7mm - 1:10 Barrel - Stability Factor - 150gr. Bal Tip

[Broncazonk]

Hey all:

Broncazonk here, a new member.

I need help!

I have a 7mm Rem Mag. with a 26" 1:10" barrel.,I had the 1:10"
put on years ago, before the early A-Max and other VLD bullets came out.)

Anywhoo... it don't shoot a A-Max worth a darn and the other VLD bullets are even worse. So the rifle has been a vault queen.

By necessity, I've now gone to the 150 gr. Nosler Ballistic Tip, which is much shorter in OAL.

I've mic'ed the bullet and entered the data on the JBM Twist Calculator and have gotten the following results:
________________________________________________________________

Input Data,All dimensions in calibers)
Diameter: 0.284 in Nose Length: 2.253
Meplat Diameter: 0.105 Ogive Radius: 9.000
Nose Type: Tangent Boattail Length: 0.352
Base Diameter: 0.845 Total Length: 4.577
Rotating Band Diameter: 1.000

Weight: 150.00 gr Specific Gravity: 10.700

Drag Function: G1 Barrel Twist: 10.000 in
Calculated Parameters
RT/R: 1.000 Sectional Density: 0.266 lbs/in²

_________________________________________________________________

Calculated Table
Mach Twist Stability Factor
_________________________________________________________________
0.500 9.3 1.286
0.600 9.2 1.274
0.700 9.1 1.250
0.800 9.0 1.204
0.850 8.8 1.163
0.900 8.5 1.096
0.925 8.3 1.045
0.950 8.0 0.968
0.975 7.8 0.921
1.000 9.5 1.342
1.100 8.9 1.197
1.200 9.1 1.236
1.300 9.2 1.274
1.400 9.3 1.306
1.500 9.4 1.333
1.600 9.5 1.356
1.700 9.6 1.376
1.800 9.6 1.393
2.000 9.7 1.421
2.200 9.8 1.443
2.500 9.9 1.469
3.000 10.0 1.500

As you can see, the Stability Factor goes under 1.0 between 1150/fps and 1100/fps,which is at about 1350 yards)

After a bullet goes under 1.0 SF, does that mean the accuracy potential beyond that range is gone?

I'm trying to get to 1,760 yards, the Mach number at that range is .862 and from the chart above the SF is back up to 1.13 +/-.

Bronc

 

[Bryan Litz Ballistics]

Bronc,
Welcome to the forum! You ask some good questions.
You're mis-understanding something about the data you're getting from the JBM site. The Sg that is calculated is valid at the muzzle only. In other words, when it gives you an Sg of 1.274 for Mach 1.3, that means if you fired the bullet with a muzzle velocity of Mach 1.3, it would have an Sg of 1.274. Firing the bullet at Mach 3.0 and letting it slow to 1.3 results in different Sg than firing the bullet at Mach 1.3.
In reality, the Sg,gyroscopic, aka, static stability) actually increases as the bullet flies downrange. Reason is because the spin rate of the bullet imparted at the muzzle decays at a slower rate than the forward velocity of the bullet decays. This results in the bullet making more turns per distance traveled downrange, so the Sg is greater.
Having said that, there can be stability problems near transonic speeds that adversely affect accuracy. However, this is not a problem with 'static' stability, but dynamic stability. Dynamic stability,Sd) is much more complex than static stability, meaning it's very hard to predict if a bullet will 'survive' transonic flight,I don't know of any online resources that calculate Sd like JBM calculates Sg). In general, higher BC bullets do better, but there are no guarantees. Only way to know for sure is to try. Unlike Sg, dynamic stability is not directly affected by spin rate, so contrary to popular belief, you can't improve the transonic dynamic stability of your bullets by increasing the barrel twist.
You'd have better luck getting to 1 mile with higher BC bullets if you can get them to shoot. Your 1:10" may be ok for the 168 grain bullets made by Berger and Sierra. The Hornady 162 is a good choice also. The lack of a 'real' boat-tail hurts the BC of the Nosler bullet quite a bit in spite of the tip.
I'm attaching a 2d drawing of the Nosler ballistic tip. My measurements vary a little from yours.
Good luck,
-Bryan

 

[Broncazonk]

Well Bryan, seeing how your reply, in five minutes, taught me more about Stability Factors, and various other ballistic matters, than I ever knew, I think that I am going to be buying Berger bullets from now on.

Thank you so much for your reply, for taking the time, I surely wish I would have found this site before, and I hope to be a positive influence.

Bronc

 

[winters7746]

i have a 284 wichester with a 1 in 10 twist,bartlein barrel),it will not shoot the 162 amax at all.but it shoots 140 and 150 grain ballistic tips extremely well at a 1000yards.the 140 nbt shoots just as good as the,150 gr.matchking) in my rifle.i have killed 2 deer at at 800 yards with the 140 nbt.my rifle shoots under 2 inches at 600 with 140 nbt....140 and 150 grain is perfect for the 1 in 10 twist.i would definately try the 150 match king for target shooting.

 

[winters7746]

7 mag does real well with 150gr.bullets using h4831 and reloader 22,,3200 fps) H4350,3100FPS)

 

[Broncazonk]

Thank you Clint for your reply.

I've been using 63 gr. of IMR 4831 in my Rem. 7mm Mag with 140-150 gr. bullets and have never been satisfied with the results.

Recently, I've read that Reloader 22 is a far superior choice AND have had people tell me that 4831 is a poor choice for Rem. 7mm. Mag.

Even IMR 4350 seems to shoot better than IMR 4831.

Why?

Bronc

 

[mildot]

With my 7mag ihave tried so many different powder and bullet combo's.

A-max's no good
4831 ok with accubonds
rl22 good with interbonds and accubonds

but i have settled with 63gr IMR4350 and 150gr Scirocco's and 168gr Berger VLD's.

 

[udiablo]

My best loads for my 7 Rem Mag, 26" heavy barreled, 9 twist, are with 168 grain Bergers,or Sierras) using either RL22 or H1000 powder.

I'm a fan of the 7 Rem Mag,after 30 years) and have owned several,still keep 2 on hand), so I've used plenty of IMR4831 and about a truckload of H870,back when it was available).

I'm sure that IMR4350 is a good powder, but I'd also suggest trying some IMR7828 if you don't want to use RL22 or H1000.