308 not obeying laws of physics, groups dropping, calling the physics police

[rockwind1]

spin off from my earlier thread http://forum.accurateshooter.com/posts/37199982/



i changed the experiment a little bit as i didn't want to load up a bunch of ammo if it didn't pan out so i shot my old lake city m118lr that i had been shooting for a while and i shot like 8 groups of 3 and i shot them even faster than the load developement rounds and they stayed pretty much straight. WTF!!!!!

so apparently, and i still don't understand it, but as i increase my imr 4064 in .3 grain increments, my groups actually drop about 3/4" at 225 yds for every RELATIVE increase in velocity.

THAT IS CRAZY!!!!!!!

i thought load developement was fairly straight forward but if this is what i have to look forward to, i am going to start bringing along a mini fridge with lots of adult beverages

 

[BoydAllen]

The laws of physics have not been violated. Try it off of a bench with a sandbag under the front. Put the sand bag near the front of the action and hold the rifle. The muzzle is rising as the bullet goes down the barrel so higher velocity shots exit earlier in that rise than slower shots. It is your shooting setup that is probably exaggerating the effect. Your forend may be flexed quite a bit from the weight of the can, and as the rifle starts to recoil it unloads like a spring, creating more muzzle rise than you might otherwise have, even though your bipod does not leave the ground. Do you have a rear sling stud that is making contact with your rear bag?

 

[gambleone]

spin off from my earlier thread http://forum.accurateshooter.com/posts/37199982/



i changed the experiment a little bit as i didn't want to load up a bunch of ammo if it didn't pan out so i shot my old lake city m118lr that i had been shooting for a while and i shot like 8 groups of 3 and i shot them even faster than the load developement rounds and they stayed pretty much straight. WTF!!!!!

so apparently, and i still don't understand it, but as i increase my imr 4064 in .3 grain increments, my groups actually drop about 3/4" at 225 yds for every RELATIVE increase in velocity.

THAT IS CRAZY!!!!!!!

i thought load developement was fairly straight forward but if this is what i have to look forward to, i am going to start bringing along a mini fridge with lots of adult beverages

If you were shooting free recoil then your situation would make some sense. In general, when shooting free recoil, I notice that as I increase powder charge my poi will shift down.

As MV increases the bullet is exiting the barrel "sooner" than previous slower loads. As such, the rifle has not moved as far under recoil and as such the angle of the buttstock has not changed the angle of the barrel in regards to the target as much as a slower load where the rifle has moved further back.

You said you are loading the bipod forward, so I don't know if this pertains to you, though instinct makes me think this is indeed the case.

Would be easy to check though, just shoot your test loads in reverse, from highest to lowest charge weights. If the groups rise instead of dropping then you will know it is not heat at the very least.

Why not try what Shrum suggested in the earlier post? Also Boyd posted before i sent this, so i would read his post and take note!

 

[Toby Bradshaw]

Nothing unusual (or physics-defying) in having POI drop with increasing velocity. At the range of velocities that you're shooting, your barrel is moving upwards, so that faster bullets (shorter barrel time) are impacting lower on the target (i.e., negative compensation).

Here are some other examples from a quick Google search on "ladder test rifle":

 

[Richard Coody]

what i believe you are experiencing is the relationship of line of sight and bullet trajectory with ever increasing velocity.

 

[rockwind1]

I Took the can off after the first episode of the groups dropping and I put on a break I had laying around so the second time I did it I used a break. I’m just sort of kidding about the laws of physics, I mean I watched it happen right in front of me I’m just very very shocked I always thought that as you increase the muzzle lastly the bullets would travel flatter and thus hit higher. But it’s painfully obvious that there is a Nother affect that takes precedent over that. I guess it’s a good sign that it’s so consistent I would assume that means that my shooting is very consistent. I do have a stud at the Bottom of my rifle stock, but my rear bag is so soft, although maybe it’s not soft when I have it squeezed. I will check and see where it lands. I didn’t exactly do what Schrom said but I think it was a successful experiment in proving that it’s the ammo and not my gun. So now I will load up the ammo again and I will do the test again backwards like Josh recommended

 

[Twoboxer]

Lower POI at higher velocities is a very common occurrence in handguns where the barrel inevitably recoils upwards. Faster bullets exit the barrel sooner and have a lower launch angle.

In any case, physics says faster bullets out if your suppressed 308 have a lower launch angle. Look for upward recoil or some other factor that causes the position of the crown to allow this.

 

[Ggmac]

If it was easy to understand we would not have many volumes and continued experiments going on .
For an interesting read go and search the British SMLE at 1000 yds .
It takes much less elevation than a Springfield 30-06 . Positive compensation .

 

[ClassCat]

Harmonics

 

[BoydAllen]

I don't care how soft your bag is you never want to have a sling swivel stud touch a bag, either initially or as the rifle recoils. Period!!!!! I usually take the back one out for that reason. Cups on the side of the stocks are another option. (I knew you were kidding about the physics)

 

[Brians356]

Nothing unusual (or physics-defying) in having POI drop with increasing velocity. At the range of velocities that you're shooting, your barrel is moving upwards, so that faster bullets (shorter barrel time) are impacting lower on the target (i.e., negative compensation).

Here are some other examples from a quick Google search on "ladder test rifle":

FWIW (to the OP) here are some links I posted some time ago to some of Varmint Al's pages on barrel motion and related issues. Not endorsing his analysis or opinions, just for illustrative purposes, to highlight the complexity of the topic, and to suggest there could indeed be a counterintuitive relationship between velocity and POI shift:

http://forum.accurateshooter.com/th...idth-of-accuracy-nodes.3937213/#post-37084098
-

 

[rockwind1]

I don't care how soft your bag is you never want to have a sling swivel stud touch a bag, either initially or as the rifle recoils. Period!!!!! I usually take the back one out for that reason. Cups on the side of the stocks are another option. (I knew you were kidding about the physics)

man, if i could solve all my problems by removing a sling stud that would be great!!!!!

 

[Dusty Stevens]

man, if i could solve all my problems by removing a sling stud that would be great!!!!!

if you have a stud touching a bag you can

 

[CaptainMal]

Do not remember what distance you were shooting. Guessing it is not much farther than say 400 yards. Guessing because even if the nodel vibration pattern has faster bullet exit on a lower barrel vibration pattern, the velocity difference will take over farther out. 600 should show it easily. If not, 1,000 has got to as the slower bullets will be in flight longer and therefore hit lower than those traveling the distance in less time.

 

[Bart B.]

Smallbore. 22 rim fire match winners oft times have a tuner on their barrel muzzle they move back and forth to get positive compensation for 50 yards at one setting and 100 yards at another setting. If they forget to set the barrel "tune" for 50 yards after shooting at 100, their 50 yard groups will string more vertically.

The opposite will happen shooting at 100 yards after 50; more vertical spread at 100.

They're changing the barrel's muzzle axis 3rd or 4th harmonic frequency so bullets leave at the correct angle to the LOS on the bore axis upswing.

 

[gambleone]

man, if i could solve all my problems by removing a sling stud that would be great!!!!!

if you have a stud touching a bag you can

If these two gentlemen told me it could be a sling stud problem, I think I would heed their advice.
You've just gotten advice from two of the most notable members on this forum. BoydAllen & Dusty. JMHO

 

[Bc'z]

I've seen this before. Cant tell you why but harmonics makes seance to me.
This target is from load development for my 06' with Rl26 & hornady 178eldx.
Notice top left target lightest charge, roughly 2" high and about 1" to right.
Now look at bottom right target.
1/2" low & about a 1/4-1/2" left.
No turret adjustment was made, just an increase of powder in .3 gr increments.
Point of aim remained the same, point of impact lowered and shifted left.
Target was shot off of bags, 3x9 Bushnell Elite @ 100 yards. Don't know if this helps you, but thought you should see it.
After you find the load your weapon likes adjust poa to match poi.

 

[rockwind1]

If these two gentlemen told me it could be a sling stud problem, I think I would heed their advice.
You've just gotten advice from two of the most notable members on this forum. BoydAllen & Dusty. JMHO

oh, i am definitely removing it, i just meant that i am having other problems and i wish it would fix those as well but it is on another rifle, hey wait, it might have a sling stud also.

 

[rockwind1]

I've seen this before. Cant tell you why but harmonics makes seance to me.
This target is from load development for my 06' with Rl26 & hornady 178eldx.
Notice top left target lightest charge, roughly 2" high and about 1" to right.
Now look at bottom right target.
1/2" low & about a 1/4-1/2" left.
No turret adjustment was made, just an increase of powder in .3 gr increments.
Point of aim remained the same, point of impact lowered and shifted left.
Target was shot off of bags, 3x9 Bushnell Elite @ 100 yards. Don't know if this helps you, but thought you should see it.
After you find the load your weapon likes adjust poa to match poi.View attachment 1045398

well, mine was dropping a lot more consistently and further than yours but it sure does look very, very similiar. however, you are 100 yds and i am at 225 so maybe yours would be just like mine at futher range. i still am wrapping my brain around it. i feel betrayed. i am now going to therapy because i started reloading.

 

[Bc'z]

I've read your other thread and this one also, and that's what got me thinking about last trip to range and that target. I'll take a look at some targets from 260 and 6.5 to see if I notice the same pattern or not.
FWIW I'm not a competitor just a hunter, and I too have a sling stud on my stock as i do when hunting. Curious to see if removing it would change things on paper.

Next trip to range is a seating depth target, I'll be leaving stud in place. I'm a firm believer in only changing 1 thing when testing to know results of improvement or not. If you change multiple items at once makes it hard to determine which 1 worked.