Range Report .308 trajectory from JBM doesn't match actual

[Namster]

Greetings everyone, I'm new here. Joined this forum to gather information.

I'm also relatively new to shooting this .308 that I picked up so wanted to see if anyone here could help with a few questions.

..please don't knock my choices in gear I decided to use to put together, this post is to find out about trajectory.

I have a Savage 10 Bas-K, .308 with a 1 in 10" and 24" barrel. Stock. I attached a magpul prs to the rifle and put on a Tasco SS 16x fixed scope on it.

I was given a box of GM308M from a friend to start with. So I looked up the trajectory on my ipod with the ballistic calculator. I think I found the correct load in the database: 168gr Sierra MatchKing BTHP under the "Federal Listing", the velocity listed in the program matches the box. I have no clue which drag model to use.

So, went to shoot it at a 100yd range here in Southern AZ and didn't input temp or altitude..though I think the altitude is about 4k feet.

So for a personal reason (cuz I wanna shoot golf balls) I set the vitals to .9" and was attempting to Zero the rifle for 160 yards, which keeps me within the vital range up to 184 yards.

Targets set with a laser rangefinder.

So I started at 25 yards and put made sure it shot about -1.19" just like the charts, moved to 50 and made sure it shot at -.21" again, just like the chart. Went back to 25 and it was holding the same shot. So I took a shot 100 expecting it to be at +.76" but it landed consistently at +1.76" ...

The range is about 100 yards wide and 100 yards deep with a 10" berm at the end and on both sides. There was a little wind, perhaps under 8 mph coming from different directions.

Each shot at 25, 50 and 100 overlapped each other basically forming a figure 8..but sideways.. I guess an infinity symbol.

What gives? Could someone shed some light on this?
Am I using the wrong BC model?
Does the muzzle velocity assume a certain length barrel?
For some reason, I considered the MV could be faster than advertised due to the longer barrel, but assumed if so, at 100yd I would impact a touch lower.

My friend was shooting his 300 WINMAG today and all his shots were consistent to the ballistics program at 25/50/100 yards.

 

[George63]

Re: .308 trajectory from JBM doesn't match actual

Does the muzzle velocity assume a certain length barrel?

yes, to use a program well you will need to accurately measure MV, get your other info right, temp, elev. sight over bore height etc. measure bullet drop over the longest distance you can: 50 yd to 300 yd is excellent then adjust the BC # on the calculator to match the measured data, now the calculator will be very accurate providing the load does not change

 

[goober]

Re: .308 trajectory from JBM doesn't match actual

first off all please fill out your profile-always nice to know where your from and other stuff relevant about you.

Save yourself time and buy a chronograph and find your average
speed. down load the Berger ballistics program free
and fill it out properly.measure centre bore to centre of scope for scope height -find out your altitude,temp,barometric pressure for where you shoot and input to program.use a known projectile like a berger who advertise their g1 and g7bc and their you have it

 

[EMorr]

Re: .308 trajectory from JBM doesn't match actual

While I’m far from an expert and I have not contacted the individual ammunition makers to verify this I was under the impression that the majority of ammo makers tested their rifle ammo through a 24” length barrel unless otherwise noted. I know Double Tap lists the barrel length the ammo was tested in on the boxes but most other makers do not. That is not to say you cannot see an increase over the published velocity but without out a chronograph or taking detailed drop measurements at varying distances to confirm it there is no point in guessing.

In response to your thoughts that a faster bullet impacting lower on the target this is incorrect. If you fire 2 bullets, one moving at 2000 FPS and one moving at 3000 FPS along the same trajectory the faster bullet will always strike higher on a target at identical distances from the shooter. Conversely if you adjust your scope for the faster bullet to hit at the same point at let’s say 200 yards but measure the bullets impact at 100 yards. The slower bullet adjusted scope will impact higher than the faster bullet adjusted scope.

In other words slower bullets need more or a lob to impact at the same point on a target as a faster bullet, but as I said earlier the faster bullet fired along the same trajectory will always impact higher.

All of this being said I don’t think it’s very likely that a velocity difference would have a one inch POI change at 100 yards. From my math you would need an almost 250 FPS difference in MV to result in a one inch shift in point of impact at just 100 yards, Based on a 300 Yard and even more based on a shorter zero.

 

[Namster]

Re: .308 trajectory from JBM doesn't match actual

Thanks for the information, I appreciate it.

From the center of the scope to the firing pin on the bolt (assuming the strike point is dead center) is 2.5"

I am new with the Ballistic Calculator and there doesn't seem to be much help on it. So when you mentioned adjusting the BC #, am I just adjusting it for the avg MV that I recorded during my session...along with the other environment variables?

 

[Namster]

Re: .308 trajectory from JBM doesn't match actual

I just filled out some information in my profile. Looks like you hail from Auckland eh? Lovely place. I jumped from the Skytower twice when I was there. Did you know it's free for subsequent jumps if it's a slow day?

I will take a look at the free program you indicated.

 

[goober]

Re: .308 trajectory from JBM doesn't match actual

Funny ,Ive never been up there as Im scared of heights -leave it to the young and tourists .The Berger program is good but also get your own chrony as theY are vital if you play with different loads

 

[timelinex]

Re: .308 trajectory from JBM doesn't match actual

Your info out is as good as the info in.
You would need to know your exact MV, weather conditions,slope and everything else to get an accurate drop chart.

My generated drop charts are always slightly off, but thats because I estimate my MV, and weather conditions are based off local readings and not exact positions I think(iphone app finds them)... So after all that, it usually gets me pretty close and I have to adjust from there. For now, thats ok for my purposes. If you want to be within an inch, you need to be much more serious about the data put in, as it makes a big difference.

 

[Lowlight]

Re: .308 trajectory from JBM doesn't match actual

For new users of Ballistic Programs,

The next time you go to the Grocery, have the cashier <span style="font-style: italic">"guess"</span> on the price of every item you purchased and ring those numbers up. Then go back in line and put the actual prices into the register and see if they match total based on her <span style="text-decoration: underline">guess</span> ?

That is essentially what you are doing, regardless of where you get the data if it is not specifically from your rifle, specifically from your location <span style="font-style: italic">(ie using a handheld weather station)</span> all you doing is putting less than perfect information in asking the computer to match your rifle and ammo combination. It only knows what you tell it, and when you take the time to tell it factual information it gives a much better answer.

 

[Namster]

Re: .308 trajectory from JBM doesn't match actual

Ahh, I get it.. the ballistics program is supposed to get me close or a decent idea.. makes sense now. Thanks for the information.

Is there such a program that can take real world results and graph them and then allow us to adjust trajectory in that manner?

Lets say I have variables such as MV, atomspheric conditions, elevation and 10, 25, 50, 75 and 100 yard statistics? Could this technically be plugged into something and then we request data.. sorta backwards ish. Just a thought.

 

[goober]

Re: .308 trajectory from JBM doesn't match actual

Do as I have prescribed earlier and you will be bang on.

Get these basics done and you'll be on target for as far as you need to shoot.do the basics and get results

 

[Namster]

Re: .308 trajectory from JBM doesn't match actual

Ch'e the Chrono is on order and I will probably take a look at some Berger rounds after I finish with my set of federals.. Though, the Federals do shoot really nicely and I can basically put them in the same hole at 100 yards.

thanks again for the responses!

 

[SScott]

Re: .308 trajectory from JBM doesn't match actual

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">For new users of Ballistic Programs,

The next time you go to the Grocery, have the cashier <span style="font-style: italic">"guess"</span> on the price of every item you purchased and ring those numbers up. Then go back in line and put the actual prices into the register and see if they match total based on her <span style="text-decoration: underline">guess</span> ?

That is essentially what you are doing, regardless of where you get the data if it is not specifically from your rifle, specifically from your location <span style="font-style: italic">(ie using a handheld weather station)</span> all you doing is putting less than perfect information in asking the computer to match your rifle and ammo combination. It only knows what you tell it, and when you take the time to tell it factual information it gives a much better answer. </div></div>
+1,
This little blurb needs to be a sticky, at the end of the loading section, as well as the
Equipment section.
I'm still chuckling.
SScott