1 IMPROVED RIFLE ACCURACY Jim & Jerry Bennington INTRODUCTION Tuners work on rim fire rifles. Do they work on center fire rifles and if so are they repeatable and how much improvement do they provide? Accuracy is about managing the barrel vibrations. Of course, this assumes that there is an accurate scope and it is properly mounted. It also assumes that the shooter mounts the rifle properly on the bench and does his job of reading the conditions and firing the rifle. The greater the accuracy demanded, the more the skill required of the shooter. As you read the gun reviews in the gun magazines, the authors typically try several factory loads and shoot five shot groups. They then list the best, worst and average group sizes for each load. They do this because EVERY individual rifle has an inherent dynamic response, primarily barrel vibrations, to the forces generated by given load. Since the resulting groups are a function of the shooter, environment (wind, mirage…) and the rifle dynamics, there is an unavoidable randomness to the group sizes. The challenge to load development in reloading is finding the loading combination of primer, powder, load, neck tension, seating depth, and so forth, that has the best average group size AND smallest variation. Most of us have a cut out of a target with a great group in our billfold. But one great group isn’t enough; you need to ensure that the performance is consistently repeated to have an accurate load. In a similar manner, changing the rifle to improve its dynamics can improve the accuracy. Free floating a barrel is one example that is widely used. Barrel tuners are another tool that have the potential to make a good rifle even better. We will share our experience with developing and testing tuners. They show that tuners on several center fire rifles can consistently and with repeatability improve accuracy. This project started with Larry Tolksdorf, Eric Bostrom and Jim Bennington shooting at the Manatee Gun & Archery Club (MGAC) in Myakka City, Florida. The Manatee Club has a 1,000-yard range and holds monthly gong matches at both 1,000 yards and 560 yards. Gunsmith Eric Bostrom has been building very accurate custom Savage long-range rifles that have been dominating these long-range matches. The authors have been interested in rifle tuning since before Browning introduced their BOSS system in 1995. These interests resulted in the development and patent applications for the Rifle Accuracy System, which is our combination tuner and muzzle brake. After discussing the design needs for sometime, Larry T & Eric Bostrom built the first prototype. The pictures below show the final product. Initial applications of the Rifle Accuracy System showed significant improvements in accuracy but were not conducted in a structured fashion to allow consistent evaluation. This test program described here is intended to provide the initial data to better understand the tuning system and its relationship to accuracy.
Transcript
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IMPROVED RIFLE ACCURACY
Jim & Jerry Bennington
INTRODUCTION Tuners work on rim fire rifles. Do they work on center fire rifles and if so arethey repeatable and how much improvement do they provide?
Accuracy is about managing the barrel vibrations. Of course, this assumes thatthere is an accurate scope and it is properly mounted. It also assumes that the shootermounts the rifle properly on the bench and does his job of reading the conditions andfiring the rifle. The greater the accuracy demanded, the more the skill required of theshooter.
As you read the gun reviews in the gun magazines, the authors typically tryseveral factory loads and shoot five shot groups. They then list the best, worst andaverage group sizes for each load. They do this because EVERY individual rifle has aninherent dynamic response, primarily barrel vibrations, to the forces generated by givenload. Since the resulting groups are a function of the shooter, environment (wind,mirage…) and the rifle dynamics, there is an unavoidable randomness to the group sizes.The challenge to load development in reloading is finding the loading combination ofprimer, powder, load, neck tension, seating depth, and so forth, that has the best averagegroup size AND smallest variation. Most of us have a cut out of a target with a greatgroup in our billfold. But one great group isn’t enough; you need to ensure that theperformance is consistently repeated to have an accurate load.
In a similar manner, changing the rifle to improve its dynamics can improve theaccuracy. Free floating a barrel is one example that is widely used. Barrel tuners areanother tool that have the potential to make a good rifle even better. We will share ourexperience with developing and testing tuners. They show that tuners on several centerfire rifles can consistently and with repeatability improve accuracy.
This project started with Larry Tolksdorf, Eric Bostrom and Jim Benningtonshooting at the Manatee Gun & Archery Club (MGAC) in Myakka City, Florida. TheManatee Club has a 1,000-yard range and holds monthly gong matches at both 1,000yards and 560 yards. Gunsmith Eric Bostrom has been building very accurate customSavage long-range rifles that have been dominating these long-range matches. Theauthors have been interested in rifle tuning since before Browning introduced their BOSSsystem in 1995. These interests resulted in the development and patent applications forthe Rifle Accuracy System, which is our combination tuner and muzzle brake. Afterdiscussing the design needs for sometime, Larry T & Eric Bostrom built the firstprototype. The pictures below show the final product.
Initial applications of the Rifle Accuracy System showed significantimprovements in accuracy but were not conducted in a structured fashion to allowconsistent evaluation. This test program described here is intended to provide the initialdata to better understand the tuning system and its relationship to accuracy.
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SAVAGE 260 AI TEST RIFLE
Rifle Accuracy System- RAS
Rifle Accuracy System & Precision BR Tuners
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HISTORYMUZZLE BRAKES
Muzzle brakes were probably first used on military canons and cancurrently be seen on a wide variety of them. Magna-Port was one of the first to adaptmuzzle brakes to handguns and rifles. There are currently muzzle brakes in a wide varietyof designs available. While a muzzle brake can significantly reduce recoil it alsosignificantly increases the noise to the shooter and those near him. There is no free lunch;recoil energy is exchanged for pressure (noise) energy. Muzzle brakes play an importantrole in long-rang shooting. They aid the shooter in seeing the impacts of his shots. Inshort range shooting, a scope can see the shots; this is not true at longer ranges like 1,000yards.
TUNERSTuners are a more recent innovation. Browning introduced one of the early
commercial tuner systems with their BOSS system in 1995. Most competitive .22 RFshooters use tuners to match their ammunition to their rifles. It is widely understood thatone cannot be competitive in .22 RF matches without a tuner. Short-range bench restshooters have recently used tuners to improve their accuracy. Now, with the popularity oflong-range competition and varmint shooting, tuners are increasingly being used.
THEORY There are two independent types of barrel vibrations. The pressure inside the barrel causing it to expand and contract in aharmonically defined fashion (much like the string on a violin vibrates when it isplucked) as it moves along the barrel causes the first type of vibration. When thebullet leaves the muzzle when it is at its smallest diameter, accuracy is improved. Whenthe muzzle has expanded and is larger than the minimum, it acts like a bad crown withthe bullet smaller than the bore. Formulas to calculate Optimum Barrel Time areprovided in the appendix. This vibration mode depends on only barrel length. With theuse of the Quick Load and Pressure Trace products barrel times can be estimated. The second and most significant type of vibration type is the bending mode ofthe barrel. Think of clamping a steel ruler to a bench and then bending and releasing thefree end. The ruler will vibrate based on its characteristics and the force applied to startthe vibrations. A gun barrel functions in the same, but much more complicated way. In the case of our ruler one can write a differential equation of motion to describethe motion, velocity and acceleration of any point on the ruler. To do this for the riflebarrel is far more complicated.
Let’s consider the series of events from the time the trigger is pulled until thebullet leaves the muzzle. Each of the following events happens at a small, millisecondtime lag after the previous event.
a. The trigger is pulled to release the searb. The sear releases the firing pinc. The firing pin moves forward, striking the primerd. The primer and cartridge are driven forwarde. The primer ignites causing it to move rearwards, striking the bolt facef. The powder begins to burn and causes the bullet to move forwardg. The bullet strikes the rifling and begins to spin
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h. The action and barrel moves rearward in recoil until the recoil lug is stoppedby the stock
i. The entire rifle recoils
The list could go on, but the reader should get the point. Each of the events can bedescribed with a differential equation of motion, each spaced slightly different in time.All of the differential equations need to be simultaneously solved to get a picture of thebarrel vibration. Solving the equations is not important, but knowing they exist is. Whatthe shooter sees is the sum of all these. You might ask why this is important. We will tryto answer that question shortly. The barrel will vibrate with a number of modes or harmonics. Simply put, eachsuccessive mode (harmonic) will cross the centerline (the nodal point) of the barrel onemore time than the previous mode (harmonic) and be at a higher frequency. For exampleif the second mode (harmonic) were 445 cps the third mode might be 1,246 cps. As theharmonics increase, the final crossing point gets closer to the muzzle. A good example ofthis can be found on Varmint Al’s web site www.varmintal.com. His barrel tuneranalysis will demonstrate this effect.
Jim Boatwright wrote an excellent article in Precision Shooting a few years agodescribing how this barrel vibration causes aiming errors. The closer the nodal point is tothe muzzle when the bullet exits, the less the error. The problem is that the nodal point isa function of the barrel length. Shortening or lengthening the barrel still leaves the nodalpoint some distance before the muzzle. When we add a muzzle brake the barrelthinks it is longer, and the nodal point moves closer to the muzzle. That is why mostrifles shoot better with a muzzle brake or suppressor. If the muzzle brake is too long, thenodal point can move forward of the muzzle and accuracy will decrease. As described earlier the Rifle Accuracy System adds a movable weight to themuzzle brake. Slight 5-degree (.00045") changes of the location of the tuning weight canmake significant changes on group size. Why is that? Think about all the variousinfluences on the vibrations listed above. The slight movements of the tuner weightlocation can change all of them, either increasing or diminishing the magnitude of themuzzle motion. The concept is much like noise canceling headphones, which create avibration just the opposite as the one to be eliminated. It is much easier to test this than itis to calculate it.
TEST GOAL .The testing has three goals:
1. Show that tuner settings change the average group size2. Show that tuner results are consistent across shooting sessions3. Illustrate a procedure for determine a “near optimal” tuner setting4. Illustrate the effects of the uncontrollable variations within the two shot
groups used in the tuning procedure
If one were trying to measure improvements in a rifle shooting 1 ½" groups and the tunerreduced the group to ¾" that would seem to be a big improvement. In this case the skillof the shooter has a lesser impact on the results. In the case of accurate rifles that shootunder ½" the skill of the shooter becomes a much bigger factor in evaluating changes to
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the system. In the real world, if the shooter’s skill was the only variable, there would besome minimum group size that the shooter could consistently maintain. Consider thevery best of the bench rest competitors their averages can be in the low 0.200’s or high0.100’s. Typically, in competition, five sets of five shot groups are averaged todetermine accuracy.
The goal for this test was to provide data as to the degree of accuracyimprovements to be expected with the Rifle Accuracy System and the repeatability ofthese measurements. The sensitivity to tuner adjustment as it related to accuracy wasexplored. The use of the tuner system and other tuners were used to examine a range ofapplications
The barrel will wear a little with every shot changing the test results with time. Itwas decided to try to minimize that problem with this test. Tests were conduct with two(2) shot groups. “If two shots don’t group, three shots will not improve it.”Unfortunately, a great two shot group DOESN’T mean that the resulting three or fiveshot group will be small. Hence, multiple groups must be shot to determine the averagegroup size and variation for a given tuner setting. By shooting multiple two shot groupsat each tuner location and averaging the results, we could determine the relationshipbetween tuner location and accuracy. To further verify this additional three or more shotgroups could then be fired
TEST CONDITIONS The tests reported in this article were conducted on a private range in northeastMichigan. The range lies north to south and is surrounded by pine trees. During theduration of this test the conditions were calm winds and temperatures from the mid 70’sto high 80’s. Velocities were measured with a CED chronograph located 20 feet from themuzzle. Test were conducted at 100 yards and verified at 285 yards (maximum for therange). It should be noted that all the rifles used in these test were very accurate customrifles.
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Bench Set Up for Savage Test
TEST RIFLES & LOADSSAVAGE 260 AI-Eric Bostrom Gunsmith
Savage 12 LRPV Left Port Right Bolt Action, Kreiger SS 6.5mm, 30", 7:1twist barrel, Rifle Basix trigger, Nightforce 8X32 NXS Scope, Customstock by Eric Bostrom and a Rifle Accuracy System by Precision RifleSystems, LLCLoads: Remington brass, 210M primers, Ramshot Hunter powder, andmoly coated Sierra 142 MK bullets set .015 off the lands.Velocity about 2,900 fps
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Savage 260 AI with Rifle Accuracy Systems
TEST RESULTSSAVAGE 260 AI
RAS-Tuner Test-360 deg Rotation-Cust Savage260AI
0.0000.1000.2000.3000.4000.500
0 40 80 120 160 200 240 280 320
Tuner Location
Gro
up-in
7/17-Test #1
Graph #1-Group Size vs. Tuner Location
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The Rifle Accuracy Systems was initially set two turns, 720 degrees, back fromits maximum forward position.
Initial groups were shot using 40-degree rotation (.0035") between groups for onefull 360-degree rotation.
The group sizes ranged from the largest (.421") at 80 degrees to the smallest(.073") at 0 degrees and (.062") at 240 degrees.
Experience has shown that with each revolution of the tuner similar results willoccur.
Graph #1 gives us a starting location to meet our test goals.
The next step was to fire a series of groups from 200 to 280 degrees in 10-degree(.0009") increments. There were 13 tests, 42 groups or 84 shots fired to define the tunersperformance with the Savage rifle. The result of all these groups is shown in Table #1below. As can be seen, it is hard to get a specific answer to our goals from the data asshown. It can be seen that the all the groups tried to show a harmonic oscillation shape.
To better define the data, the average, maximum and minimum group sizes weredetermined for each tuner setting. Graph #2 shows this average results and the range ofthe data. Graph #2 also shows that the smallest average groups were achieved at tunersettings at 230 degrees. The minimum variation in groups was also achieved at the 230setting.
Now to address our test goals:1. Show that tuner settings change the average group size.
The data clearly shows that group sizes change with tuner settings.2. Show that tuner results are consistent across shooting sessions.
By averaging the test data it can be seen that there is consistency at the tunersettings within the bounds of the range of results.
3. Illustrate a procedure for determine a near optimal tuner settingAs described at the beginning of this article, a 360 degree rough test willprovide a starting point for the best setting. Then tests in smaller incrementsabout the small group as a starting point will provide near optimal accuracy.Additional test, may improve the results. Shooting multiple 2 shot groups isessential because we are trying to find a tuning point that is partially obscuredby the variation in group sizes due to other factors, principally wind, mirage,and shooter error. The more 2 shots groups shot at a given tuner setting thegreater the statistical confidence in the results.
4. Illustrate the effects of the uncontrollable variations within the two shotgroups used in the tuning procedure
The maximum, minimum data shown in table 1 & graph 2 give an indicationof the variations and how they relate to tuner settings.
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285 YARDSSince one of the goals of this project was to improve long-range accuracy groups weretested at the maximum distance of the range, 285-yards. The following target shows thatthe tuner setting needed to be adjusted by 10 degrees, from 260 degrees to 250 degrees,for best accuracy. What is interesting is the groups shot with two different powders at thesame best tuner setting, 250 degrees. The RL 19 group was 0.0751 MOA and the Huntergroup was 0.1305 MOA. Since all the 100 yard data was with the Hunter powder,because it was faster, one might wonder how much better the 100 yards groups mighthave been if the RL 19 powder had been used.
285 Yard Target for Savage 260 AI
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The next question.How does Rifle Accuracy System tuning change with a different rifle?A very accurate bench rest rifle was chosen. This rifle was originally a 6PPC Bench Restrifle made by Dwight Scott. The rifle has a HALL action. The barrel was changed forthis test to a Broughton 6.5 mm, 28", 8:1 twist barrel chambered for a 260 AI with thesame reamer used on the Savage barrel. The rifle has a Jewel trigger, T36 WeaverScope and the Rifle Accuracy System. The Hall’s only common feature with the Savageis the 260 AI cartridges. The Hall action has been glued into the Adamowicz stock.The load was MRP powder, 139gr moly coated Lapua bullets, 210M primers, Remingtonbrass..What was surprising is that the best tuner locations were within 0.0009” of the bestlocations on the Savage.
Hall 260 AI Rifle
Graph #3- 260AI HALL TEST DATA
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What happens to a rifle that doesn t need a muzzle brake and just uses a tuner? To answer this question a tuner was adapted to a very accurate LES BAER AR 15..223 Lake City Brass, 205M primers, Ramshot TAC powder, Sierra 69gr moly coatedMatch King bullets, OAL=2.248Les Baer 24", 9:1 twist, SS barrel with modified Harrell Tuner
Les Baer .223 with TunerWithout the tuner this load typically shoots 5 shot groups under 0.400”. The addition ofthe tuner substantially reduced these groups.
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The 22 RF bench rest shooters have long used tuners to match their rifles to theirammunition. What happens if we put a tuner on a Ruger 10/22? A SuperTune tunerwas modified to adapt it to the barrel of the 10/22.CUSTOM RUGER 10/22-22 RFWolf 22 LR Ammunition
Ruger 10/22 with tuner
Graph #5 Ruger 10/22 with Tuner Data
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CONCLUSIONSv Tuners work on center fire rifles and are the results are repeatable.
v Tuners and the RAS specifically, will make substantial improvements in rifleaccuracy.
v The tuning requires little effort to improve accuracy. Maximizing the accuracytakes some additional effort. Tuner locations may require slight changes tomaintain their advantage as conditions, distance and loads change
NEXT STEPv Shoot 5 & 10 shot groups at longer range, 565 and 1,000 yards, while changing
tuner locations.
v Determine sensitivity to tuner location at 200 yards. Testing was done with 10-degree (.0009") increments in tuner location. Will smaller increments improveaccuracy?
APPENDIXEric Bostrom-Myakka City, FL, 941-322-2487Precision Rifle Systems, LLC, - Myakka City, FL, 941-322-2487
#Harrell’s Precision-Salem, VA, 541-380-2683, www.harrellsprec.com#SUPERTUNE-Time Precision, Bethel, CT, 203-744-1052
www.shootingsoftware.comChris Long has explained the concept of Optimum Barrel Time on his web site,
www.the-long-family.com/obt, in great detail.#Rifle Accuracy Facts by Harold Vaughn-published by Precision ShootingHarold Vaughn in his book Rifle Accuracy Facts explains some of the sources
of the vibrations.Varmint Al-www.varmintal.comVarmint Hunter Magazine April-June 2009, pages 170-174
