Ballistics Mystery

[JackEllis]

A friend helped me set up some handloading equipment over the weekend and we turned out 20 rounds of .30-06 using 150 grain Hornady GMX bullets with a 56 grain H-4350 powder charge.

With a bullseye point of aim the rounds landed 2.75-3.25 inches high at 50 yards. No big deal except...firing a Wolf 145 grain round at the same point of aim at the same distance landed it on the bullseye. The rifle had been zeroed for Remington Core Lokt Express 150 grain ammo to be 1.75 inches high at 100 yards - and Wolf rounds land pretty close to the Remingtons. I've done the test comparing Wolf ammo and my hand loads twice and in both cases the results were very similar.

I'm trying to understand why the GMX lands so much higher than the Wolf round. They weigh the same and ballistics coefficients aren't that much different. The points of impact should be closer than they are. I'd be interested in any ideas that might explain the difference.

[bsumoba]

A friend helped me set up some handloading equipment over the weekend and we turned out 20 rounds of .30-06 using 150 grain Hornady GMX bullets with a 56 grain H-4350 powder charge.

With a bullseye point of aim the rounds landed 2.75-3.25 inches high at 50 yards. No big deal except...firing a Wolf 145 grain round at the same point of aim at the same distance landed it on the bullseye. The rifle had been zeroed for Remington Core Lokt Express 150 grain ammo to be 1.75 inches high at 100 yards - and Wolf rounds land pretty close to the Remingtons. I've done the test comparing Wolf ammo and my hand loads twice and in both cases the results were very similar.

I'm trying to understand why the GMX lands so much higher than the Wolf round. They weigh the same and ballistics coefficients aren't that much different. The points of impact should be closer than they are. I'd be interested in any ideas that might explain the difference.

easy...the higher the powder charge, the higher the point of impact. most factory ammo, especially wolf are underpowered.

[baih777]

powder

[gc44]

'06

easy...the higher the powder charge, the higher the point of impact. most factory ammo, especially wolf are underpowered.

^^^
This member is spot on. Sounds like you need to work up a load for this bullet/rifle. Try Ken Waters "Pet Loads" book. Will save you lotsa time, and is a good read. Well worth the price, and then some....good luck and happy shooting!

[JackEllis]

My first thought was also that the hand load was hotter. If the target had been at 200 yards or better that theory might make sense but at 50 yards, I've convinced myself there's just no way. For one thing, 56 grains of powder is supposed to produce 2800 FPS at the muzzle with the GMX round while the Remington muzzle velocity is supposed to be 2910. I also used Gundata.org's ballistic's calculator to compare the Remington load with a Hornaday load that approximates the one I made up and at 50 yards they're on top of one another.

If the difference in impact points was less than an inch I'd adjust my scope and move the target back but a 3 inch difference at close range defies logic and it's not consistent with results from two different calculators.

Very strange.

[bsumoba]

My first thought was also that the hand load was hotter. If the target had been at 200 yards or better that theory might make sense but at 50 yards, I've convinced myself there's just no way. For one thing, 56 grains of powder is supposed to produce 2800 FPS at the muzzle with the GMX round while the Remington muzzle velocity is supposed to be 2910. I also used Gundata.org's ballistic's calculator to compare the Remington load with a Hornaday load that approximates the one I made up and at 50 yards they're on top of one another.

If the difference in impact points was less than an inch I'd adjust my scope and move the target back but a 3 inch difference at close range defies logic and it's not consistent with results from two different calculators.

Very strange.

Do not rely on book data to tell you what you should be getting for velocities. For that matter, don't rely on ballistic calculators to give you exact point of impact data as well.

All of that is done most likely, with different firearms, different barrel lengths, different conditions, different reloading components, etc. Even firearms that are "identical" will have variations in the manufacturing of certain components of that firearm, leading to slightly different performance.

It sounds like you are assuming your velocities. You need to chronograph your load and then use that to calculate your come ups on the calculator. If you have not chronographed your velocities, then you are just throwing darts with your eyes closed.

[micro911]

Re-zero for the bullet and it will be good. I usually put a tape on the side of the stock for zero info for couple of different bullets.

[NapalmCheese]

Barrel harmonics are messing with you.

[SkyHawk]

You're flying blind without a chrony

[btm]

Barrel harmonics are messing with you.

Great point, entirely posssible. Also I would expect that given the same bullet weight the slower load would print higher, not lower. More dwell time in the barrel.

[vinconco]

Great point, entirely posssible. Also I would expect that given the same bullet weight the slower load would print higher, not lower. More dwell time in the barrel.

+1.
Heavier bullet slower load, higher impact.

[pacrat]

Do not rely on book data to tell you what you should be getting for velocities. For that matter, don't rely on ballistic calculators to give you exact point of impact data as well.

All of that is done most likely, with different firearms, different barrel lengths, different conditions, different reloading components, etc. Even firearms that are "identical" will have variations in the manufacturing of certain components of that firearm, leading to slightly different performance.

It sounds like you are assuming your velocities. You need to chronograph your load and then use that to calculate your come ups on the calculator. If you have not chronographed your velocities, then you are just throwing darts with your eyes closed.

^^^TRUE THAT^^^


The only solid data you have at this point is bullet weight. Everything else is by approximated by guess and by golly. Far to many variables for any published data rule of thumb quesstimations to be relevant.


JM2c

[vinconco]

I rarely use the chronograph any more for working up dope on my rifles. I zero at 100 and shoot at 200, 300,400, 500 and use the drops to generate the dope and velocity. I've been lied to too many times to count by chrono's and in the end the trajectory tells the story. Most chronographs are little more than a "well educated" guess.

[Epaphroditus]

Chrono is better than nothing and does verify ballpark velocity. But I'm with vincono - actual trajectory is king.

As for barrel harmonics it is my understanding that generally the group will open up as opposed to being thrown in a particular direction (although that does happen).

My 7 mm Wby mag Mk V thin whippy barrel is very particular and even a 0.002" spread in OAL is noticed not to mention changing from one bullet to another even if the same weight. Actual bullet geometry and jacket composition/thickness has an effect - sometimes pronounced. Same case same powder but different bullet geometry/composition (although same weight) can have significant differences in pressures as well as velocity.

[bsumoba]

+1.
Heavier bullet slower load, higher impact.

It really depends on what your zero is...

I rarely use the chronograph any more for working up dope on my rifles. I zero at 100 and shoot at 200, 300,400, 500 and use the drops to generate the dope and velocity. I've been lied to too many times to count by chrono's and in the end the trajectory tells the story. Most chronographs are little more than a "well educated" guess.

Chronographs are good to get an initial velocity to run your ballistics, especially if it is a new caliber, load, bullet, powder charge, etc.

It is also good to see what kind of ES/SD you are getting. If your ES/SD looks like crap, then you might need to work on something in your reloading process or work up another load. In general, this translate to larger vertical spread and groups as you go out further and further, but not always. This assumes a good chrono like a Magneto or Oehler. I am done with ones that are around $100 bucks.

Density altitude (atmospherics) throughout the day will also affect your trajectory. It is not that your velocities are wrong, but your atmospherics make it feel like you are shooting at higher or lower elevation, thus affecting your POI. I have seen the same load shoot 24 minutes high in the AM and then shoot 25.5 minutes high in the afternoon to hit the same target.

As for barrel harmonics it is my understanding that generally the group will open up as opposed to being thrown in a particular direction (although that does happen).

Barrel harmonics...big topic.