.308 Reload Velocity Calculations

[texasjohn]

I started doing some reloading for my .308 recently and I have a question about velocity.

First, I do not (yet) have a chronograph, and I am not super concerned about absolute velocity, except that it is interesting to me.

I am loading 168gr Sierra Game Kings with 43gr or Varget and CCI Primers and Federal brass. This is out of a 20 inch barrel on a Savage PC Model 10.

The data I have says my velocities should be 2600 - 2700fps. So, I plugged this and the bullet data into a ballistics calculator to find my estimated drop so I could move from 100 to 200 yards easily. That was 2.2MOA which is roughly 9 clicks UP for me.

I put in 8 clicks, and was shooting high, dropped down 2 clicks and started hitting pretty close. This is a total of 6 Clicks up or 1.5MOA.

So, when I try to reverse engineer me velocity from my data, and I get 2950fps! I didn't have time for a third datapoint (300 yards) but it seems like this must be right. But it seems very fast for a 168gr out of a 308.

Am I missing something?

 

[USSR]

No way in hell your 43gr of Varget load is generating 2950fps out of your 20" barrel. I would suggest you do a box test of your scope to determine if your W&E adjustments are actually what the manufacturer is saying they are. Normal up adjustment for a rifle such as yours is +2MOA to go from a 100 yard zero to 200 yards.

Don

 

[No4Mk1*]

Did you enter your scope height into the calculator?
More likely I think you probably had some error in your sight in. Maybe you are really a half inch too high or low at one or both ranges. That would produce much error in your reverse engineered velocity.
Was the barrel a different temp when you sighted in at a different range?
Just some random thoughts because I imagine your velocity is around 2500-2600.

 

[Dave P]

Quickload says 2550 fps.




I say, from 100 to 200, any normal 308 drops 3 MOA.

If your scope is .5 minute clicks, then this fits your data perfectly. Are you sure your scope is .25 minute clicks??

 

[texasjohn]

Yeah, that is exactly what I was thinking. There isn't even load data for pushing the bullet that fast (nor was I trying to ).

The scope is a Leopuld Vx3, so I assume its adjustments are true. It may be that I am just shooting low at the 200 yard target, making my adjustments off.

I have a feeling I will be buying a chronograph one of these days...

 

[texasjohn]

Yes, it is 1/4 MOA adjustments.

 

[texasjohn]

Did you enter your scope height into the calculator?
More likely I think you probably had some error in your sight in. Maybe you are really a half inch too high or low at one or both ranges. That would produce much error in your reverse engineered velocity.
Was the barrel a different temp when you sighted in at a different range?
Just some random thoughts because I imagine your velocity is around 2500-2600.

Ahh, I will check that. I left it at the default. Where do you measure the scope height to? Center of the objective?

Also, I should note that I have a 20MOA base on the rifle. I didn't think this should effect anything, but maybe it does?

 

[rcmodel]

You simply can't reverse engineer velocity by measuring drop.

Drop is not only dependent on velocity, but also by the Ballistic Coefficient of the bullet.

The BC can change drastically from what the manufacture says it is by simply shooting it in a different barrel with a different, or even the same rifling twist.
Other factors that effect it are altitude, humidity, and barometric pressure

If it starts out completely stable it will have a higher BC.
If it starts out wobbling and then settles down further out, it will have a lower BC.
If the air is denser, it will have a lower BC.

rc

 

[texasjohn]

That makes sense.

Basically, I can measure my bullet drops to determine how my rifle is shooting at various distances so I can create a table to help me remember adjustments for my specific rifle, but I don't have enough data (true BC, etc.) to make declarative assumptions about velocity.

That being said, any recommendations on a decent Chronograph? What would a basic setup need? The couple I have shopped around for have had some different options, but I wasn't sure what it really the best thing for me to get.

 

[rcmodel]

Shooting Chrony or PACT maybe?

Just be sure you get one that has the read-out & buttons on an extension cord on the shooting bench.

The least expensive Shooting Chrony and others have the LCD display and re-set buttons mounted on the box that also holds the sky-screens.

You don't want that one.

rc

 

[Zak Smith]

Well, you can reverse engineer muzzle velocity from drop if everything else is known. The actual BC values on most match bullets is known, although I'm not sure how close the GK matches or has been independently tested. Many people who try to do this, do it wrong, usually by making a procedural or measurement mistake along the way.

Short answer, buy a chronograph. It'll pay for itself in very short order.

 

[texasjohn]

Yeah, that is the kinda info I need Makes sense that not having to walk back and forth a lot to see readouts would be useful.

Anyway, thanks everyone for the help.

 

[JDGray]

I run that same load from my Savage 10FP 20", except I'm shooting SMKs, load gives me 2640fps avg. Hows that load of yours grouping?

 

[Jim Watson]

Well, you can reverse engineer muzzle velocity from drop if everything else is known.

I recall an old Gun Digest article titled 'The Trajectory Chronograph' that gave methods for estimating velocity from POI at different ranges. As Zak said, you have to know EVERYTHING else and make meticulous measurements.
The price of consumer chronographs these days make it hardly worth the time on the nomographs and slide rule.

 

[kludge]

Yeah, that is the kinda info I need Makes sense that not having to walk back and forth a lot to see readouts would be useful.

I put my Chrony at 15 feet. I can see the readout just fine. I think I paid $80 at Midway.

Best money I ever spent on a piece of reloading equipment. More useful even than a reloading press.

 

[texasjohn]

I run that same load from my Savage 10FP 20", except I'm shooting SMKs, load gives me 2640fps avg. Hows that load of yours grouping?

The rifle I think is capable of sub MOA. I have on occasion done 3 shots in a group less then an inch at 100 yards, and less the 2 inches at 200. Though I tend to not be patient, and then go for a quick 4th and 5th shot and mess the whole target up.

At this point it is me that is holding things back I am convinced.

The first pic is my latest 200 yard target, and the second is my 100 yard.

In the middle of the 100 yard target, I adjusted my bases a bit to get soem more eye relief but ended up with that nice 3 shot group on the bottom left. On the 200 yard, the top of the target was me shooting and adjusting to the right drop. I couldn't see where I was hitting at first (high) and then the bottom right was after I came back down a little.

I have a lot of work to do to get everything working better, but like I said, i think it is me, not the rifle