Velocity question

**brickmanDan** · by **brickmanDan** on Mon 2 Nov - 20:09

Using published data and the ballistic tables provided in Nosler's Reloading Manual, I guesstimated a velocity for a particular load and dialed it in for a 300 yard zero. Here is my question: I dialed it in to be 3.5" high at 100 yards but when I shot it, I was actually 4.125" high at 100 yards. Does this indicate higher or slower velocities than I anticipated??? My initial thoughts were that I'm getting much higher velocities...but now I'm second guessing that line of thinking.

Is there a way to determine velocity based on ballistic tables, how much you dialed in, and how much higher you actually were.?.?...keep in mind that the 3.5" was based on a guesstimated velocity.

Thanks!

Joe B. · by Joe B. on Mon 2 Nov - 23:08

Did you account for height of scope center above bore center, very noticeable difference at 100 yards with a 300 yard zero. If you are using the ballistics chart online that I told you about the default sight height is .5" for iron sights, most scopes are centered 2" to even 3" above bore.

Also what is the actual ballistic coefficient of the bullet and actual muzzle velocity. Look like you are a bit slower than you estimated or BC is not as high as it should be for the type of bullet..

If you are zero at three hundred, play with the BC until your 100 yard height matches then turn it around and play with velocity till it matches, and see which one is more feasible. Older book velocities and pressure were often a crap shoot and also guesstimates, "safe in test gun only, reduce loads and work up" and not tested in pressure barrels. Speer #11 manual is notorious for this.

Example:
.25-06 120gr BT 2945 fps 300 yard zero 3.5" high at 100, BC=.507

Change BC to .312 and height at 100 is 4.125. A BC somewhere between a round nose and a soft point flat base spitzer or a hollow point possibly?

Or assume the boattail BC of .507 but velocity of only 2777fps for 4.125" high at 100, while zeroed at 300

Either your bullet or your velocity does not measure up to your guesstimate.

Joe B.

Joe B. · by Joe B. on Tue 3 Nov - 9:20

Yep, that hollow point you IMed me about ate your lunch due to the low BC. Match bullets are very accurate at specified target ranges with their rearward center of gravity, but not necessarily accurate out to really long ranges. The HP loses 100fps more velocity than the BTSP in 100 yards.

ronryder · by ronryder on Tue 3 Nov - 14:35

Unless you had a chrono graph, your ACTUAL velocity can be 75 foot either way from what the book load shows, thus, you had a WAG at best.

If you were that close, I would call it pretty darn good all in all. And until you shoot several groups for proof, you still cant make book on any end result.

Generally, one cant make predictions on velocity due to impact on target. Not 100% at least. Higher impact may result from more recoil, thus one could guess higher velocity, but dont take any of this too the bank.

If you hold the gun lightly, or in a death grip, all these can affect vertical impact.

and for another variable, unless you have perviously tested, you dont even know if the scope gives accurate 1/4 minute clicks or what ever the scope claims it gives..

Now I dont know if you mean you "dialed it in" by shooting, or you referred to the tables, and counted clicks, and trusted the scope to come up the desired amount.
I have seen all sorts of scopes, that you crank X number of clicks, and see NO difference on target unless going twice the adjustment.

Again, the scope in play, is just another variable to consider in getting shots on paper. Among the dozens of other variables. Cool

Velocity question

Velocity question

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