July 2011 Cover Page

  July 2011
  
The Rifleman's Journal
A Collection of Articles Dealing with Rifle Accuracy Topics

John Chilton of the United States F-Class Rifle Team in Ireland

This Month:
Hap Rocketto - Hap's Corner
Germán Salazar - Rifles and Reloading


15 Cents 

Cartridges: The .308 Heavy Bullet Conundrum


The .308 Heavy Bullet Conundrum
by Germán A. Salazar


The .308, unlike many other cartridge commonly used for long-range shooting, presents us with a true conundrum: it is simpler to get good 1000 yard performance from lighter rather than heavier bullets (within reason). This odd situation arises from the unfortunate combination of a short case neck and relatively limited case capacity with which the .308 is saddled. There's little benefit to my repeating the obvious merits of the .30-06 here, so let's get on with this analysis of the little .308 which, of course, is the required cartridge for Palma and F-TR shooting, thus making this concern a fairly important one to many competitors.

The 155 gr. bullets from Sierra, Berger and Lapua will do a perfectly adequate job at all distances and are a very common choice. Notwithstanding the simplicity of loading the 155's, there are many of us who favor the heavier bullets. The two principal reasons for using heavy bullets are: enhanced ballistic performance and reduced probability of blown-up bullets that can result from the high muzzle velocity that is necessary to keep those light bullets at or above 1.2 mach at 1000 yards. Add in the basic fact that the Sierra 190 gr. Match King is widely available, is reasonably priced and is a fabulous performer at 600 yards and we have defined one basic parameter for our analysis: the rifle must be suitable for the 190 gr. Sierra.

Pressure Basics
A fundamental rule of internal ballistics is that to extract maximum performance from increasingly heavier bullets, one must use increasingly greater amounts of slower burn-rate powders. This allows the pressure curve to rise in a normal fashion despite the longer engraving time of the heavier bullet and its increased momentum which resists acceleration.

When a faster than optimal powder is used, maximum pressure will be reached too quickly and the heavy bullet will fail to reach the muzzle velocity it might have otherwise reached. This happens because the relatively smaller amount of fast-burning powder that will create maximum pressure will not generate sufficient gas volume to power the bullet down the barrel to the same (high) MV that a larger amount of slower burning powder can generate. Just switching to slow powders isn't a universal answer, however; because unless you can reach the powder's normal operating range (55,000 psi to 60,000 psi) it won't burn efficiently enough to generate its potential gas volume. The powder burn-rate and the bullet must be closely matched for optimum performance.

To understand this a bit more clearly, remember that while the heavy bullet is engraving and struggling to move, the powder behind it continues to burn and build pressure. The rate of pressure increase (the powder's burn-rate) must closely match the bullet's ability to accelerate up the bore. Consider the opposite situation: too slow a burn-rate for the bullet, for instance H4350 with a 155 gr. bullet. In that situation, the bullet will move forward before the powder can build pressure up to the normal operating range (55,000 psi to 60,000 psi). Each millimeter of movement increased the total combustion chamber size (cartridge case volume plus volume of the bore behind the bullet) which, of course causes pressure to drop. Although there is a range of powder burn-rates that can be used effectively with any given bullet, that range is not as wide as the novice reloader might initially believe if one is to create a load capable of accurate long-range performance within the pressure limitations applicable to the cartridge.


Palma 95 Chamber. Sierra 190 and Berger 175 shown seated to just touch the lands.



Stretching the Case
The slower powders appropriate for use with heavier bullets tend to be somewhat bulkier than the faster powders, consuming more case volume per unit of weight. Accordingly, less of the slower powder will fit in the case - however, we need more of it! If only the case were larger... But wait, there's another fly in the soup - the heavier bullets are longer and intrude deeper into the case, thus further reducing case capacity. The Palma 95 chamber, quite widely used in match rifles, is a good example of this limitation. Although that chamber is fine with any 155 gr. bullet and even with the 175 gr. bullets, you can see in the picture above that the 190 really intrudes into the powder space with the end of the bearing surface well below the base of the case neck.

Clearly, we need a larger case, and there are two ways of accomplishing this. The first is to re-form the case in an enlarged chamber, the so-called "improved" case. This is not permitted by the Palma or F-TR rules, so we can discard it without further consideration. The second method is to lengthen the chamber throat, allowing the bullet to be seated further out along the case neck and maximizing the available case capacity for powder. We will focus on this method as it is the only practical approach.

Berger 210 Chamber. Berger 201 and Berger 175 shown seated to just touch the lands.

Lengthening the chamber throat has its limitations. Because the case neck itself is fairly short, an excessively long throat will not accommodate the lighter bullets that may, at times, be desirable. For instance, I have a .308 barrel throated for the 210 gr. Berger in which a 175 gr. Berger bullet has only a few thousandths of shank in the case. As it is, there isn't enough engagement to hold the bullet securely in the case and if I were to try to jam it in the rifling, as I prefer, it would simply fall out of the case. The 190 gr. Sierra should work reasonably well in this chamber, but it is about as light as you could use and still have enough bullet engagement in the case neck. Because this project was based around the 210 gr. Berger, I never tried the Sierra 190 in it. In fact, to really make the 210 work well, the throat should be even longer than it is as I was a bit powder limited when working with this combination using Reloder 17. Most competitors need some degree of flexibility in their rifles, making a modest throat lengthening practical, but not much more.The super-long throat and very heavy bullets aren't a good approach when one considers the cost of bullets and practicality.

0.114" Freebore Chamber. Sierra 190 and Berger 175 seated to just touch the lands.

Just Right...
The chamber reamer with which I've had my barrels chambered for the past few years is a good example of a moderate throat increase. With a 0.114" freebore, it accommodates 175 gr. to 190 gr. bullets well, can be made to work with some (but not all) 155 gr. bullets, but is inadequate for anything above 190 gr. Although the 190 Sierra is seated 0.050" further out than in the Palma 95 chamber, this isn't enough to get an adequate charge of IMR 4064 or a similar powder into the case for 1000 yard shooting. If the throat was a little longer to allow enough powder to keep the 190 above 1.2 mach at 1000 yards, it would be too long for the 175 or any 155 to be used, thus losing a significant degree of practicality. However, this 0.114" freebore length is perfect for 500 and 600 yard shooting with the 190 Sierra using IMR 4064 and for 1000 yards with the 175 gr. Berger, with a faster powder, such as H4895.


1000 Yard Choices
The result of all of this measuring and weighing, is the odd situation - our conundrum - that although the 190 gr. Sierra bullets are superbly accurate and a great choice at 500 and 600 yards, they are not very practical for 1000 yards in the .308. You may remember our earlier article on the importance of keeping the bullet at 1.2 mach or above at 1000 yards. It has been my experience that keeping the 190 gr. Sierra at or above 1.2 mach at 1000 yards with a reasonable throat length and commonly available powders is not generally feasible. However, there are other bullets perfectly suited to that purpose, most notably the Berger 175 gr. and 185 gr. long-range match bullets.

Although the 185 gr. Berger LRBT is only a few grains lighter than the classic Sierra 190, the reason it can hold 1.2 mach at 1000 yards is a higher BC created by stretching the bullet's nose, which also happens to reduce the bearing surface - and that is the key element. The reduced bearing surface means the bullet engraves more quickly and thus allows the use of a faster, denser powder that fits within the available case capacity. The same applies to the 175 gr. Berger which is, in fact, my preferred bullet for 1000 yard shooting with the .308.

Bearing Surface
Bryan Litz's book, Applied Ballistics for Long Range Shooting, shows the bearing surface of the Berger 185 gr. to be 0.360" versus 0.393" for the Sierra 190 gr. However, measuring my lot of older Sierra 190's reveals a bearing surface of 0.460". Bryan's data is based on current production bullets so it's reasonable to conclude that Sierra has updated their design over the years, but it also points out the need to measure your particular bullets. With a 0.100" reduction in bearing surface, you can see a good part of the reason why the 185 gr. Berger is a better performer at 1000 yards than my old lot of Sierra 190's. Please don't confuse measuring bearing surface to compare various types of bullets with sorting a particular bullet by bearing surface variance which is a separate discussion.

A bullet's bearing surface is a major contributor to the pressure level that will be generated with a given powder charge. Comparing the Berger 185's 0.360" bearing surface to the Lapua 185 gr. D46 (a very old design) at 0.408" is a solid indication that the Lapua will create higher pressure with the same powder charge; therefore, those two bullets should not be interchanged without an appropriate load adjustment. How about that old stand-by, the Sierra 180 gr. Match King? About 20 years ago, Sierra redesigned that bullet, going to a blunter, shorter boat tail design. The old long boat tail version has a 0.400" bearing surface whereas the new version has a 0.462" bearing surface. These are significant differences, it pays to check before making substitutions!


Wrapping Up
The .308 suffers from some design limitations that make balancing throat length, bullet weight and powder burn-rate a particularly important task. Seemingly small differences in seating depth and bearing surface are actually quite significant and will have a material effect on pressure levels. A chamber throat that allows the use of the economical 190 Sierra for mid-range while remaining short enough to use other bullets will likely not allow that 190 to generate enough MV for 1000 yard use. However, the more expensive Berger 175 gr, and 185 gr. LRBT bullets will do the job at 1000 and you can still use the solidly accurate and lower cost 190's at 500 and 600 yards. This medium length throat is just about ideal for the heavy bullet shooter trying to make the most of the .308 from 500 to 1000 yards.


F-TR: Scoping It Out

F-TR: Scoping It Out
by Germán A. Salazar
 
Three months past the day of the rotator cuff surgery on my right shoulder, I find myself shooting and enjoying F-TR matches. As of today, I've fired four matches, all at 500 yards during the Phoenix "Summer Season" which is limited to that distance. It has been a true learning experience, more challenging than expected in some ways, perhaps not as much as I feared in others. I'll share some of my observations and experiences, none of which will be news to our experienced F-Class readers, but they may answer a few questions for sling shooters contemplating a busman's holiday into F-Class.
 
My four match scores to date, on the NRA MR65F target are: 594-22, 594-27, 589-28, 596-26. All were fired with the same rifle, a Gilkes-Ross action in a Robertson/Sitman Highpower Prone stock with an Anschütz 5020 trigger set at 2.5 lb., a 30" Krieger 1:11" twist barrel chambered by Clark Fay of Raton New Mexico, with a Leupold BR 24 scope and a Rempel bipod. This is simply my old Palma rifle with a scope and bipod attached. I'm also using an old Smallbore kneeling roll under my chest for support as my right arm is not yet strong enough to rest on while shooting.
 
Carpet Under the Bipod
The two ranges at which we shoot during the Summer have very different firing lines: one is relatively soft red dirt, the other is concrete. I quickly learned that a piece of carpet was an essential component under the bipod. Without the carpet, the bipod tends to dig into the dirt with every shot, resulting in odd elevation shots. On the concrete it isn't quite as essential, but it smooths the recoil movement appreciably and is worthwhile. John Lowther gave me a short-nap carpet remnant to use for this, but my car floor mat also worked well.
 
Scopes
I shot the first three matches with a Leupold BR24 with a 1/4 moa dot. Tha dot size is a bit bigger than ideal as it tends to obscure the entire X ring, especially in heavy mirage. For the most recent match, I switched to a Leupold BR24 with a 1/8 moa dot; this one is a bit too small, but it allowed for greater precision in aiming. Somewhere out there is a "Goldilocks" just-right dot size, but for now I'll stick to the 1/8 moa dot.


Gilkes-Ross rifle with Leupold BR24 scope.
  
John Lowther again jumped in to give me a hand with scopes by lending me his Nightforce NSX 12-42 scope. I tried it during a recent practice session, but unfortunately, the slots in my rifle's scope rail seem to be on the low end of the width tolerance and the crossbar in his Farrell rings wouldn't quite go in properly. Although I couldn't shoot with it for score, I was able to evaluate the effect of the additional magnification. In short, I can certainly appreciate the enhanced ability to hold-off with precision that the additional magnification provides; but... for now, I remain a clicker not a shader, so this is of less importance to me than it might be to someone who prefers to hold off. Another factor against the Nightforce in my case, is that it would take my rifle/bipod combination over the F-TR weight limit. Although I could reduce weight with a buttplate assembly replacement, there's really no urgency to do so because I'll stick to the Leupold for now.
 
A small item to note is that my shooting glasses were just as useful with the scope as they are with irons. Having the lens centered in front of the eye was just as useful with the scope as it is with irons.
 
Clicking or Shading
That brings us to the question of clicking the scope versus holding off. I've been shooting iron sights and clicking sight knobs for most of my life; trying to hold off made me very uncomfortable and the reflex pull of the trigger just wasn't there. Once I returned to holding center and clicking, I was more comfortable and was able to execute my shots more quickly and cleanly. By zeroing the windage knob I was also able to easily return to a previous setting when conditions warranted.
 
The most serious limitation of the Leupold BR24 scope isn't the magnification, it is the 1/4 moa clicks. My method of shooting, with iron sights and the standard NRA target with its 2 moa 10 ring, is to use 1/8 moa clicks and to click constantly - always working to center the shot and adjusting for the slightest shift in the wind. Unfortunately, when that style of shooting is applied to the F-Class 1 moa 10 ring and a scope with 1/4 moa clicks, overcorrection and lost points happen quickly. In the most recent match (596-26) I dramatically reduced the amount of clicking and it paid off with a nice 200-7 in the first string and a good overall score. The X count was down a bit, but for the first time, I didn't lose any points to overcorrection.
 
Reading the Wind

Viewing through the spotting scope.
Ultimately, any rifle match comes down to reading the wind. All of our work on equipment, loads and shooting technique don't count for much if we miss a significant wind change and needlessly lose points. In conversations with other F-Class shooters in our club, I found that few were using a spotting scope to see mirage; they were largely relying on the rifle scope. However, the rifle scope is focused on the target, as it must be to eliminate parallax, and thus cannot show mirage with the same clarity as a spotting scope that is focused roughly halfway down the range. After our initial discussion of this topic, F-Open shooter Charles Gooding began to use his spotting scope for mirage reading and his scores have taken a distinct upward bounce.


Aiming through the rifle scope.

I position the spotting scope in the same manner as I did when shooting from the sling. It is very close to me and can be used without moving the head from the cheekpiece. The object, as always, is to minimize movement in order to maintain a consistent position and to minimize the time lost between the last glance at the mirage and breaking the shot.
 
This picture shows the basic positioning of the spotting scope in relation to the rifle.
 
Sling vs. F-TR
A number of friends have asked me to compare the two forms of shooting, and that is a very interesting question.
 
In a nutshell, sling shooting is certainly more taxing physically, but F-TR places a higher premium on perfection in wind reading. I'm certain that my wind reading skills will improve as a result of shooting F-TR. The accuracy standards required for success in either are roughly equal; all competitors have a top level rifle and perfect ammunition. Continuous refinement of equipment, ammunition, shooting technique and wind reading are the keys to success in either category. In reality, there are more similarities than differences and competitors in either one should try the other when the opportunity presents itself - there is always something new to learn.
 
Obviously, I have a great deal yet to learn about F-TR shooting, but I'm looking forward to it. Based on my rate of recovery from the shoulder surgery, it may be a year before I can shoot in a sling again, so there will be plenty of time to work on it.
 

Hap's Corner: And He Was Asking Me?

And He Was Asking Me?
by Hap Rocketto
Distinguished Smallbore Prone Rifleman, Distinguished Rifleman, Distinguished Pistol Shot.
Badges courtesy of German Salazar

When I was a young cub chasing Distinguished there were many of my fellow shooters who looked upon my quest as a life misspent. One who would give me an occasional good natured jab about the barbaric nature of the life of a high power competitor was my friend Jay Sonneborn. Jay and I go back a long way and we even traveled to Camp Perry to shoot smallbore one year. Jay, a bit older and more experienced, took a good part of the 12-hour drive trying to get me to see the error of my ways. Brother Love’s Traveling Salvation Show had nothing on Brother Jay’s preaching to the pagan on Interstate 80 West. As it turned out all was for naught as Jay went his way and I went mine.

Twenty-five years later the both of us had met most of our shooting goals. Jay had become Distinguished with the smallbore rifle in both prone and position, been on several Dewar Teams-including coveted appointments as both Coach and Captain of that prestigious team, won the 2001 NRA Intermediate Senior Three Position Smallbore Rifle Championship, and retired after a long and successful career as a banker. On the other hand I had become Distinguished with the service rifle, the smallbore rifle in position, been a line coach on several Dewar Teams, won the 2002 NRA Intermediate Senior Three Position Smallbore Rifle Championship, and retired after a long and successful career as a teacher. I am seeking my final leg on prone Distinguished, the all important Perry leg and Jay is looking for his first service rifle leg. We had both come 180 degrees.

I confess to a little surprise at Jay’s hunt for Distinguished. However, it made sense for him to do so as he was a smallbore shooter who had nothing more to prove to himself in that game and was seeking a new challenge. Much to my astonishment he came to me for advice. Little did Jay know that my minor successes in shooting, greatly magnified in my retelling, came in spite of myself. For some reason I am viewed as some sort of Grand Panjandrum of shooting when in reality I am more like the Great Oz, a magnificent façade hiding a less than imposing reality.

He wanted to know how to deal with 300-yard rapid fire. Knowing that Jay had the best of equipment and hand loaded his ammunition with the precision of a bench rest shooter the answer had to have him focus on the few remaining aspects of the position and its mechanics. I first reminded him that 300 yards is shot prone and was rewarded with a well-deserved tart look saying, “What do you take me for, a fool?” Jay is no fool, and I know that, but the first step to solve the problem is it to put the 300-yard stage in perspective. Jay knows prone.

“The National Match Course is 60% prone and you are an excellent belly shooter” I reminded him. “Your knowledge of the position and wind doping is extensive. For 300 rapid prone you need a high tight sling to keep your position from breaking up through the magazine change, to maintain your natural point of aim, and to negate the effects of recoil.”

“A regular cadence, with a short “dog pant” breath taken between each shot, will insure that each shot is executed in a consistent manner. Additionally the regular breathing will insure that you have sufficient air in your lungs to keep your vision clear and your nerves calm.” All of this is important, I reminded him, but it is mechanical and should become second nature through correct training.

“The most important thing in rapid fire, in my humble opinion,” I pontificated, “is how you deal with the sights. With good position sight alignment becomes automatic. Once eye, rear sight, and front sight are in line the next issue is sight picture and it is possibly the most important thing in rapid. Once you establish either a “Navy” hold, favored by shooting legend Gunner D.I. Boyd, or a pumpkin on the fence post picture, you must concentrate on keeping the post in focus.”

“Your eye cannot focus on two points simultaneously. If you switch focus back and forth you have introduced inconsistency into your sight picture, and inconsistency is the kiss of death in shooting. If you keep the post in focus you go a long way to insuring that your sight alignment will not change. It is OK to have the bull look like a blur, you still will shoot Xs and tens, but to loose sight alignment is disaster.”

“That all there is to it” I concluded. “Now what advice can you give me to be a success at shooting smallbore prone?” I asked, expecting just as elaborate and detailed directions down the road of success as I had given him.

Jay, my favorite banker, cut right to the point, “Don’t shoot nines.”

 

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