What do I do to minimize POI changes with different loads? I have a Rem 700 barrel that will change 6" or more just by changing bullet weights.

Now it is totally floated, guess I better try some tip pressure next.

couldnt help my self
physical concept of forces and moments, the flowfield in the vicinity of a bullet most generally consists of laminar and turbulent regions. The flowfield depends in particular on the velocity at which the bullet moves, the shape of the bullet and the roughness of its surface, just to mention the most important factors. The flowfield obviously changes tremendously, as the velocity drops below the speed of sound, which is about 1115 ft/s (340 m/s) at standard atmospheric conditions.
Generally, a body moving through the atmosphere is affected by a variety of forces. Some of those forces are mass forces, which apply at the CG (center of gravity) of the body and depend on the body mass and the mass distribution. A second group of forces is called aerodynamic forces. These forces result from the interaction of the flowfield with the bullet and depend on the shape and surface roughness of the body. Some aerodynamic forces depend on either yaw or spin or both.
Summary: The wind force, which applies at the center of pressure, can be replaced by a force of the same magnitude and direction plus a moment. The force applies at the CG, the moment turns the bullet about an axis running through the CG.
This is a general rule of classical mechanics and applies for any force that operates at a point different from the CG of a rigid body.

Well Terry,

If you don't understand the concept now after reading that post......you never will.

Terry, just what did you expect? Are you one of those hunters that buy whatever is on sale and go deer hunting and never re-sight for each lot or brand?

Terry, different weight bullets will have a different effect on the barrel flexing while the bullet moves through the barrel. A 150 gr bullet @ 2700 f/s will cause a barrel flex that will let the bullet exit the barrel at point x, while a 180gr bullet @ 2500 f/s will have a different barrel flex, causing the bullet to exit the barrel at point Y. The amount of barrel flexing have a huge impact on the POI. It is quit possible to have more than a 6" difference in the POI, depending on the weight differece and the speed at which the bullet is propelled.

To get bullets of different weights to impact on the same place will take a lot of work and even more patience.

It will be far easier to develop different loads and know how to dail your scope to switch between the different bullets.

quote:

Terry, just what did you expect? Are you one of those hunters that buy whatever is on sale and go deer hunting and never re-sight for each lot or brand?

Ouch
While I have had a couple rifles that would group to close enough POI to allow me to switch with no adjustments. Normally if I have a rifle that I switch loads for I keep a cheat sheet. Allows me to adjust the scope from one load to another. If you do this test at the range to insure your scope moves the same each time.

I have to tell Terry I am sorry for saying what I did. I was NOT being mean to him. I just run into this all the time with both customers and friends that do not know a change in any one single thing can mean a vast difference in where thier gun will shoot. I have seen as much as 10" difference in different brands of bullets of the same weight. I really hope no one else was offended! I am old and can get a little nasty.
Sorry, Terry.

quote:

Originally posted by bfrshooter:
I have to tell Terry I am sorry for saying what I did. I was NOT being mean to him. I just run into this all the time with both customers and friends that do not know a change in any one single thing can mean a vast difference in where thier gun will shoot. I have seen as much as 10" difference in different brands of bullets of the same weight. I really hope no one else was offended! I am old and can get a little nasty.
Sorry, Terry.

Well said, thanks, no harm.

Terry Just pokin some fun.
this may be a little more help.
I use an external ballistics progran to plot point of impact.
I use an average velocity for each load that I have developed and chronographed.
There are a lot of other factors involved.
I have had some luck by developing loads that are close in velocity. for example a 130gr load at 2700 f/s and 140gr load at 2700 f/s will group pretty close. the heavier bullets tend to go hi,
Dave

Terry dont listen to those people

The fact is that if you have a perfect produced rifle, with a straight barrel with the hole centered in the middle of the barrel, and if all lugs are bearing, and the action is symetrical in the stressed area.
Then you would get a predictable shift in POI, according to the following rules:
Light and fast bullets = short flighttime = high POI

Heavy and slow bullets = long flighttime = low POI

Perfect rifles will all deliver groups with almost all ammo mixed, that is about 1" wide and up to 2.5" high at 100yd

If a rifle doesnt behave like this, then it has tecknical problems, ant is not to be concideret reliable according to accuracy.

This statement might seemes provokative, but it containes mutch more truth, than many people/"experts" wants to hear

Terry--
I have found that working up loads to get similar POI for different bullet weights works for me. The key in my rifle seems to be keeping the MV similar for each round....allowing me to make 125g light loads for practice in my 30-06 shoot within 1" of the POI of the full-house 180g hunting loads, and also 150g medium loads. I believe it to be a barrel harmonics thing....if I could increase the velocity of the lighter weight load far enough I think it would come back down near the POI of the other loads. Frustrating or fun to experiment with may depend on how my day at the range is going

Cheers,

Dan

If everything on the rifle is tight, then you are getting that dramatic of swings in the POI.. I'd say that your velocity must be having some strong changes also....I see that throw off POIs a lot....

I was doing some 223 loads recently....with the same bullet weights.. at 100 yds, the slower velocity actually consistently had HIGHER POIs... and the faster velocity ones had a lower point of impact!

Can't explain that one... but I am talking the same thing, with the same powder, but 7 or 8 different types of bullets from various manufacturers...

So I just recorded the settings for elevation and windage in my load data, and just went on from there....

You can relate a lot of what happens with different weight bullets and velocities to handguns. Heavy, slow bullets with more bore time hit higher. Light fast bullets hit lower. If you eliminate barrel whip, etc, the higher the barrel rises in recoil before the bullet leaves, the higher the bullet will hit. Rifles work the same but are not as drastic due to gun weight and being against the shoulder.
The question of the same weight bullet of different brands (Or weights.) hitting off zero is due to changes in jacket material and thickness, pressure developed, friction and core softness. Some are also longer or shorter for the same weight. Then the ogive shape can be a lot different. One should never expect the same results when changing brands or weights of bullets. I have even seen a difference between lots of the same brand. Any change in a bullet means a new sighting in session.
I have found that the 30-06 and .308 are the most picky calibers. Other calibers like the 6.5 X 55 can shoot a range of bullet weights to almost the same POA. Most 7MM's are forgiving too. What you must remember is every gun is different and some will do it, others will not, no matter how the gun is built. If you are lucky and hit barrel harmonics with two different weight bullets it is the exception. You CAN, by careful loading make things work but it can mean a long process and a lot of shots. It is a lot easier to change scope settings and record settings for each weight bullet.
Use the same brass, same primer, same powder, same bullet and be happy. Change one thing and you must re-sight.
I have friends that come to shoot and every single time, they come with different loads and bullets, boolits. Some mix brass. I have sold some of them my guns and told them the load to use and they change charges. I throw up my hands when the guns will not shoot and tell them why. Next time out, they still did not do what I told them. Why they think they can better what I spent years doing is beyond me. I sold one a Ruger SRH that can hit a pop can at 200 yd's from a rest. He can hardly keep his shots on target at 25 yd's now because he changes his loads.
Another crimps his .308, why I don't know, but he deforms the shoulders and bulges the cases. Most will not chamber. Bullet runout is very bad. He will listen to me for a second and interrupt without hearing me.
So, please guys, I never mean to holler at any of you, but you can see what I go through. I have been a part time gunsmith most of my life and some of my best friends are the best gunsmiths around. I stand in thier shops and listen to customers as he explains things. It is unbelievable!!! How anyone even kills a deer is a mystery.

Terry Blauwkamp: I am assuming you are serious here and that you are finding these P.O.I. differences at 100 yards in your testing!
Over the last 47 years of doing ammo testing and load development with literally HUNDREDS of Rifles and thousands of loadings I think that 6" of dispersion with different bullet weights is NOT excessive or even notable!
I have seen Rifles (that gave me wonderful and reliable service for years!) that shot diffent brands of ammo and bullet weights into P.O.I.'s that differed in excess of 12" (again at 100 yards - where I do all my load testing at by the way).
Now some folks may decry this performance as the function of a flawed Rifle - I don't!
In fact it is probably more the norm for Rifles to shoot different bullet weights into different P.O.I.'s in excess of 6" at 100 yards.
AND!
I am not talking about strictly vertical dispersion with the different bullet weights from the same Rifle.
I am talking as well about horizontal dispersion of P.O.I. with different bullet weights!
AND!
A combination at times of both vertical AND horizontal dispersion when testing different bullet weights. I quickly scanned some of the other posts here but did not see this "combination of vertical and horizontal" P.O.I. dispersion mentioned.
Believe me - it happens as well, and in fact I find it AGAIN to be the norm!
Different bullets (especially different weight bullets) fly, travel, spin, gyrate, vibrate a barrel, oscillate and spiral differently!
They are simply gonna end up at different places!
A 6" dispersion of P.O.I. with different bullet weights at 100 yards IS NOTHING!
I have seen much greater dispersion countless times over the past 4 1/2 decades!
Fuhgeddabowwtit!
Hold into the wind
VarmintGuy

Dear Varmint:

Yes, I am dead serious. I have "never" in all my years had a rifle that would change POI 6" or more until now.

I have other guns that shoot anything into a 2" group all mixed together.


Thanks for your thoughts.

Hi, Terry,

I have had some that will change the point of impact with changes of bullet weight and velocity, but in a very predictable fashion. I have had some that were all over the place. On a recent trip, my 375HH was totally inconsistent in handling solids and soft points by the same company, in the same weight and the same velocity. I had to work my buns off to "harmonize" loads. Conversely, my 9.3x62 was unbelievably consistent with 286 RN solids and soft points hitting right behind the target dot at 100yds, the 286gr Partitions hitting 1.5"high, and the 250BT's 2.9" high.

I'd play with a little upward pressure as you mentioned. It might act to damp the barrel harmonics and close things up, and its easy to do. I think you are going to have a wonderful opportunity to burn a lot of powder before you find the right or best combination. And that isn't all bad. Kudude

I have a .308 Savage 99 that I shoot both 150 gr Ballistic tips and 180 gr RN Hornady bullets out of. The BT exit at nearly 2800 fps while the 180 are moving along at 2600 fps. When the BTs are sighted in to hit 2" high at 100yds, the 180's are about 6" low for a shift of 8". When shifting between loads I just keep track of the scope settings. I move my scope up or down 32 clicks to get back on target. Seems to work out for me.