1 in 9 twist 20 inch Hbar and 77 gr bullets.

[Davo308]

I have a 20 inch hbar chrome lined Delton upper, less than 500 rounds through it. 1 in 9 twist.

My shooting is typically done at 2k altitude and 70 degrees plus temps.

Eventually Id like to reload and figure a hot loading would do better with a 1 in 9 twist. Buffalo bore 77 gr loads are getting 2850 out of a 20 inch barrel for example.

I fired 2 boxes of 77 gr otm rounds at 100 yards and accuracy was about 2.5 moa. No key holing.

My question is can a 20 inch 1 in 9 twist adequately stabilize the 77 grain smk with my atmospheric conditions?

 

[stello1001]

Yes. I do it with greater accuracy in worse atmospheric conditions.

 

[GIXXER2000]

The 1-9 is likely to slow for 77gr. The max i shoot with a 1-9 is 62. I shoot the 77gr in my 1-7 barrel.

 

[Gohring65]

Your altitude and temps will definitely help. What’s your Sg?

 

[Davo308]

Your altitude and temps will definitely help. What’s your Sg?

What's Sg?

 

[Gohring65]

What's Sg?

Stability factor

 

[Billl223]

77s would prefer 1:8, Try some 69 Sierras. Should shoot sub MOA.

 

[John Glidewell]

I plugged a 77 otm in the Berger calculator. 2850 fps at 2000 altitude. Came back as marginal. I have a 9 twist and run 73s as it is very finiky with anything heavier. Mine shoots 73 BT bergers really well.

 

[Gohring65]

I would definitely try a bullet giving me 1.43 sg.

 

[Davo308]

Stability factor

At 2650 muzzle velocity...
Sg: 1.78
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

At 2750 muzzle veloity...

Sg: 1.81
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

At 2850 mv ( Buffalo bore spec)

Sg: 1.83
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

Bullet Gyroscopic Stability Calculator

Calculate the Gyroscopic stability of a bullet with the Miller approximation.

bisonballistics.com

Projectile length of 0.95 inches.

 

[Gohring65]

I plugged a 77 otm in the Berger calculator. 2850 fps at 2000 altitude. Came back as marginal. I have a 9 twist and run 73s as it is very finiky with anything heavier. Mine shoots 73 BT bergers really well.

View attachment 7846324

Did you plug in his projected temp?

 

[Gohring65]

At 2650 muzzle velocity...
Sg: 1.78
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

At 2750 muzzle veloity...

Sg: 1.81
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

At 2850 mv ( Buffalo bore spec)

Sg: 1.83
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

Bullet Gyroscopic Stability Calculator

Calculate the Gyroscopic stability of a bullet with the Miller approximation.

bisonballistics.com

You’re good with those numbers.

 

[John Glidewell]

Yes. I did multiple Temps. At 95 degrees it goes to a 1.52. Might be worth trying. Me, if I had 69s or 73s I would run those. 9 twist tops out (typically) at 75gr. But different bullets, barrels etc. Just my .02.

 

[Gohring65]

Yes. I did multiple Temps. At 95 degrees it goes to a 1.52. Might be worth trying. Me, if I had 69s or 73s I would run those. 9 twist tops out (typically) at 75gr. But different bullets, barrels etc. Just my .02.

I wasn’t swiping at you, I just wondered if you entered his temp or not. I figured you probably did, but wanted to satisfy my curiosity. I always ran the 68 bthp in my colt hbar 9 twist and did very well to 550 yards.

 

[John Glidewell]

I wasn’t swiping at you, I just wondered if you entered his temp or not. I figured you probably did, but wanted to satisfy my curiosity. I always ran the 68 bthp in my colt hbar 9 twist and did very well to 550 yards.

Not worries, did not take it as a swipe. I ran 73s at 2960, yes that was a bit warm, but it shot lights out. I ran that at a milk jug challenge and had impacts at 1k, 1200 and 9th round impact at 1500 yards. It was dying at 1500. Bryce ran his 223 as well and his 77gr stuck in the backer at 1500 after going through water filled milk jug. I think he was pushing the 77 at 2850.

 

[sakobob]

It's doing 2.5 moa at 100....the bullet isn't working in your barrel...plain and simple.
Buy a 7 twist and be done with it , at today's ammo prices you'll have paid for a barrel by the time you figure out your 9 twist isn't happy with a 77gr

 

[Davo308]

At 2650 muzzle velocity...
Sg: 1.78
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

At 2750 muzzle veloity...

Sg: 1.81
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

At 2850 mv ( Buffalo bore spec)

Sg: 1.83
The bullet will be well stabilized and yaw will be minimized. This is a good twist for maximum ballistic performance.

Bullet Gyroscopic Stability Calculator

Calculate the Gyroscopic stability of a bullet with the Miller approximation.

bisonballistics.com

Projectile length of 0.95 inches.

Update berger calculator using 0.994 projectile length is still 0.51 stability factor at 2750 fps.

 

[bfoosh006]

FWIW... I'd suggest making sure your barrels twist is truly 1n9.
Lots of barrels can be faster or slower than the stated twist rate.

It's doing 2.5 moa at 100....the bullet isn't working in your barrel...plain and simple.
Buy a 7 twist and be done with it , at today's ammo prices you'll have paid for a barrel by the time you figure out your 9 twist isn't happy with a 77gr

And this... is very true.

 

[Gohring65]

You should reload if you’re going after precision in a AR.

 

[Phil McGrath]

With a 1/9 tube it’s hit or miss with the 77’s. Those that I have seen that didn’t they did prefer the 75’s, so with that said your going to have too find out for yourself.

 

[Davo308]

You should reload if you’re going after precision in a AR.

This was never intended to be a precision ar.

Just a Jack of all trades with the realization that most. Of my ammo is xm193.

I didn't even know about 77 grain smks till years later

 

[SporterII]

The 69 SMK is a very good bullet. Perfect for the 9 twist. Easy button for success.

 

[Biggy]

IMHO and in barrel makers opinions, 77gr or above = 1 in 7 or 1 in 7.7 twist is best and 1 in 8 twist depending on the barrel length can be good to very good.

 

[Molon]

. . . 1 in 8 twist depending on the barrel length . . .

It’s largely Internet myth that bullet stability increases with increasing barrel length. Technically, the gyroscopic stability factor does increase with increased barrel length, however the amount that it increases is miniscule and lost in the noise of other variables.

As an example, the 55 grain bullet in M193 will have a gyroscopic stability factor of approximately 4.25 when fired from a 14.5” Colt M4 barrel. When the same M193 round is fired from the 5.75” longer barrel of the 20” Colt A2 barrel, the gyroscopic stability factor of the 55 grain bullet only increases to approximately 4.27.

While the increased RPM due to the faster velocity will act to increase the bullet stability, the increased velocity also “increases the force applied to the nose of the bullet at the center of pressure and strengthens the overturning aerodynamic torque which actually makes the bullet less stable.”* These two opposing dynamics are the reason for the miniscule increase in gyroscopic stability factor.


* From Applied Ballistcs For Long-Range Shooting by Bryan Litz


The 0.58 MOA 10-shot group pictured below was fired from a semi-automatic AR-15 with a 1:8" twist barrel, using hand-loads topped with the Sierra 77 grain MatchKing.

.....

 

[Gohring65]

This was never intended to be a precision ar.

Just a Jack of all trades with the realization that most. Of my ammo is xm193.

I didn't even know about 77 grain smks till years later

It’s hard to judge a rifles capability with one brand of factory ammo. You’re chasing your tail trying to figure out what’s wrong without using multiple brands of ammo. That’s where reloading can give you options.

 

[Phil McGrath]

This was never intended to be a precision ar.

Just a Jack of all trades with the realization that most. Of my ammo is xm193.

I didn't even know about 77 grain smks till years later

In that case you can run with just about everything you want too, just know that the 77’s might not agree with your barrel.