Can anyone explain this POI change?

one-mad-dog

one-mad-dog

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A load I worked up for my Remington 7400 in .243 Win which works very well for deer is:

40 grains of IMR 4350, CCI#200 LR primers, Nosler 100 grain partition bullets in Winchester cases. The average velocity is 2710 fps and they consistently group less than 1.5 inches at 100 yds. with the 7400.

I also have some 95 grain partition bullets so I thought I would try these with the exact same components as above. The average velocity was 2732 fps and they grouped well but 3 inches lower and 1 inch to the right of the 100 grainers.

I then tried the 95 grain partition bullets with the same components but with 41.0 grains of IMR 4350. The average velocity was 2808 fps with a 1” group but 5 inches lower and 2 inches to the right of the 100 grainers.

Can anyone explain why the point of impact is changing so drastically. I would expect a different POI with these different loads but not like this.
 
A different bullet requires working up the load from the starting load and sighting in with that bullet. Mind you, sometimes the rifle just doesn't like the bullet.
 
Barrel harmonics X2. This is why it is important to work up a load every bullet variation will perform differently. For more info, a good resource is Varmint Als web pages on the topic.

There may be several sweet spots "nodes" in a barrel especially with light weight hunting barrels. Find the best node which provides the performance you are looking for. If one inch groups fills your need then no need to look further. As they say, accuracy is everything. Enjoy...
 
one-mad-dog said:
A load I worked up for my Remington 7400 in .243 Win which works very well for deer is:

40 grains of IMR 4350, CCI#200 LR primers, Nosler 100 grain partition bullets in Winchester cases. The average velocity is 2710 fps and they consistently group less than 1.5 inches at 100 yds. with the 7400.

I also have some 95 grain partition bullets so I thought I would try these with the exact same components as above. The average velocity was 2732 fps and they grouped well but 3 inches lower and 1 inch to the right of the 100 grainers.

I then tried the 95 grain partition bullets with the same components but with 41.0 grains of IMR 4350. The average velocity was 2808 fps with a 1” group but 5 inches lower and 2 inches to the right of the 100 grainers.

Can anyone explain why the point of impact is changing so drastically. I would expect a different POI with these different loads but not like this.
Click to expand...

Don't worry about POI when developing loads... just worry about accuracy. When you find a satisfactory load, re-zero your sight for that load.
 
Thanks for the comments everyone. I was surprised by the significant amount of difference in POI small changes in components can make - but obviously they do even when velocities don't vary much. These loads all display ample accuracy for hunting but will need to be sighted in for each load.
 
The 7400 is a complicated rifle, what with the way the forestock, gas port and gas cylinder touch and hang off the barrel. It is worlds apart from a bolt action rifle that has a free floated barrel that only touches the receiver.

I am also surprised to see this much POI change, what a P.I.T.A. In particular those 95s, simply adding a grain of powder caused the POI to move down 2" and right 1".

Yes you can deal with / work around problems like this by sighting in with your chosen load, but it does not help your confidence knowing that if you have to change ammo you could be dealing with a 3-4 MOA shift in POI. Or even if you don't change ammo, knowing that shooting hot (temperature) or cold ammo might also be worth a minute or two POI shift. Though for hunting at 200 yards or less this might be something you can live with.
 
Adding to the problems of the OP is the fact that such light loads were used.
A 243, remember, is a take off from the 308 Winchester. The 308 was designed for pressures among the very highest, of any rifle calibre.
The pressures the 243 was designed for would drive a hundred grain bullet in the 31 to 3200 feet per second range.
When you get down to the 2700 fps range, anything can happen, it is now a different calibre, practiacally speaking.
Bench rest shooters have always loaded their rifles on the heavy side of heavy, to achieve those great groups.
 
I had the same thing happen with Juniors 308win when I started some reduced loads to
try and help her with felt recoil.
The bullet drop was expected, but to head to the right of center was unexpected.
I think the high and low tides have something to do with the rifle construction as well. :)
 
Whoa, necropost!!

Even though the thread is over 4 years old I'll toss in my $0.02.

A 5" change in POI with a different load at similar velocities is rather drastic, I haven't encountered that much change yet. However, almost every time I change something, the POI moves somewhere.
 
Cleftwynd said:
Whoa, necropost!!

Even though the thread is over 4 years old I'll toss in my $0.02.

A 5" change in POI with a different load at similar velocities is rather drastic, I haven't encountered that much change yet. However, almost every time I change something, the POI moves somewhere.
Click to expand...

Cleftwynd

I had a Remington 788 in .243 Winchester and went to the range to test the 100 grain Hornady flat base and the Sierra 100 grain boat tail bullets. The boat tail Sierra hit over one foot to the right of the 100 grain Hornady.

Never say never the shape of the bullet, the shape of the base "and" the muzzle crown all effect where the bullet goes.

barrelvibes-1_zpskydtbbib.jpg


boattail-a1_zps57c50062.jpg


Cleftwynd, never say never and remember $hit happens. :slap:

P.S. When I first read this post and then saw the post date I left it alone because you never know how far a bullet can move in four years. :evil:
 
Oh I sure understand the physics behind it, I just haven't had one shift that far yet. I have also had stiffer loads print much lower at 100 than a light starting load. The worst rifle I had for changing POI with different bullets was a Ruger M77 MKII in 308, that was one picky rife for some reason. But that was before I knew what I know now about pressure points on the barrel and twist rates, etc....
 
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