Charge goes up, impact goes down.....

[LoneWolfUSMC]

I was testing two groups of a FGMM Clone.

The first is 42.2gr of IMR 4064 pushing a Nosler CC 168 in Federal once fired FL sized brass and a CCI200 primer. Flash holes have been deburred. Neck Tension is .002".

The second is the same with a charge of 42.9gr of 4064.

The control is Federal Gold Metal Match 168gr Lot# 321W719.

The rifle is zeroed for the FGMM since that is what I am trying to duplicate. Since I also need to track my cold-bore the FGMM is fired fist. All Chrono testing is at 100 yards from the bench.

The FGMM is dead on. Four rounds were fired because that is what I had left in the box. The last couple of cases of this ammo has been HOT.

1. 2734 fps.
2. 2756 fps.
3. 2745 fps.
4. 2730 fps.

42.2gr IMR 4064
1. 2606 6. 2631
2. 2651 7. 2590
3. 2643 8. 2587
4. 2617 9. 2586
5. 2664 10. 2622

42.9gr IMR 4064
1. 2644 6. 2649
2. 2668 7. 2645
3. 2700 8. 2626
4. 2700 9. 2682
5. 2663 10. 2663

At 100 yards the 42.2 gr groups were 1/4" higher than the 42.9gr groups. The FGMM was dead in the center.

I then moved back to 200 yards prone and fired 10 shot groups at separate targets "round robin" style.

FGMM was dead on. The 42.2 group was 1.5" above the 42.9 group.

I find it interesting that as the velocity moves up the groups are coming down. I can only guess that barrel harmonics are to blame.

I am going to work up towards 44.6gr. That should be a little faster than the FGMM, but it should hit the OBT node for my 26" barrel.

I am about ready to go buy an off the shelf box of FGMM to chrono a different lot. It would be nice if all of Federal's current lots are running this fast. Of course it could just be my chrony, but it was reading right along with Quickload for the 42.9gr loads.

What do you guys think?

 

[jrob300]

Re: Charge goes up, impact goes down.....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body">I can only guess that barrel harmonics are to blame.</div></div>

More than likely. If you've ever done ladder testing you'll really see this. Loads can walk all over the page. But you're right. The very first time I saw a slower load hit above a faster one, I thought, "Huh?"

How did you arrive at 44.6g? Is this a known 168g load? I'd probably know this if I owned a .308.

Not familiar with OBT. What is that?

John

 

[Greg Langelius *]

Re: Charge goes up, impact goes down.....

I think there's something else at work here, which pistol shooters will recognize.

More charge equates to shorter bore transit time. This means that the bullet emerges earlier in the muzzle flip cycle, when the barrel is pointed lower, resulting in a lower POI.

Pistol shooters, who often lack adjustable sights, will use charge weight to regulate POI height, based on this phenomenon.

Greg

 

[jrob300]

Re: Charge goes up, impact goes down.....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">I think there's something else at work here, which pistol shooters will recognize.
</div></div>

I did a bit of timed steel shooting back in the '90's with a Gov't model. Because it was timed, we were looking for the load that would give the least recoil/muzzle flip (tactical class, uncompensated) and would still cycle the slide. We were using adjustable sights, so transit time was not an issue, but now that you mention it I remember it being a point of discussion.

<span style="font-weight: bold">That </span>was back in the cobwebs.

Sorry for the hijack.

John

 

[sinister]

Re: Charge goes up, impact goes down.....

The higher the velocity the flatter the trajectory.

Think softballs -- fast pitch is flatter across the plate. Same projo thrown underhand by a little girl will require some loft.

 

[LoneWolfUSMC]

Re: Charge goes up, impact goes down.....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jrob300</div><div class="ubbcode-body">
How did you arrive at 44.6g? Is this a known 168g load? I'd probably know this if I owned a .308.

Not familiar with OBT. What is that?</div></div>

44.6 should get me to the same velocity as the FGMM lot I am running. I am HOPING that the same velocity with this powder will yield the same trajectory. It hasn't with RE-15 and Varget. 4064 looks almost identical to the powder I pulled from this lot of Federal.

OBT is "Optimal Barrel Time". There is a table floating around showing the time it takes for sound to travel through barrel steel. Basically what OBT does is calculate the best time for the projectile to leave the barrel based on how the shockwave from ignition travels through the length of barrel you are working with.

Several people I have talked to stated that OBT and their best accuracy load coincide. I am just now starting to work with it.

If I can look at a Quickload output for my barrel and select a charge that will give me the optimum barrel time, and it works, then that saves a whole lot of testing.

In this case the goal is not just a good shooting load, it's a ballistic duplicate to Federal Gold Metal Match. So a bit more tinkering is involved.

The bitch is that we have one case of this lot left. Once that is gone we will be ordering a new lot or switching to Hornady TAP. If that happens I get to start fresh and work up a clone again. Talk about a PITA.

 

[MitchAlsup]

Re: Charge goes up, impact goes down.....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body">I find it interesting that as the velocity moves up the groups are coming down. I can only guess that barrel harmonics are to blame.</div></div>

Inside 300 yards, barrel harmonics (as greg aluded) cause a greater change in PoI than does MV.

 

[Fuzzball]

Re: Charge goes up, impact goes down.....

Mr. Greg Langelius has it correct; it's bore time, not harmonics.

Things will reverse to their proper order down range as the extra speed shows its proper effect.

 

[LoneWolfUSMC]

Re: Charge goes up, impact goes down.....

So I need to stop wasting my time at 200 and get out to 350 or so to verify the load.

I was planning on that after I got the close range down, but it seems as that may help me out a little more than the 200 yard verification.