Bullet concentricity and twist rate

[spitballer]

Have a question regarding bullet concentricity and twist rate: Like most guys when I look through the scope and I'm holding steady on the bull I see no reason why the bullets don't land where I'm pointing them, even if it's some distance away. But this past weekend I had a group arranged in an interesting pattern that may indicate concentricity problems, and I'd like to get an expert opinion on this:

The ten-shot group was fired at 300 yards and consisted of a core hole with three rounds fired through it, and the remaining seven arranged roughly in a circle around the center group. Although the overall size of the group wasn't bad, I can only remember pulling one of the rounds and therefore there must be a reason why the remaining nine rounds didn't go where they were pointed. My reloading manual says that out-of-round bullets tend to 'walk towards the target', not unlike an out-of-round washing machine that tends to 'walk around the laundry room'. Is this what's happening here?

I remember a respected member advising us that the top shooters only twist the bullet fast enough to stabilize it, and no more. Is this why? To preempt gyroscopic 'walk'?

I also have a related question regarding barrel design: While researching the development of M193 ammo I discovered that the rate of twist for the M16 was increased from 1:14 to 1:12 to reduce 'tumble' and excessive wounding. This was a decent decision considering the application IMO, but for purposes of range-only accuracy, wouldn't a 1:14 twist be more appropriate for the 55gr boat tail?

My present barrel was intended for 55gr BT's only and I originally specified a 1:14 twist, but agreed to the 1:12 advised by the salesman. Should I have insisted on the 1:14? Thanks in advance for valued input.

 

[Walkalong]

The best jackets are the most consistent in thickness all the way around. That makes the lead core inside more concentric. That reduces wobble because the lead core is close to being perfectly round. In a poor jacket the lead core will not be so close to perfectly round and will wobble just like an unbalanced tire. Guess which ones shoot the best.

Now, spin that bullet. Since that lead core is close to but not perfect, it wobbles ever so slightly. The faster you spin it the worse it gets, so using a twist rate no faster than needed helps.

Less consistent jackets are used on things like FMJ and other blasting ammo bullets.

These days most jackets are very good, but only the best are used for match bullets.

It is going to be hard (Very hard) for you to shoot the difference between a 1 in 12 and 1 in 14 twist, assuming all other factors are equal. First you need a gun and bullets up to the task, then it's up to you. My Sako .222 Mag has a 1 in 14 twist and will shoot 50 and 55 gr bullets great. It is iffy on 60 Gr bullets. 1 in 12 is not over rotating 55 Gr bullets enough to affect accuracy. IMHO.

 

[243winxb]

top shooters only twist the bullet fast enough to stabilize it

Less torque, means the rifle rides the bags better on recoil. http://www.bergerbullets.com/twist-rate-calculator/

 

[Bart B.]

Walkalong is on the right track. The faster you spin a bullet, the more it will jump off the bore axis from the centrifugal forces built up in it.

5 to 10 percent of the very best match bullets are probably perfectly balanced and the others a very tiny bit unbalanced. It's not easy to form perfectly uniform bullet jackets and uniformly seat a lead core in them. Which is why the most accurate benchrest rifles and ammo shoots record holding groups under 3/10ths MOA at 100 yards. A few groups will be under 1/10th MOA, some about 2/10ths MOA and a few up to 3/10ths MOA. All those thousands of other groups are larger.

Then there's the inconsistency of what's aiming and holding the rifle. Us humans are not very repeatable holding a rifle while bullets are in the barrel. I once watched a test of a dozen people shooting a 308 Win match rifle that tested about 1/3 MOA at 100 yards in an accuracy cradle shoot 5-shot groups ranging from 3/4 inch to near 2 inches. No wonder benchrest folks shoot their rifles in free recoil and don't hold them against their shoulders.

 

[spitballer]

Sincere thanks for top advice. I've taken away a few helpful tidbits from this thread:

1) Wobble is exacerbated by excessive twist.
2) Rifle movement at the bench explained in part by torque.
3) Repeatability during brief trip down the barrel.

All three of these areas are inter-related and worth focusing on, 243winxb your comment on torque shed big light here thx

 

[Walkalong]

One of the reasons some folks did not like .40 S&W when it came out was "torque". All weapons have some torque, but most don't have enough to really notice much. Some really twist on you. It depends on velocity, twist, and bullet weight, compared to the guns weight and design.

 

[Bart B.]

Rifle torque can be a tool to help folks learn how to keep their aiming eye open while the rifle fires so they can call their shots properly.

Watch the end of a scope reticle at the edge of the field of view. If you see it jump counterclockwise as the rifle fires, then your aiming eye's open when the rifle fires. More easily seen shooting magnum cartridges; they torque more than lesser ones.

It'll jump clockwise with a left hand twist.

 

[Jim Watson]

While researching the development of M193 ammo I discovered that the rate of twist for the M16 was increased from 1:14 to 1:12 to reduce 'tumble' and excessive wounding. This was a decent decision considering the application IMO, but for purposes of range-only accuracy, wouldn't a 1:14 twist be more appropriate for the 55gr boat tail?

That does not agree with contemporary accounts. The reason then given was that the M16 rifling twist rate was increased from the traditional .22 centerfire 14" to GI 12" was for adequate stability of the relatively long 55 gr FMJ BT in cold air. Work done in the Cold Regions Research and Engineering Laboratories, Alaska.

I think applying benchrest and target shooting criteria to shooting 55 gr FMJ BT is a misdirection of effort. Why are you locked into the type? But since you are, your 12" twist is about right for the 55 gr FMJ BT. The best results I have seen reported were with the Hornady 2267.

A 14" twist shooting 50 - 55 gr flatbase spitzers or 52 gr BT HP is a proven combination for moderate range accuracy.

I saw an old article about a benchrest shooter who used some sort of concentricity gauge on his bullets. He marked them for the direction of error and shot them with half "up" and half "down". He got two distinct groups, maybe a quarter of an inch each, about a quarter of an inch apart.

 

[Bart B.]

Dr. Franklin W. Mann drilled holes in the side of black powder bullets and showed how four different groups could be shot spaced 3 hours apart on a clock face after being oriented the same in the chamber. That was over a hundred years ago.

The .30-06 had too fast a twist (1:10) for the 150-gr spitzer bullets it first used. Was better in the 1920's with the 173-gr boattail machine gun bullet. Those bullet weights did better, accuracy wise, with a slower 1:12 twist in a case shooting them out 100 fps slower in a 2 inch shorter barrel using the new 7.62 NATO round.

Military teams got the best long range accuracy with the 7.62 shooting arsenal match ammo with its 173-gr. bullet replaced with a 180-gr. one in M14NM's 1:12 twist 22 inch barrel. Garands shooting the same 7.62 primed case and IMR4320 under 190-gr. bullets did best in their 24 inch barrels with 1:12 twists.

 

[spitballer]

55 gr FMJ BT is a misdirection of effort. Why are you locked into the type?...

...this would be a tipped or hollow point, not FMJ. For use in 30" target bbl. Obviously it would be a close call and I'd have to research it further, but IMHO specifying a twist rate to match a favorite bullet would be more practical and rewarding than simply using a bullet that most closely matches a commonly available twist.

 

[Jim Watson]

...this would be a tipped or hollow point, not FMJ.

Ah, ok. Mention of M193 in the same paragraph tripped me up.

The .222 Rem Magnum with then-standard 14" twist was factory loaded with a 55 gr flatbase which is shorter than a boattail.
The Berger stability calculator says you NEED that 12" twist for a Sierra 55 gr BTHP.

 

[fotheringill]

Another POSSIBLE thing to add into the mix. When you were seating the bullets, was there a noticeable difference in the pressure needed on the downstroke to seat? That would indicate a possible difference in neck tension.

 

[spitballer]

The Berger stability calculator says you NEED that 12" twist for a Sierra 55 gr BTHP...

...yes, and the JBM calc isn't much different. Keeping in mind that a figure of 1.5 is considered stable enough to satisfy the military, a 12" stabilizes my favorite bullet at around 1.46 or so, while a 14" twist only gives me very marginal stability at 1.087. However, there are certain rifling configurations from my barrel mfr that are available in a 13" twist which dial in at around 1.25...

But I'll be the first to admit that all this tweaking is unnecessary and useless if I can't stick to the fundamentals and hold steady over the target. Right now I've got another challenge I'm dealing with also, a pretty basic one, reconfiguring distance to the lands. Normally I simply seat on the lands in lieu of a crimp for consistency, but lately Bart B's comment from some months ago about a gentle start is proving prophetic and I'm trying to avoid too much pressure too early by backing off to .015". Hopefully scores will improve.

 

[ArchAngelCD]

......
I think applying benchrest and target shooting criteria to shooting 55 gr FMJ BT is a misdirection of effort. Why are you locked into the type? But since you are, your 12" twist is about right for the 55 gr FMJ BT. The best results I have seen reported were with the Hornady 2267.
......

I own and shoot a Howa 1500 in .223 that has a 1:12 twist barrel.

While I have no doubt that Hornady #2267 bullet is a good shooter I found a bullet that shoots great in my rifle. The bullet is a Sierra GameKing 55gr HPBT #1390. In my rifle that bullet paired with Varget shoot sub 1/2MOA groups out to 250 yards. (the longest distance I have available to me)

In the same rifle other 55gr bullets can shoot up to 1.5 MOA @100 yards. That is with the same brass, same primers and same charge of Varget. Yes, the bullets you shoot most definitely make a difference.

I will have to give those Hornady bullets a try, thanks. They seem to be cheaper than the Sierra bullets for the range.

 

[Jim Watson]

While I think the Hornady FMJ is an accurate bullet as far as FMJs go, I do not think it will compete with a closed base bullet like HP, SP, or nose cone.

 

[spitballer]

a possible difference in neck tension...

...good point, when I'm holding the bullet up against the lands neck tension has a powerful effect on accuracy. The key point is consistency, and these days I'm using turned necks and very light tension which helps with consistency when seated into the lands.

But nothing is as simple as it seems, and although I may be getting consistent starts by seating against the lands, I may also be shortening the power curve by delaying the bullet there. Not what I need for a lengthy target rifle...

 

[Bart B.]

The bullet is a Sierra GameKing 55gr HPBT #1390. In my rifle that bullet paired with Varget shoot sub 1/2MOA groups out to 250 yards.

That's equal to or better than benchrest records for many 5- or 10-shot groups in aggregates.

Is it true you don't get groups larger than 1/2 MOA?

 

[ArchAngelCD]

That's equal to or better than benchrest records for many 5- or 10-shot groups in aggregates.

Is it true you don't get groups larger than 1/2 MOA?

With that bullet and rifle its rare I get large groups when I do my job but the group's talking about are only 3 rounds, not 5 or 10. I should do some testing and see how I do with real groups of at least 5 shots.

The range I shoot at is on PA Game lands and they have a lot of rules. You can not load more than 3 rounds in long guns so I have to break my hold to load more ammo and when I do it's hard to get back to where I was shooting. It's a shame because I fitted my Howa 1500 with the capability of using a 5 or 10 round magazine. If the game warden catches you with more than 3 rounds in your rifle its a very larger fine. I'm not willing to take that chance.

 

[Bart B.]

OK, I understand now.

My take on accuracy is the largest groups fired. One almost always does that well or better. It's hard to duplicate the smallest groups fired, but it's a popular way to claim accuracy. I've put 3 consecutive shots inside 2/10ths MOA at all ranges up to 1000 yards a few times with both scoped and iron sighted rifles. But I don't claim them as any accuracy level. I know they happen when only one of two things happens for all shots fired; everything is perfect or all the variables cancel each other out.

And nobody knows when the largest or smallest few-shot group of several will happen.

 

[spitballer]

I'm trying to get away from cherry picking targets also. Groups of ten works out well for me because I schwab after every ten shots anyway.

Got a question for you guys: my last group of ten showed a core group of three, in the center. I was using new cases with lots of neck tension so I didn't want to seat too far into the lands with them. I thought I was seated about .002" into the lands but after re-checking measurements I think I was actually right at the lands. This would mean that some of them were just barely touching and some were just barely not touching. Anyone care to guess which ones did better?

 

[ArchAngelCD]

OK, I understand now.

My take on accuracy is the largest groups fired. One almost always does that well or better. It's hard to duplicate the smallest groups fired, but it's a popular way to claim accuracy. I've put 3 consecutive shots inside 2/10ths MOA at all ranges up to 1000 yards a few times with both scoped and iron sighted rifles. But I don't claim them as any accuracy level. I know they happen when only one of two things happens for all shots fired; everything is perfect or all the variables cancel each other out.

And nobody knows when the largest or smallest few-shot group of several will happen.

I agree, no argument here but right now I have no choice. I would like to shoot larger groups and further too. Maybe someone will open a "real" range here in NE PA some day.

 

[Bart B.]

ArchAngelCD,

You have the choice of loading one round at a time and not using a magazine. Virtually all detachable box magazine rifles shoot more accurately that way; unless you've tested each magazine for the accuracy it allows.

Even bolt action tube guns used in competition are tested with several magazines then use only those that produce best accuracy and shoot to the same point of aim.

All that was not a problem until the USA armed services got their M14 match rifles rebuilt to shoot most accurate. All of them used matched magazines for rapid fire matches that shot most accurate and to the same point of aim. I don't think your Howa 1500 is any different.

Spitballer,

I don't think .002" difference in bullet position relative to the rifling will matter much. Where the .223 Rem bullet sets relative to the rifling is controlled by the shoulder as it's hard against the chamber shoulder. The case head's not against the bolt face when its's fired. It's the distance from case shoulder to bullet ogive diameter at land contact that's critical.

 

[spitballer]

Spitballer,

I don't think .002" difference in bullet position relative to the rifling will matter much...

... doggone it I knew you were gonna say that. Well, I've got three options as I see it:

A) Seat .002" into the lands
B) Seat .002" off the lands
or
C) Seat well off the lands, say .015" - .030"

From the standpoint of consistency my guess would be either A) or B), because C) has the potential to introduce slop...

But before I load up this week's batch of ammo, let me ask you about option C): is there any advantage to seating well off the lands if I'm not using a crimp? I mean, the crimp is what helps to reduce start pressure by helping to spit the projectile through the choke point, right? And isn't this what allows the bullet to travel further down the bore before maximum pressure is achieved?

Otherwise I'm inclined to go with option B) because any inconsistency while seated just off the lands would have less of an effect than a similar inconsistency while seated just into the lands...

 

[ArchAngelCD]

Bart B,
I appreciate your advice and will act on it. After all, you have been there - done that...

 

[Walkalong]

A) Seat .002" into the lands
B) Seat .002" off the lands

I agree with Bart, and you'll never actually seat exactly .002 on or off the lands consistently.