couple of times to check give or take a meter. Nothing which would drastically affect the shot

 

[Smellydog]

Does it matter

 

[tom308w]

Hi I shoot 150 sako super hammerhead loaded with 44 gens n140 . Chrono at 2650 to 2700

 

[paul o']

What is the brass telling you post picture of the hot case , my Sako 85 with a standard barrel ran at 2870 very happy with a 150 Pro hunter.

 

[ejg]

I bought a chrono a few years back off E-Gun in Germany, best buy. Took all the guessing out especially when choosing factory ammo. They differ hugely.
edi

 

[andyk]

Someone (Virtus maybe) did a thread recently talking about how BC changes with velocity and how BC figures are averages across the range of velocities the bullet is used at.

IIRC the figures were better at higher velocity and dropped as the bullet slowed. That could mean that you’d have a better BC at close range and a worse one at longer.

I can’t recall it all so that may be wrong.

Does strike me that while your approach is useful. It’s going to have a margin of error. Potentially quite a big one as even a large change in velocity isn’t going to make a big difference at those sorts of ranges.

 

[Hook'N'Bullet]

Does it matter

What? Am I not allowed to be curious?

 

[Hook'N'Bullet]

Someone (Virtus maybe) did a thread recently talking about how BC changes with velocity and how BC figures are averages across the range of velocities the bullet is used at.

IIRC the figures were better at higher velocity and dropped as the bullet slowed. That could mean that you’d have a better BC at close range and a worse one at longer.

I can’t recall it all so that may be wrong.

Does strike me that while your approach is useful. It’s going to have a margin of error. Potentially quite a big one as even a large change in velocity isn’t going to make a big difference at those sorts of ranges.

Yeah you make a good point. Thanks for the input

 

[Hook'N'Bullet]

I bought a chrono a few years back off E-Gun in Germany, best buy. Took all the guessing out especially when choosing factory ammo. They differ hugely.
edi

Yeah I just bought a cheap Caldwell one.