Twist Rate and Bullet Stabilization Problems

[JFrank]

Had a fella come to a 600 yard informal shoot awhile back bringing his 308 IIRC a 1-12 twist shooting 175 gr bthp , I suggested he may have a problem after checking that calculator, well im hear to say the damn thing shot just fine.
Go figure

 

[HowieG]

Had a fella come to a 600 yard informal shoot awhile back bringing his 308 IIRC a 1-12 twist shooting 175 gr bthp , I suggested he may have a problem after checking that calculator, well im hear to say the damn thing shot just fine.
Go figure

It would sure be easier if this shooting stuff was a precise science, but it isn't. Right when you think you have a handle on it, it throws you a curve. Keeps life interesting though.

 

[wombat13]

My Savage .25-06 shoots 100 gr bullets to about 1" at 100 and 117 gr and 120 gr bullets to about 4". The berger twist calculator indicates the bullet should be stabilized, but I doubt it. I shoot at about 500' of elevation.

 

[HowieG]

My Savage .25-06 shoots 100 gr bullets to about 1" at 100 and 117 gr and 120 gr bullets to about 4". The berger twist calculator indicates the bullet should be stabilized, but I doubt it. I shoot at about 500' of elevation.

They are making straight round holes? Or keyholes?

 

[HowieG]

My Savage .25-06 shoots 100 gr bullets to about 1" at 100 and 117 gr and 120 gr bullets to about 4". The berger twist calculator indicates the bullet should be stabilized, but I doubt it. I shoot at about 500' of elevation.

Also, I guess I should be asking what bullets are shooting poorly. And have you measured your twist?

 

[Jim Watson]

Things used to be simpler.
.22 centerfire varmint rifles generally had 14" twist for flat base spitzers.
The .222 was introduced with a 50 gr bullet and shot the 52-53 gr boattails well. The .222 Magnum and commercialized .22-250 brought that up to 55. Sierra made a 63 gr semispitzer and Speer a 70 gr roundnose that are short enough to stabilize like a 55 gr spitzer.

My 14 twist .22-250 is most accurate with a 52 gr SMK or the ugly little 50 gr Remington Power Lokt. It would shoot 70 gr Speers pretty well and one 60 gr Hornady but not the other, which was longer.

Then the Army got into the game and now people worry about twist rates All The Time.

High SD, high BC bullets demand faster twist.
My contribution to the madness was when I made a .223 into an F class rifle, 6.5" twist for 90 gr boattails. A great little 600 yard rifle, not so hot at 1000 no matter what the computer said.

 

[HowieG]

Things used to be simpler.
.22 centerfire varmint rifles generally had 14" twist for flat base spitzers.
The .222 was introduced with a 50 gr bullet and shot the 52-53 gr boattails well. The .222 Magnum and commercialized .22-250 brought that up to 55. Sierra made a 63 gr semispitzer and Speer a 70 gr roundnose that are short enough to stabilize like a 55 gr spitzer.

My 14 twist .22-250 is most accurate with a 52 gr SMK or the ugly little 50 gr Remington Power Lokt. It would shoot 70 gr Speers pretty well and one 60 gr Hornady but not the other, which was longer.

Then the Army got into the game and now people worry about twist rates All The Time.

High SD, high BC bullets demand faster twist.
My contribution to the madness was when I made a .223 into an F class rifle, 6.5" twist for 90 gr boattails. A great little 600 yard rifle, not so hot at 1000 no matter what the computer said.

So, now we have the 224 Valkyrie to shoot 90 and still feed through the magazine. And it has twist problems. The 90 likes a 6.5 twist, but a 7 twist is better for the rest of the bullets and doesn't shoot the 90 that well.

Meanwhile, the PRS comp people (mainly bolt guns) have been going the other way and getting smaller. They have gone from 30 cal through 7mm and now 6.5mm is YESTERDAY. 6mm is the answer in that competition now with the 6mm Dasher et al., and they are trying to tighten the twists to handle longer and longer 6mm needles.

Makes you just want to stand back and chuckle.

 

[wombat13]

Also, I guess I should be asking what bullets are shooting poorly. And have you measured your twist?

No, I have not measured the twist of the barrel. Hornady SST 117 gr., Sierra GK 117gr., and Federal Fusion 120 gr. All produced 3+" groups at 100 yards. No keyholes. 100 gr., TTSX, Ballistic Tip, and Partitions all produced groups about 1" at 100 yards.

I don't know much about this, so maybe my interpretation is wrong. Do poorly stabilized bullets always produce keyholes? Is it possible that I could shoot 5 shots of a poorly stabilized bullet and not have a single keyhole?

 

[HowieG]

No, I have not measured the twist of the barrel. Hornady SST 117 gr., Sierra GK 117gr., and Federal Fusion 120 gr. All produced 3+" groups at 100 yards. No keyholes. 100 gr., TTSX, Ballistic Tip, and Partitions all produced groups about 1" at 100 yards.

I don't know much about this, so maybe my interpretation is wrong. Do poorly stabilized bullets always produce keyholes? Is it possible that I could shoot 5 shots of a poorly stabilized bullet and not have a single keyhole?

No. They can have marginal stability and just shoot poorly. If you moved down range to 300 or 400 yards, they might just begin to wobble and keyhole after they slow down. Or not. Your rifle may not like those bullets. You ought to run the JBM calculator on those bullets and see what that tells you about them. Are you at low altitude?

JBM - Calculations - Stability (jbmballistics.com)

 

[wombat13]

I shoot at about 500 feet of elevation (at the range). The JBM calculator yields a stability factor of about 1.5 for the 120 Fed Fusion. The others are similar, although the SST will be a bit lower since it is a longer bullet.

 

[MCMXI]

Had a fella come to a 600 yard informal shoot awhile back bringing his 308 IIRC a 1-12 twist shooting 175 gr bthp , I suggested he may have a problem after checking that calculator, well im hear to say the damn thing shot just fine.
Go figure

According to Berger's twist rate calculator a Sierra 175gr BTHP at an easy 2,600 fps with a typical 1.240" overall length is comfortably stable in a 1:12 barrel, even at 30F and sea level ... go figure!

 

[HowieG]

I shoot at about 500 feet of elevation (at the range). The JBM calculator yields a stability factor of about 1.5 for the 120 Fed Fusion. The others are similar, although the SST will be a bit lower since it is a longer bullet.

Okay. So if your rifle actually has the twist you say it does you should be good in the stability department. You could measure your twist to see if it is what it should be. If it is, that means you have another problem. Could be the bullets don't like the powder you are using or your rifle just doesn't like those kinds of bullets. Any number of things. Rifles are strange. I have one that likes Sierra bullets but is not too fond of Hornady. Who knows why....

 

[wombat13]

Okay. So if your rifle actually has the twist you say it does you should be good in the stability department. You could measure your twist to see if it is what it should be. If it is, that means you have another problem. Could be the bullets don't like the powder you are using or your rifle just doesn't like those kinds of bullets. Any number of things. Rifles are strange. I have one that likes Sierra bullets but is not too fond of Hornady. Who knows why....

Yes, I know they should be stable, but... that's why I replied. IIRC your OP asked if anyone had problems stabilizing bullets that their rifle should stabilize. Anyway, any problem is unlikely due to the ammunition loading process. These were all factory ammunition that I tried. All three factory ammo with 100 grain bullets (Partition, Ballistic Tip, TTSX) shoot great. All three factory ammo with 117/120 grain bullets shoot very poorly (SST, SGK, Fusion). I don't know much about Hornady ammo, but I have always had good results with Federal, except this time. The 117 SGK is a Federal Premium load, and the 120 gr. Fusion is Federal Fusion.

 

[JFrank]

According to Berger's twist rate calculator a Sierra 175gr BTHP at an easy 2,600 fps with a typical 1.240" overall length is comfortably stable in a 1:12 barrel, even at 30F and sea level ... go figure!

Ya
Go figure.

 

[HowieG]

Yes, I know they should be stable, but... that's why I replied. IIRC your OP asked if anyone had problems stabilizing bullets that their rifle should stabilize. Anyway, any problem is unlikely due to the ammunition loading process. These were all factory ammunition that I tried. All three factory ammo with 100 grain bullets (Partition, Ballistic Tip, TTSX) shoot great. All three factory ammo with 117/120 grain bullets shoot very poorly (SST, SGK, Fusion). I don't know much about Hornady ammo, but I have always had good results with Federal, except this time. The 117 SGK is a Federal Premium load, and the 120 gr. Fusion is Federal Fusion.

I have seen factory ammo shoot poorly in rifles, but 4 inch groups are pretty bad. I'd say the next step is to measure your barrel twist and see if it really is in spec.

 

[Blue68f100]

A neighbor of mine had a gun that was shooting very inconsistent , 3-4" groups. I traced the problem back to the scope. Once it was replaced all was good. Make sure the mounts are tight.

 

[HowieG]

A neighbor of mine had a gun that was shooting very inconsistent , 3-4" groups. I traced the problem back to the scope. Once it was replaced all was good. Make sure the mounts are tight.

That is a valid point, but I don't usually think 'scope problem' if a rifle is shooting some bullets well, and some not so well. In my experiences with bad scopes, they under perform with all ammo. It is always worthwhile to check your basics regularly anyway.

What I have learned from experience is when all else fails, shoot what the rifle likes, whatever that may be. After all else has failed, if you just can't stand it, get another gun that isn't so picky.

 

[wombat13]

I have seen factory ammo shoot poorly in rifles, but 4 inch groups are pretty bad. I'd say the next step is to measure your barrel twist and see if it really is in spec.

That is a valid point, but I don't usually think 'scope problem' if a rifle is shooting some bullets well, and some not so well. In my experiences with bad scopes, they under perform with all ammo. It is always worthwhile to check your basics regularly anyway.

What I have learned from experience is when all else fails, shoot what the rifle likes, whatever that may be. After all else has failed, if you just can't stand it, get another gun that isn't so picky.

It's definitely not the scope or mounts. The rifle shoots very well with every 100 grain bullet, but not 117 gr or heavier. I have some 110 gr. Accubonds loaded. It will be interesting to see how they shoot.

I could measure the twist, but it would be just for learning's sake. I'm not going to get rid of a $200 Savage Axis II because it only shoots 100 grain bullets great! I'll just use a different rifle if anything needs more killing than 100 grains of Partition @ 3300 fps!

If the 110 gr Accubonds shoot well, then I think this rifle has the bases covered. I can't think of anything I'd ask a .25-06 to kill that couldn't be handled by one of the following: 100 gr Ballistic Tip, 100 grain Partition, 100 grain TTSX (all confirmed accurate in the rifle) or a 110 gr Accubond (accuracy TBD).

 

[HowieG]

It's definitely not the scope or mounts. The rifle shoots very well with every 100 grain bullet, but not 117 gr or heavier. I have some 110 gr. Accubonds loaded. It will be interesting to see how they shoot.

I could measure the twist, but it would be just for learning's sake. I'm not going to get rid of a $200 Savage Axis II because it only shoots 100 grain bullets great! I'll just use a different rifle if anything needs more killing than 100 grains of Partition @ 3300 fps!

If the 110 gr Accubonds shoot well, then I think this rifle has the bases covered. I can't think of anything I'd ask a .25-06 to kill that couldn't be handled by one of the following: 100 gr Ballistic Tip, 100 grain Partition, 100 grain TTSX (all confirmed accurate in the rifle) or a 110 gr Accubond (accuracy TBD).

I totally agree. I was hoping you would measure it just for information purposes. I wouldn't/couldn't even change a barrel on a $200 dollar rifle without going outside economic boundaries to do so.

 

[wombat13]

I totally agree. I was hoping you would measure it just for information purposes. I wouldn't/couldn't even change a barrel on a $200 dollar rifle without going outside economic boundaries to do so.

How hard is it to measure twist? I'm not even sure how to do it.

ETA: Okay just read how to do it. I'll try to do it this weekend.

 

[kcofohio]

How hard is it to measure twist? I'm not even sure how to do it.

ETA: Okay just read how to do it. I'll try to do it this weekend.

Easiest way it to take a cleaning rod with cleaning bristle, mark off the inches and one make to note the revolution. Let the rifling turn the rod. When it makes one revolution, you'll know the twist.

 

[HowieG]

How hard is it to measure twist? I'm not even sure how to do it.

ETA: Okay just read how to do it. I'll try to do it this weekend.

A brush on the rod works, or a tight patch. You can do it from the breech or the muzzle. I usually do bolt actions from the breech, but it's fine either way.