Why 180g bullet shoot 3" right of 150g at 100 yards?

[jimhoodag]

I have been shooting my .300 WSM with a 150g Hornady BTSP with Superperformance powder at 3,250 fps with 3 to 4 inch groups at 500 yards. In preparation for an elk hunt, I loaded some 180g Speer BTSP with H4350 at 3,000 fps; however, the point of impact moved 3 inches to the right at 100 yards. Why?

The 168g Berger VLD's are shooting left about 1.5 inches at 100 yards, moving at 3,200 fps. I could understand a lower point of impact from the heavier bullets, but not the horizontal changes. I think it has something to do with the rifling spin rate, so I will go ahead and state that I think my Christensen Arms gun has a 13:1 rate.

Thanks,

Jim

 

[Win94ae]

If Gyroscopic (spin) Drift was the only culprit, the heaviest/slowest bullet would be most right, (more specifically the direction of twist,) and the lightest/fastest most left. It looks as if you are saying the medium weight/velocity bullets impact most left.

[edit]Oops. I just noticed you said "at 100 yards". Spin drift would be so minute at those distances, you would not notice them.

I'll go with differing barrel harmonics.[/edit]

 

[KMart]

I have seen the same things when shooting 155 AMax and 155 Sierra BTHP. With everything the same. Same brass, same powder and charge, same primer, same rifle, same day. The difference in point of impact moved 4-5 inches when going from one bullet to the other. I chalked it up to different ogives and barrel harmonics.

 

[jcampbellsmith]

I have been shooting my .300 WSM with a 150g Hornady BTSP with Superperformance powder at 3,250 fps with 3 to 4 inch groups at 500 yards. In preparation for an elk hunt, I loaded some 180g Speer BTSP with H4350 at 3,000 fps; however, the point of impact moved 3 inches to the right at 100 yards. Why?...

I think you are looking at the problem from the wrong perspective. Why would you expect to be able to predict the performance of one bullet from the performance of another? Your question highlights the importance of checking your zero for your rifle shot by you every time you change ammunition.

Have fun. JCS

 

[Fat Albert]

Why did it shoot 3" to the right? Because that is were the barrel (not the sights) was pointed at when the bullet came out of the barrel.

 

[300 RUM]

Most likely you have discovered the difference in recoil between the two

 

[desertrat]

I have had the same type of results from different weights with my 06. About 5-6 high n right from light 110 grain small critter vs the 165 gr deer hunting type. Couldn't make it play nice with that low weight bullet. I'd say it is not uncommon for a rifle to throw certain type of bullets like that.

As an after thought, I switched to 125 grain for light shooting and they performed excellent in the 06 and they only 15 grain heavier.

 

[Erik Cortina]

Barrel harmonics. Play with seating depths and you will see the POI change as you move the bullet in or out of the case.

 

[wapiti25]

For hunting elk, I recommend the 180 gr bullet. Zero your rifle with the load you select. A Berger 185 Hunting VLD would be a good choice if it will fit into your magazine. You will have to work with the seating depth to find the "sweet spot" and fit in the mag. assuming it is a short action.

 

[amlevin]

Most likely you have discovered the difference in recoil between the two

I would go more with this than barrel harmonics. Call it "shooter harmonics". More recoil, more movement.

 

[300 RUM]

Most likely you have discovered the difference in recoil between the two

I would go more with this than barrel harmonics. Call it "shooter harmonics". More recoil, more movement.

Each bullet weight will have a different recoil effect, but I believe it is a combination of both barrel harmonics and true recoil that causes the POI change. Even if you shoot the same weight bullet but hold less or more into your shoulder you will see a POI change.. I struggle with this myself at times (X X X X X 9! WTF )

Like I have said before and so have many others one hole groups don't happen on their own.

Also, keep in mind that a 155 Zero on the scope or sights doesn't mean a 185 zero!