Horizontal dispersion?

K

kilotanker22

New member
Okay so I have a 6.5 PRC. Leaves me scratching my head a little bit. With the ELDM 140 grain bullet I can repeatably shoot sub. 5 moa groups with a particular powder.

Switch to the ElDX bullet and my horizontal dispersion is crazy! Just today I tested more loads with this bullet. Velocity spreads are the same as with the ELDM, but I am seeing a 2-3" horizontal dispersion with this bullet. Could it be that this particular rifle just really does not like the ELDX?

The Ogive from the ELDM and the ELDX are definitely different, but I have never aeen so much horizontal dispersion from one bullet to the next.

With the ELDX bullet the best I can get it to shoot is around 1.5 moa. And that is seated around .120 from the lands. The closet I go the worse the dispersion gets.

The ELDM bullet did not seem to care where I seated it in relation to the lands, but shot best around .025 off. Maybe my answer here is just stick with the ELDM?
 
I am certain that the barrel is not contacting the stock as the channel is rather large and the stock really does not flex. I guess that I will load up some more of the ELDM bullets and see if the problem persists. Since switching to the ELDX I have not shot any of the ELDM bullets. I plan to do it this way to potentially rule out the optic. I figure, if I load some more of the ELDM and it goes right back to shooting well, then it is not the optic and it is most likely that the rifle just does not like the eldx
 
Kilotanker adjust your seating die to give you the same jump with the 2 bullets. I went through the same thing with the 190gn ablr and the 190gn aclm.
 
Very interesting issue. Is the ELD-X the 143-grain? Have you measured the velocities? I am thinking this bullet is giving you an unfavorable barrel time that is allowing a horizontal recoil moment to influence the muzzle position at bullet exit. Are you seeing this both off the bench and with a tight sling in prone?
 
Unclenick said:
Very interesting issue. Is the ELD-X the 143-grain? Have you measured the velocities? I am thinking this bullet is giving you an unfavorable barrel time that is allowing a horizontal recoil moment to influence the muzzle position at bullet exit. Are you seeing this both off the bench and with a tight sling in prone?
Click to expand...
yes it is the 143 grain eldx. Average velocity is 3130 fps. I have tested this with seating depths ranging from .005 off to .120 off. .120 off was the best shooting load, although velocity was the same.

Velocity with the 140 ELDM bullet was lower by about 100 fps with my best shooting load, but also had a half grain less powder. I am gonna try a new test with both bullets.

First I am going to bring the ELDX down to the velocity that I originally shot the 140 ELDM and test groups there with both bullets.

Test two will consist of bringing the ELDM up to the velocity at which I am shooting the ELDX.

In both loads the standard deviation was single digit and extreme spread was less than 20 fps.

Oddly enough Unclenick, when my Magneto speed is attached the groups tighten up with the ELDX. With the Magnetospeed horizontal shot dispersion is around 1.25 inches, while without the Magnetospeed, Horizontal dispersion is anywhere from 2"-5", 5" being the dispersion I saw when .005 off the lands.
 
I have not shot it in the prone, only from the bench. I have had the same results both with a loaded bipod and a front and rear bag.
 
Total number of shots down the tube is 100 at this point.

If Barrel time really ends up being my issue, then I should be able to prove that point in my above tests. Or at least support it. If the ELDM bullet does not shoot well at 3130 fps and the ELDX does shoot well when I slow it down to 3040. That result would support a barrel time issue IMO. Also I feel like the Magnetospeed reducing the variation also supports your theory, as that would certainly change the way the bore expands as well as how long the wave would take to reach from end to end of the barrel.
 
The Magnetospeed's influence on the barrel sounds like that of a barrel tuner. Barrel tuners work by changing the timing of the muzzle deflection from recoil, so that makes it sound like you are, indeed, dealing with a barrel time issue.
 
I will load some up later tonight and slow them back down a bit. I was a little stunned when I saw that a half grain of powder yielded 100 feet per second change in velocity.
 
One thing that tells you for sure is that you are outside a velocity flat spot where small changes in powder charge have almost no effect on velocity over a short span. Such flat spots often overlap vibrational accuracy nodes and are a good place to start looking for an accuracy load.
 
"...around 1.5 moa..." The whole ELD line are hunting bullets. However, group size is more about the rifle and its user than anything else. And 1.5" at 100 is good.
"...from the lands..." Forget that whole thing until you have the load worked up. The off-the-lands stuff is a load tweaking technique that is 100% trial and error and will be different for every rifle.
ELD M bullets are not suitable deer hunting bullets. They're Match/Target bullets. Good for varmint hunting ONLY.
 
I see where you are coming from T'Oheir, but 1.5 moa is bad imo when that is opening what I know the rifle is capable of by a factor of 5. I disagree that the ELDM would not be suitable for deer sized game. There are plenty of thinly jacketed bullets with a lead core that are devastating on medium game. I am not trying to penetrate through thick hide and bone. Designed as a match bullet maybe, but for deer, yotes and Chuck's I assume that they will perform quite well.

Unclenick, I can see your point. The previous 100 fps took nearly 2 grains of powder to achieve. In fact from 55.5 grains to 57 grains I only saw around 65 fps gain. Then jumping to 57.5 grains of powder I saw a 100 fps gain. From 57.5 to 58 grains I saw only 10 fps gain.
 
Maybe i will buy a box of 130 grain pills. Maybe I can push them to the next barrel time node above the one I am outside of now. With the 140 class bullets I can't push them fast enough to get into the next node.
 
I have all the rounds loaded up to test both bullets around both velocity levels. If it really is a barrel time issue I should see the horizontal dispersion of the ELD-X reduced tomorrow when I go to the range. I should also see the horizontal dispersion of the
ELD-M bullet increase when brought to the velocity where I saw those results with the ELD-X
 
Well, again the ELDM shot the best. At the higher velocity the ELDM shot 1.3 MOA. The ELDX was around 3 MOA

The lower velocity turned out not to be that much lower. Since putting around 100 rounds down range I have picked up 60 fps or so from when I only had 20 rounds down range. However slowing the ElDX down by 40 fpw did improve the groups a little. I would stack 3 on top of one another and then send one an inch right then send number five an inch left.

I think that if I stick with the ELDM bullet and slow it back down to 3040 or so it should shoot much better.

When I first shot this load and shot multiple .2-.5 moa groups the average velocity of that bullet was 3039 I think. Today with the same load I was getting 3089 average, but velocity spread was great. Es of 14 and SD of 5.

I feel like with this particular rifle I just can not achieve the desired velocity while maintaining what I believe to be acceptable accuracy.

So my options to get this to shoot well are several.
I can just slow this bullet down, I can buy some lighter bullets to get into that next barrel time node up, or I can buy something heavier and get into the next barrel time node down.
 
And no my velocity gain was not from temperature, in fact it was warmer outside when I tested the first loads at this charge. I attribute the increase in velocity to copper fouling.

Unclenick, I have been wondering. If I removed the Muzzle brake that should definitely change how that barrel expands and bends when the pressure wave bounces back and forth right? Although I understand that it may not be in a way that is beneficial
 
Removing the brake will definitely change muzzle deflection timing. If you look at the Browning B.O.S.S system, it is a muzzle brake with a micrometer adjustment system to let you use it as a barrel tuner (though a plain non-brake version is now available, too) specifically to get it to make that adjustment. They list what adjustment numbers on the scale are the best range for different bullet weights. With such a system, you can shoot a charge weight ladder to identify a velocity flat spot close to the velocity you want, and then tune the brake to get the muzzle deflection flat spot lined up with it. Tuning seating depth would then be the last source of improvement.

Taking the brake off may also lower the magnitude of the deflections and address the stringing, so I think taking it off is a good idea. While you are at it, inspect the muzzle crown for any signs of asymmetry. Some time ago, a couple of guys got one of Dave Manson's muzzle crowning tools and did, IIRC, over a dozen muzzles on guns belonging to fellow shooting club members and found about half of them shot tighter groups afterward, suggesting production barrel crowning is typically handled too loosely. Just another place to look.
 
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