Long bullets in almost-fast-enough twist bbls?

[mwsenoj]

Getting ready to load some high BC in my 1 in 9" 20" .223. If some of the bullets are rated for a little bit faster twist than I have and I am wondering if I can overcome my lack of twist rate can be padded by higher fps? I am sure that I will to some extent but will it be of any significance?

 

[mwsenoj]

No replies yet? Something came up and I didn't get to load any. Hoping to get at least a few people's opinions/knowledge by load time

 

[USSR]

While you can "make up" some bullet stabilization with a too heavy (too long) bullet in a too slow twist rate rifle by driving the bullet faster and increasing the RPM's, there is no way of quantifying it other than trying the load in your particular rifle.

Don

 

[NCsmitty]

I am wondering if I can overcome my lack of twist rate can be padded by higher fps?

The short answer is yes, if you're working with a border line potential. The problem with many 223's, especially AR types, is the 16" barrels that limit that potential, and the velocity needed to stabilize the longer bullets.
Your rifle at 20" will gain at least 100fps over a 16" version, as more barrel length is free extra velocity, as most people know.
The only way you'll know is to try, but you can only push that 20" so far.


NCsmitty

 

[snuffy]

It would help a great deal to know exactly what "high BC" bullet we're talking about. If it's a 75 grain BTHP, then maybe, If it's a 85/90 grainer, then there's no way Hose.

The problem is you're limited by maximum pressure you can load to. It would be no problem if we didn't have a pressure limit.

 

[mwsenoj]

Yes, I have some 68 - 75 gr offerings. Berger, SMK and Hornady.

 

[Clark]

75 gr Vmax in 1 in 14" twist 6mm barrel will stabilize for me at 50 yards, but keyhole at 100 yards.

So if I hunt long range once or twice a year, and then go to local 100 yard ranges, I need lots of safety margin on the twist rate.

In 2010 I had 180 gr VLD 7mm bullets in a 9.25" twist barrel that got tiny groups at 100 yards and missed deer at 500 yards.

 

[kelbro]

All three of those will shoot fine in a 20" 1:9 without going over max.

 

[mwsenoj]

Ok, the test came back. I did not have any real point of aim for the target so don't pay attention to group size. I shot at 100yds and hopefully these bullets will be stable out past that as well.

 

[snuffy]

What I see on that "target" is a waste of components. You proved nothing, only gained some trigger time.

The paper is so thin that you can't see if the bullets were making nice round holes, instead of a slot. A scribble for a bullseye shows no clue as to group size.

Buy some decent targets, go back, start over. Well, at least you have some fire-formed brass!

 

[P-32]

What I see on that "target" is a waste of components. You proved nothing, only gained some trigger time.

Snuffy, I don't totally agree. Yeah trigger time, but he also did not have key holing either which in this case could have been a waste of resources loading bullets that key hole.

 

[MtnCreek]

I have to agree with waste of components. The target looks like it is showing some signs of bullet instability, but it could also be a sign of poor target backing. The bullets do not have to impact sideways to show instability.

What are the bullets you're using?

 

[mwsenoj]

Ya, the sole point of the test was to see if the rifle would stabilize the heavy bullets. I had heard that key holing sometimes doesn't happen at closer distances (i.e. 50 yds) I wasn't investigating group sizes or anything else besides key holing. I bought 5 each of the Berger 68, 70, 75 Hornady 75 match and I have some 15 year old 69gr SMKs that my dad never used.

Also, the targets were not backed well, but they were fairly thick pieces of paper - good enough to show that they weren't spinning off centerline.

I would also like to add, I am just doing this stuff when I have a chance. It may not be the best way to figure out if these bullets are capable of bughole performance, but I have a new job and a house that I am fixing up and time is my most precious comoditiy. I shot these tests well after sunset with the car lights shining on the target and they answered my question for now. I will try to load up a few more and shoot @ 200 yds next chance I have.

 

[helotaxi]

The 68 and 69gn will be fine. No worries there.

The heavier Bergers are VLD profiles and will require some real investigation. Same with the 75gn Hornady BTHP. Chances are good that the 75gn Berger will not work and the 70gn will depend on atmospheric conditions so not entirely suitable. Anything longer than those is going to be a "no go" in a 1:9. That includes the 75gn AMax.

Clark- if it keyholes at 100 is wasn't really stable at 50. A bullet that is stable (really stable, not early in a divergent oscillation) at 100yds is going to be stable all the way to the target unless it goes transonic somewhere in between. Bullets don't go from stable to unstable as they slow down within the supersonic range.

 

[USSR]

Bullets don't go from stable to unstable as they slow down within the supersonic range.

Au contraire. If the RPM's are not high enough enough, a bullet that exhibits stability at a shorter range, can become unstable at a longer range even though it remains supersonic.

Don

 

[helotaxi]

Then it wasn't really stable to begin with. Bullets don't just suddenly become unstable. The upset forces on the bullet are the highest as it leaves the muzzle and is moving the fastest. If it is spinning fast enough to be stable at this point is isn't going to get worse from there. The velocity is going to decrease relatively quickly and the RPM isn't. The net result is that the bullet's rotational moment will become more stable as the velocity decreases.

A bullet is either stable or not. If the RPM isn't high enough for it to really be stable, it is in the early stage of a divergent oscillation and isn't actually stable at shorter range. This isn't the bullet going unstable. This is the bullet never being stable.

 

[35 Whelen]

I directed this same question to my buddies with whom I shoot High Power as I shoot a 1-9", 20" AR. Their answer was everything lighter than 75 gr., I'd be OK, 75 gr. bullets, MAYBE, especially the shorter length bullets such as the 75 gr. Hornady HP's. Anything over that, no way. I trust these guys because part of the X-course competition involves shooting out to 600 yds.

I have a Rem. 788 in .223 with a 1-14" barrel. According to Greenhill's formula, that twist shouldn't stabilize a bullet over .7" in length. Undeterred, yesterday I loaded some 60 gr. Hornady SP's (.75" long) and some Sierra 65 gr. SBT's (.8" long IIRC). Velocity of both was a shade over 3100 fps. The Hornady's grouped 5 shots right at 1", while the Sierra's, though striking the target perfectly straight, grouped 5 shots over 2 1/2".

I also have a 6.5x55 my Dad built years ago that has a 1 in 9" twist. Shooting 160 gr. RN's, I was getting nice, small groups, but noticed each bullet hole in the paper would show a slight smudge on one side of the hole. I tried them at 200 yds. and still got nice, small groups, but still the smudge. The bullets obviously weren't stabilizing, but were still accurate to at least 200 yds.

I said all that to say this: try them, but don't judge the stability, or lack thereof, by how round the bullet hole is in the paper.

35W

 

[USSR]

Bullets don't just suddenly become unstable.

Sure they do, if the RPM's drop below what is necessary to stabilize a bullet. They can be marginally stable at close range at high velocity, and then as the velocity drops at long range, become unstable. Bullet design also comes into play. The Sierra 168gr MatchKing bullet with it's 12-13 degree boattail is a perfect example of a bullet that is a superb bullet out to 600 yards and bit beyond, but becomes unstable at long range. That's why most good LR bullets have 9 degree boattails.

Don

 

[helotaxi]

The Sierra 168SMK is known to have an issue with transonic stability. It has nothing to do with spin and everything to do with the shape of the bullet reacting poorly to entering the transonic region as the shockwave works its way forward along the body of the bullet. That is the exception to bullets becoming unstable down range. If you read my earlier post I specifically mention that.

A bullet that is stable (really stable, not early in a divergent oscillation) at 100yds is going to be stable all the way to the target unless it goes transonic somewhere in between. Bullets don't go from stable to unstable as they slow down within the supersonic range.

The rate of spin on a bullet decays so slowly that it can assume to be essentially constant over the flight range of the bullet.

 

[35 Whelen]

I'd suggest you post this question over at the National Match Competition forum. Those guys in compting in High Power deal with this regularly and you'll get a positive answer based on first hand experience rather than hypothesis and conjecture (mine included).

35W

 

[USSR]

The Sierra 168SMK is known to have an issue with transonic stability. It has nothing to do with spin and everything to do with the shape of the bullet reacting poorly to entering the transonic region as the shockwave works its way forward along the body of the bullet.

Sorry, but the Sierra 168SMK becomes unstable well before it enters the transonic phase. I've personally spotted for a guy that was using it on a 1,000 yard range, so this is first hand empirical evidence, not something 2nd hand. If this wasn't the case, all you would have to do is run the 168SMK at a high enough velocity where it wouldn't go transonic at 1k, and all would be fine. However, this is not the case.

The rate of spin on a bullet decays so slowly that it can assume to be essentially constant over the flight range of the bullet.

While the rate of spin does not decay at anywhere near velocity, to say it is essentially constant is ridiculous. If the rate of spin did not go down over range, a bullet would remain nose up and not nose over to align with it's trajectory as velocity drops (i.e. the bullet would hit the target with it's meplat hitting the target slightly above the base and not produce a perfectly round hole).

Don

 

[243winxb]

223 Rem. 1-9 Twist Barrel.

The Savage Axis 1-9 twist is perfect, for the 69gr Sierra or 68 gr Hornday. So Savage instruction say. Test firing proves them correct.

 

[35 Whelen]

Sorry, but the Sierra 168SMK becomes unstable well before it enters the transonic phase. I've personally spotted for a guy that was using it on a 1,000 yard range, so this is first hand empirical evidence, not something 2nd hand. If this wasn't the case, all you would have to do is run the 168SMK at a high enough velocity where it wouldn't go transonic at 1k, and all would be fine. However, this is not the case.

Need more information. You state it becomes unstable prior to entering the transonic phase. OK, becomes unstable when fired from a barrel with a 12" twist? 14"? 10"? At what MV? At what range? At what velocity? At what RPM?
If what you're saying is true, then a 168 gr. MK, or any bullet for that matter, would be unstable at some unknown, relatively low velocity which I know to be untrue. This because I've fired a few hundred, maybe more, 205 gr., .30 caliber cast bullets at velocities as low as 1100 or so fps out of "standard" 30-06 chambered rifles and they're always very stable. ALWAYS. I can show you many 100 yd. targets where these bullets, started at MV's of 1400-1500 fps, shot nice groups over and over. In fact, one of my buddies loads my cast bullets in his '06 for our local HP reduced course matches with good success.

This can easily be proven or disproven. I have in my shop a bag of Sierra 175 MK's. You tell me at what velocity this type bullet becomes unstable, and I'll load several to the muzzle velocity you believe they become unstable and fire them at my 100 yd. target or if you prefer, the 200 yd target. I 100% promise you they'll be stable AND accurate. I've fired way too many "slow" heavy .30 caliber bullets to believe differently.

35W

 

[USSR]

This is just a blanket statement, or rather a guess, without more information. You state it becomes unstable prior to entering the transonic phase. OK, becomes unstable when fired from a barrel with a 12" twist? 14"? 10"? At what MV? At what range? At what velocity? At what RPM?

Fired by a Remington 40X, 1-12" twist at 2700fps at the 1000 yard range during an F Class Match at Bodine, PA. The rifle and load was performing fine at 600 yards, prior to moving to 1000 yards. Exterior ballistics software estimates the velocity at 1000 yards was 1279fps, which is more than 150fps above the point at which the bullet goes transonic.

If what you're saying is true, then a 168 gr. MK, or any bullet for that matter, would be unstable at some unknown, relatively low velocity which I know to be untrue. This because I've fired a few hundred, maybe more, 205 gr., .30 caliber cast bullets at velocities as low as 1100 or so fps out of "standard" 30-06 chambered rifles and they're always very stable.

You simply cannot compare cast bullets at low velocities shot at short distances with spire pointed jacketed bullets shot at high velocity at long range. I cast lead bullets as well, and I have a 240gr .30 caliber bullet that will stabilize at 1500fps in my 1-10" twist .30-06. But, a 240gr Sierra MatchKing at 1500fps would not stabilize at some point, whether it would be at 50 yards or 250 yards, I do not know.

I have in my shop a bag of Sierra 175 MK's. You tell me at what velocity this type bullet becomes unstable, and I'll load several to the muzzle velocity you believe they become unstable and fire them at my 100 yd. target or if you prefer, the 200 yd target. I 100% promise you they'll be stable AND accurate. I've fired way too many "slow" heavy .30 caliber bullets to believe differently.

You're talking about 100 - 200 yards, and I'm talking about shooting long range. I shot competitively in 1000 yard F Class for 6 years, so I can tell you that results at a couple hundred yards do not translate to what you can expect at 1000 yards. For example: I was shooting at Reade Range in Pennsylvania at an F Class Match with a friend. He had a .308 load that used the Berger 210gr VLD bullet. Now, this bullet has a BC of .640, so there is no way in hell that it is gonna go transonic at 1k at 2500 - 2600fps at the muzzle (retained velocity at 1k is estimated at about 1450fps). We fired the 600 yard match first, and he was doing REALLY well. Then we move back to 1000 yards, and things went all to h3ll. Now, I have shot with this guy before, so I know it was not due to operator error. The only possible explaination is that the heavy bullet was marginably stable at close and mid-range, and as the velocity and rpm's dropped, the bullet became unstable at long range.

Don

 

[helotaxi]

Fired by a Remington 40X, 1-12" twist at 2700fps at the 1000 yard range during an F Class Match at Bodine, PA. The rifle and load was performing fine at 600 yards, prior to moving to 1000 yards. Exterior ballistics software estimates the velocity at 1000 yards was 1279fps, which is more than 150fps above the point at which the bullet goes transonic.

You confuse the term "transonic" and "subsonic" The transonic region begins in the mid 1300 FPS range. The 168SMK goes unstable in the transonic region.

You simply cannot compare cast bullets at low velocities shot at short distances with spire pointed jacketed bullets shot at high velocity at long range. I cast lead bullets as well, and I have a 240gr .30 caliber bullet that will stabilize at 1500fps in my 1-10" twist .30-06. But, a 240gr Sierra MatchKing at 1500fps would not stabilize at some point, whether it would be at 50 yards or 250 yards, I do not know.

Actually you can make the comparison. And the 240SMK is a dynamically stable bullet, unlike the 168SMK and makes the transonic transition very well.

Pick up Applied Ballistics for Long Range Shooting by Bryan Litz. He discusses this in great detail in Chapter 10. To quote the take-aways from the end of that chapter "3. Avoid the .30 caliber 168 grain Sierra MatchKing for long range shooting (over 800 yards)...the Sierra 168 MatchKing has a unique dynamic instability that prevents it from flying properly at long range."

"4. Most bullets don't have problems with dynamic stability until they reach transonic speed (~1300fps)."

Dynamic stability has nothing to do with spin or the bullet being "marginally stable" at the muzzle.

If you want to argue with him go right ahead. BTW, he used to design missiles, won the US Palma in 2008 and is the chief ballistician for Berger bullets.

Additionally, your friend's 210 Berger VLD has an average BC of .631 over the entire velocity range, the .640 number is only good at 2500fps. The BC decays with velocity. Depending on the atmospheric conditions, the bullet could have been entering the transonic region around 900yds.