Bullet Stability in a 1-10" 7mm/.284 barrel

[Broncazonk]

I have a question about bullet stability and its lack thereof. I have a 7mm Magnum with a 26-inch HS Precision barrel (fluted, stainless, cryogenically stress relieved) with most importantly a 1-10" twist. I limit MV in it to a chronographed 3050 FPS.

When I put this rifle together, I lived in Wyoming at an elevation of 7,000 feet and I had no problems with the original Hornady 162 gr. A-Max bullets. (I'm talking the first-generation, r-e-a-l-l-y long A-Max bullets.) The Hornady tech guys said I needed a 1-8.5" twist to shoot them, and I thought they were full of BS. Then, when I moved down to an elevation of 899 feet, I was shocked to discover those tech guys were not full of BS at all. At 899 feet, my 1-10" twist barrel scatters 162 gr. A-Maxs' all over hell and gone. From the JBM website, the Miller stability factor of this bullet is 1.364 in a 1-10". That's well above the 1.3 bare minimum SF, but it's not good enough for darn sure.

The 162 gr. Hornady SST yields a 1.397 SF and I can't get those to fly worth a damn either.

The 154 gr. Hornady SST yields a 1.582 SF at 90 degrees F and they are the cat's ***. But...however...they are only a 1.409 SF at 30 degrees F.

Is 1.409 SF going to cut it this fall??

Thanks everyone.

Bronc

 

[mikecr]

Are the bullets tumbling?
I don't see any reason why they would..

 

[Broncazonk]

Thanks Mike,

Both the 162 A-Max's and 162 SST's are hitting randomly within about a 40-inch circle at 600 yards. The bullets are not exactly tumbling, just wobbling (not making circles in the plate) and difting every which way. The 154 SST's are very accurate at 90F, but I'm not sure where they will be at 30F.

Bronc

 

[Mickey D]

You might try 140-Berger vld. Back in the late 60s I shot a 10 twist 7mm mag with 139 g Hornadys. Just a guess but you seem to be running out of twist with the heavy bullets. The 140s may help you out.

 

[bheadboy]

Competetive shooter have found altitude, temp, humidity have a major effect on stability,

I thinkd lilja web has and article on the subject - you are not going to stabalize at that level with 10x

Bob

 

[mikecr]

While his stability is marginal, it should not cause that issue in itself.
I think there is something else going on that he needs even higher stability to overcome.
Maybe a problem in the bore, or at the crown??

 

[Broncazonk]

Thank you very much, mikecr, bheadboy, and mickeyd98 for responding and trying to help.

Question: do the plastic tips on bullets like the A-max, SST and Ballistic Tip effect, change or alter the Stability Factor calculation. (The actual abbreviation for Stability Factor is Sg, so I will stop using SF!) Anyway, when measuring the length of the bullet, the plastic tip does not have the same density as the rest of the bullet.

Bronc

 

[frankgreen]

Bronc/guys, I don't care what the tip of the bullet looks like/made of for the most part. I look at the overall length of the bullet to come up with twist recommendations.

Off hand I don't shoot any of the 154's but a 1-10 it should be fine. To feel better about it I would have to do a twist calc.

You should also talk to the manufacture of the bullet and ask them what they recommend.

Is there something else going on? Bad crown, problem with bore like was said? Possible but if it shoots the lighter/shorter length bullets fine I'm going to say it's the twist. If the crown was damaged or the bore it would effect even the shorter length bullets.

This is also an example of what I always tell shooters. You have to pick the twist that's going to stabilize the longest heaviest bullet you are going to shoot. I tend to somewhat lean towards a little faster twist than running the twist on the edge. Why? Just what you are saying. Different altitudes, temp's etc....can effect stability of a bullet especially if the twist is on the edge of being stable for the bullet you are shooting.

Also I say this. A slightly faster twist and if your shooting good quality bullets/ammo doesn't seem to hurt you at all like some people think it will.

It also goes back to the question. What are you doing with the gun? What kinda accuracy are you expecting with it? etc.....

In some cases you cannot have your cake and eat it too. What do I mean by that? A guy wants a 6.5 twist .224cal. barrel to shoot 90gr. bullets but then complains it only shoots 5/8" groups at a 100 yards with a 55gr ballistic tip but the 90's shoot sub 1/2moa. Just an example! Shooting a lighter weight/shorter jacket length bullet in a faster twist might not shoot quite as good as the longer bullet but it doesn't throw the accuracy completely out the window.

If it where my gun I would have a 1-9 twist barrel on it for the 162's to 180's. Even if you decide to shoot some 150's thru it I highly doubt you would see any real great difference in accuracy etc...to complain or worry about.

Later, Frank
Bartlein Barrels

 

[JCHEDJ]

Reply

It is the middle of summer in Texas...105 F seems to be the norm

MY 7MM 08 with a 1 in 9" Shilen barrel is shooting like a Bargain Store Basement markdown special.

The AMAX's ( 162 gr ) are tumbling at 1,000 yds. Normal powder is H 414. Changed to H 4895 and the bullets are falling 20 feet short of the target. In Nov thru march ...NO PROBLEM with powders ( H414, H 4895 and IMR 4895) and hitting target dead center at 1,000 yds

Have changed the bullet seating depth from 2.800" , to 2.827",2.850" and the Hodgdons page length of 2.875"

I am also reducing the load by .5 grains...will post results after I work up the courage to sit in the heat.

Very interesting article about temp and pressure is on Hodgdons Extreme powder page.

http://www.hodgdon.com/smokeless/extreme/page3.php#top

The more I learn the more I realize just how little " I " know

hope this helps a little