Ballistics: Heavy Bullets in the 1:13" Twist .30-06

Take It To the Limit: Heavy Bullets in the 1:13" Twist .30-06
by: Germán A. Salazar


How heavy of a bullet can you shoot in a .30-06 with a 1:13" twist barrel?  Can you shoot a 175?  How about a 185, a 190 or even a 200 grain bullet?  What's the limit?  I've been wondering about that for a while because I have just such a rifle (click here for related article).  Although the 155 gr. bullets shoot extremely well in this barrel, I like an interesting project and with a good assortment of bullet weights on hand, exploring the limits of stability promised  to be a fun and educational endeavor.

The conventional wisdom among competitive shooters holds that the 1:13" twist barrel is well suited to shooting the 155 gr. bullets and little else - or is it?  Some years ago at a 300 Meter ISU match at Fort Benning, Tom Tamas, then the best 300 Meter shooter in the U.S., was shooting next to me with his .30-06 free rifle; naturally we began to talk about it, especially about loads.  Tamas told me that his rifle had a 1:14" twist barrel and he was shooting the Sierra 190 at a very low MV, around 2300 fps.  This is about the MV level of the famed Frankford Arsenal International Match ammo of the 1920's and 1930's, but that FA match ammo used the 173 gr. government match bullet and was fired in 1:10" twist barrels.  Interestingly, Tamas also mentioned that his rifle and load combination didn't shoot very well at all beyond 300 meters.  The successful use of the 190 at a low MV and with a slow rate of twist really caught my attention as it was not something that should work based on most shooters' understanding of external ballistics.  Yet, notwithstanding conventional wisdom to the contrary, here was Tamas soundly dominating the match with this improbable combination.


My objective with this test is to determine how heavy a bullet will reliably stabilize in the 1:13" barrel on this rifle.  This isn't simply a theoretical exercise, but a practical one to learn about bullet stability for Highpower competition.  I'm confident that the 175 gr. bullets will work, but from there on, I don't know.  I will test 175, 180, 185, 190, 197 and 200 grain bullets from a variety of manufacturers.  Because stability is related more to a bullet's length than to its weight, I've measured a selection of useful bullets for this test and obtained the following results:

The first length column shows the average of five samples of the actual bullets I shot.  The second length column and the G7 ballistic coefficient are taken from Bryan Litz's book Applied Ballistics for Long Range Shooting.  My measured lengths correlated well with Litz's measurements.  Litz defines projectile stability as "the ability of a projectile to maintain its point-forward orientation in flight, and return to that orientation if disturbed" (Litz, at p. 147).  That is what we'll try to evaluate with these tests.  I highly recommend a close reading of Chapter 10 of this book, it covers gyroscopic stability (Sg) and dynamic stability is detail but without overwhelming the reader with mathematics.  The concepts presented by Litz will be summarized here as they form the basis for our understanding of the test results.


Click table to enlarge.
The last column in the chart show each bullet's calculated gyroscopic stability factor (Sg), which is a measure of the spinning bullet's ability to resist the aerodynamic force on its nose that tries to make it tumble.  The faster the bullet spins, the greater the gyroscopic stability will be.  Gyroscopic stability can be quantified (a program for that purpose is included with the book) with Sg 1.0 being a bare minimum if a bullet is to avoid tumbling right out of the barrel and Sg 1.4 being recommended as a practical minimum to ensure adequate gyroscopic stability. 

If a bullet has an adequate Sg, which Litz defines as being no less than Sg 1.4, then gyroscopic stability will increase as it travels downrange because the force acting on the nose decreases with velocity but the rate of rotation which counters that force stays almost the same.  Therefore the rotation's countering force to the aerodynamic force is proportionally greater downrange.  Increasing the MV of a bullet will only slightly increase gyroscopic stability because the aerodynamic force trying to overturn the bullet increases in almost perfect proportion to the increased spin rate's ability to counter it. Accordingly, MV plays a very small role in gyroscopic stability.

If a bullet's Sg is between 1.0 and 1.4, the bullet should be stable enough not to tumble, but it won't necessarily settle into a stable flight.  This marginal level of gyroscopic stability means that the initial pitch and yaw motion that every bullet has won't damp out; consequently, the bullet will have a lower effective BC because of the higher drag induced by the pitching and yawing motions and accuracy will be compromised.  These problems will increase as the bullet travels downrange, so unlike the bullet with adequate gyroscopic stability (Sg at least 1.4) which gets more stable, the marginally stabilized bullet will become less stable as it travels downrange.

The Sg is most easily increased for a given bullet by speeding up the barrel's twist rate.  As an example, to increase the 190 Sierra's Sg from 1.39 to 1.50, all we have to do is change the twist rate from 1:13" to 1:12.5".  However, if we were to try to do it by increasing MV, we would have to raise it from the usual 2800 fps to 3500 fps - not an easy task and certainly not possible with the .30-06.  Increased MV is not the best way to find stability.

The other element of ballistic stability, known as dynamic stability, deals principally with the effect on a bullet of going into the transonic/subsonic region and is principally a factor of bullet design.  Most long range shooters are aware of this and load their ammunition to a sufficiently high MV to avoid this problem and it is not relevant to this test.  We are focusing on gyroscopic stability because that is exactly what is affected by the barrel's rate of twist and it is not as widely understood. 

Moving on to the testing, there were some decisions to be made as to bullets and loads.  Because I've already shot both of the Berger 155 gr. bullets through this rifle at 500 yards, and I know they shoot very well, I didn't include them or the Sierra 155 in this test.  There was no reason to, really, because the object is to see how heavier bullets perform in the 1:13" twist; that the 155's will perform well is no mystery.

For the sake of simplicity, I decided to use the same charge of IMR 4350 for all bullet weights.  This was a relatively light charge, about 2 grains below what I normally shoot with a 200 gr. bullet and 4 grains below my load for the 175.  No effort was made to tune the load or seating depth to any particular bullet because this isn't an accuracy test, it's a stability test.  The Wilson seating die was adjusted for each bullet, but only to maintain a uniform 0.010" jump to the lands for all of them, not to tune any bullet to an optimum level.  The goal was simply to see at what bullet weight we would fall below a minimum practical level of gyroscopic stability.

Saturday morning rolled around right on schedule and I met some fellow members of the Desert Sharpshooters Rifle Club at the Ben Avery range for our regular practice, chrono and test session on the 100 yard range.  After setting up the reloading equipment and firing a few shots to confirm the zero, we got down to the business at hand.  Temperature in the low 90's, humidity 10%, a moderate strength, variable wind from the left and bright sun were the prevailing conditions.  For those who want to do some math later, the range is at approximately 1640 ft. elevation.  All firing was done prone, with iron sights, at 100 yards on the NRA 100 yd. Smallbore target (A25) which has a 2 inch 10 ring and a 1 inch X ring.







First up was the 175 gr. Berger match boat-tail bullet (Sg 1.52).  As I fired, the wind kept pushing my shots to the right.  I made several small adjustments, but generally the group formed up at about 5:00 in the 10 ring with a couple to the left of that during some let-offs in the wind speed.  The load seemed very mild, primer edges well radiused, no stickiness on extraction.  I wasn't too surprised that they all went in point-first with no signs of instability; I didn't think the load was particularly accurate, but it was also very light, so I won't judge the bullet's potential by that.  I shoot the 175 Berger in my Palma rifle quite a bit and know it to be an excellent bullet when it has the right load behind it.

Next in line was the Sierra 180 Matchking (Sg 1.49).  The 180 is only slightly longer than the Berger 175, and given the results from the 175, I had no reason to think the 180 would not be stable.  That turned out to be a good assumption, with the 180's forming up nicely in the middle of the target.  The group strung vertically, but again, it was a very light load and that is a common characteristic of light loads.  Vertical dispersion was about the same as the 175 Berger, and all bullets went in point-first with no elongated holes or other signs of impending instability.



Continuing up the weight scale, I next loaded the 185 gr. Lapua D46 (Sg 1.44), a FMJ mach bullet designed in the 1930's and still a darn good choice.  Not as sleek as many modern designs, the 0.309" diameter D46 has a way of performing well in all sorts of barrels - but would the 1:13" twist stabilize it?  As the picture shows, the D46 shot better than the previous two bullets.  In part we can attribute that to the fact that the standardized load for the day was slightly closer to a normal load for the D46 than for the lighter bullets.  In any event, once more, all bullets went in point-first and cut nice round holes.


As we moved to the Berger 185 match boat-tail (Sg 1.33) I was beginning to think we were near the end.  The 185 Berger is quite a bit longer than the D46 and the Sg was now below 1.4.  The first shot went into the X ring at 4:00, the next one made the hole slightly larger, the next two barely increased it.  I couldn't believe what I was seeing.  The next six shots stayed pretty tight and the group was excellent.  What really surprised me is that the Berger 185 was shooting better from this barrel than it has from the 1:10" twist and 1:11" twist barrels in which I've previously tested it. 


Now we were close to Tamas' old 300 meter load with the Sierra 190 gr. Matchking (Sg 1.39) in the 1:13" twist barrel.  The wind from earlier in the day was back and I made a few corrections trying to keep the shots centered, but mostly succeeded in scattering them horizontally.  The load didn't exhibit any pressure signs (nor should it at the level I was using) and the shots followed my adjustments of the windage knob.  Other than one shot at 10:00 (which is a normal "bad shot" location for me) the 190 Sierra held great elevation, all shots went in point-first and all holes were nice and round.  I was surprised, this was not at all what I expected; perhaps I should have been more confident, after all it wasn't too different from the Tamas load that worked so well.

Next up was the 197 Western match hollow point (Sg 1.51).  Although the 197 is heavier than the Sierra 190, it is a shorter bullet, with a fairly blunt nose, thus the Sg was back into the "safe" zone.  This bullet was originally loaded in Western match ammo in the late 1950's and early 1960's; mine are from WCC 60 .308 match ammo which I pulled down.  Because they're shorter than the 190 and because they were intended to work in a 1:12" twist, as is standard for the .308 ammo they came in, I expected good results and I got them.  The 197 Westerns wadded up nicely with 6 shots in a single hole in the X ring and the remaining 4 close by.  No surprise at this point, but it sure was satisfying and the other club members were definitely becoming more interested in the test.

Now we reached what would be the end of this phase of testing simply because we were down to the heaviest bullet I brought: the 200 gr. Sierra Matchking (Sg 1.33).  Whether they went in point-first or not, we were out of bullets to test!  With the load being a lot closer to normal than it had been for the other bullets, the rifle took on a more characteristic bark with each shot and the bullets began to tear out the X ring at 8:00.  I was almost laughing as I looked in the spotting scope after each shot, because the hole kept getting bigger, but the bullets weren't going anywhere else!  By the time I got to the tenth shot I was trying a bit too hard and that one snuck out of the X ring, but a 100-9X was pretty darn good for a combination that the conventional wisdom would have rejected for use at any distance.  Although the Sg of 1.33 is marginal, at this short distance, the inability to damp out the pitch and yaw motions wasn't having any particularly negative effect.

I loaded six more of the 200 gr. Sierras and headed over to Norm's chronograph as he was wrapping up his load testing.  My normal load for the 200 gr. Sierra yields 2760 fps with H4350; the light test load I was using gave 2630 fps.  I didn't chronograph any of the other loads, but since the powder charge never varied, we can safely assume that all the other bullets, which were lighter, had a higher MV than the 200, although still below their normal MV in the .30-06; probably closer to the MV one would expect from a .308.  The heaviest and slowest combination was the least likely to stabilize, but it did and it did so in grand style with that 100-9X.

Seven different bullets, ranging in weight from 175 gr. to 200 gr., fired through a barrel with a rate of twist that we generally consider unsuitable to stabilize them and not only did they all stabilize, they seemed to shoot better as they got heavier!  You might properly conclude that I was very happy with the way the first phase of the test went.  Although the results are counter intuitive to us at first, they are consistent with Litz's theoretical work and are good evidence that a marginally stabilized bullet can shoot well at short range.  The more interesting side of the testing is the mid-range or long-range test.  The next phase would be to try some of these combinations at 500 yards to see if the stability would hold up or if  they were too marginal and would deteriorate with distance.

Tamas' old comment about poor accuracy beyond 300 meters for his slow twist 190 load was prominent in my thoughts.  My calculations yield an Sg of 1.14 for the Tamas load, that's even more marginal than any of my loads and yet it shot well at 300 meters.  I decided to shoot at 500 yards with the 190 gr. Sierra (Sg 1.39) and the 200 gr. Sierra (Sg 1.33) because the 190 was almost exactly at Litz's recommended minimum Sg and the 200 was just below it.  Those two loads should show if Sg 1.4 is a meaningful recommendation for a minimum Sg.  I increased the powder charge by 1.0 grain to bring it closer to my normal load for those bullets in order to avoid load-related accuracy issues.  That charge increase should raise the MV by about 60 fps, so the 200 were probably in the 2690 fps range and the 190's at about 2750 fps.  This is still about 50 fps below normal, but close enough without detailed testing of this lot of powder and this barrel's preferences.  The test ammo was loaded at home, in the usual manner with all charges weighed.

Early on Sunday morning, Mark Ahern met me at the Phoenix Rod & Gun Club for the 500 yard test.  For those who want to do some calculations, temperature was about 75 degrees, humidity 20% and elevation is about 1150 feet, so conditions were a bit less favorable for stability, but not a particularly significant amount.  The target is the NRA MR65, the 500 yard slow-fire target with a 10 inch 10 ring and a 5 inch X ring.  As always, shooting was done prone with iron sights.

After I took a few shots to sight-in, Mark pasted a fresh target face and I fired 15 shots with the 190 gr. Sierras (Sg 1.39).  As the picture shows, they scored a 150-8X and at one point, blew the spotter out in the X ring, doing some damage to the target face.  There was briefly a slight wind to the left, but not particularly troublesome, then it stopped and the flags hung on the poles.  I was zeroed a bit low as it turns out, but I was trying to leave the elevation alone in order to gauge the load.  Overall, I would say that the 190's stabilized at the bare minimum Sg worked very well at 500 yards. 



Mark then ran up a second target for the 200 gr. Sierras (Sg 1.33).  Again I fired a few sighters, Mark put up a fresh target face and we began.  My optimism faded quickly as it became apparent that although the bullets were going roughly into the center of the target, they just weren't doing so accurately.  Shots were off call, with a larger elevation spread than the 190's and some were going out to the sides with no wind to account for it.  The score of 146-4X wasn't very impressive, especially considering how well the 200's shot at 100 yards. 


Mark reported that the bullet holes from the 200's were not perfectly round.  However, after having a spotting disc spindle inserted, then a paster put over the hole and later removed for the photos, that was impossible for me to see the slight elongation.  Mark is a tool and die maker with a fine eye for detail, so I have no doubt that what he said is an accurate observation and indicative that at an Sg of 1.33 the 200's losing some degree of stability and no longer at peak accuracy by 500 yards.  I finished the morning by firing a few left over 190's and 200's into a single target at 100 yards; for what it's worth, they all went into a tight group similar to the previous day's shooting at the higher elevation of the Ben Avery range.

That the 1:13" twist just wasn't enough for peak accuracy with the 200's at 500 yards was no real surprise.  That it kept the 190's shooting well was surprising to me but in line with Litz's recommendation of Sg 1.4 as the bare minimum for good downrange stability.  When I first read the stability chapter in Litz's book, I wondered how accurately derived that recommendation of Sg 1.4 might be - was it just 1.0 plus a big safety margin or was it experimentally derived?  I don't know how Litz got there, but my testing certainly supports that figure as being a very useful guide when evaluating potential bullet and barrel twist combinations.

UPDATE - June 5, 2010

This target shows the 155 Berger VLD  (Sg 1.44) fired at 500 yards.  I decided to shoot one target with this bullet because it has produced scores of 200-14X and 200-17X for me at 500 yards in this rifle in recent weeks.  This time it was a 199-12X under hot and breezy conditions.  Although I think there was more vertical dispersion than usual, all my loads were acting a bit hot on this day (about 100 to 102 degrees while firing) so that may be a big part of it.  This target is a useful baseline for evaluating all the others.
Next up was the Sierra 180 Matchking (Sg 1.49).  Like the 155, it has an Sg over 1.4, Litz's minimum recommended number and therefore should shoot well.  In fact, it did, most of the vertical dispersion you see occurred in the first few shots when the barrel was settling in to the powder change from the previous string (H4350 with the 180 from surplus 4895 with the 155).  The score was a 199-7X, not as good an X count, but shots were on call and I lost a few X's to the settling-in

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The last bullets fired was the 197 gr. Western (Sg 1.51) which gave me a 197-10X.  Despite it's heavy weight, this bullet is very blunt, that's why the Sg is high and the BC is low.  Therefore, it really drifts in the wind, about the same as a 168 Sierra is my best estimate.  Accordingly, I got pushed out the sides a bit.  There was also some settling-in as I switched to H4831sc for this load.  Overall, the 10X count shows that the bullet performed well in the 1:13" twist as the high Sg indicated that it would.  Compare this to the target fired with the 200 Sierra (Sg 1.33) and you'll see what a difference a higher Sg can make.  All shots after the settling-in were on call and predictable.

My conclusion from this test is that Litz's recommendation of a minimum Sg of 1.4 is a very sound piece of advice and if followed, will allow you to use some bullet/twist combinations that run counter to conventional wisdom, but will perform very well.  I also learned that while stability and bullet weight are closely related, bullet form has a lot to do with it also and muzzle velocity has very little to do with it.  Notice that the last three bullets fired at 500 yards each increase Sg as they increase weight!  Why?  Because they are also becoming more blunt as they go up in weight; that's just a function of the three bullets I picked, of course.  Had they all been from the same maker, that would not likely be the case.  I still prefer a 1:11" twist for my general shooting with the .30-06, but it's interesting to know what can be done with the 1:13" twist.  I set out to learn something new and I learned even more than I expected about what works, what doesn't and most importantly - why they do or don't work.

Bryan Litz's book can be bought directly through his website http://www.appliedballisticsllc.com/ and I highly recommend it for those with an interest in external ballistics.  If you would like to use the Litz program to calculate Sg or for general drop and drift calculations but don't have the book, Bryan has generously given permission to our club to host an online version of the program at our website http://www.desertsharpshooters.com/ , just click the Ballistics Software menu item.

My sincere thanks to Mark Ahern for his help with target pulling and photos, to Oliver Milanovic for more target pulling, to Jack Arnold who also helped with the photos and to Jason Christ who put the data chart into a useable format - thanks guys!


 

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