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The following article was published in Precision Shooting Magazine May 1996

What Makes For Peak Rifle Accuracy?

By Mike Bryant

What makes one rifle accurate and another inaccurate? That is a question that many people have asked over the years. The basic principles of rifle accuracy are the same whether you are dealing with a high-grade benchrest rifle or a bolt-action hunting rifle.

Most of my experience with rifle accuracy has been in competition benchrest shooting. I shot my first match in the summer of 1982 and have been shooting in registered competition ever since. My experience with rifle building began a little before I began shooting benchrest. But in general, I have been stocking and later barreling the benchrest rifles that I shoot in competition since I started shooting competitively. There is nothing like building tight necked 6 PPC's and Waldogs to teach you what it takes to make a rifle accurate.

There are no deep dark secrets to obtain peak rifle accuracy, only top quality mechanics. For the purpose of this discussion, the rifle will consist of three major parts, the barreled action, the stock and the scope. You cannot have top accuracy without all three parts pulling their weight.

The Barreled Action

The relationship of the barrel and action can best be thought of as how every part of the barreled action relates to the centerline. The centerline is an imaginary line passing from the center of the bore through the center of the action. The centerline may be imaginary but can readily be determined with the aid of a good 0.0001" dial indicator. Of primary importance, the chamber has to be absolutely concentric and parallel to the centerline of the bore with the bolt face being an absolute 90-degree perpendicular to the centerline. The kicker here is that this is hard to accomplish because it is dependent on three different separate parts, the bolt, receiver and barrel and how they fit up to each other.

The bolt body, firing pin, barrel threads and chamber should be completely in line with the centerline of the bore. The action lug recess, bolt lugs, bolt face, receiver face and barrel shoulder need to be 90 degrees to the centerline. The bolt lugs and action lugs should make full contact on all locking surfaces. The action threads and barrel threads should make full contact on the bearing surface of the threads with no contact between the top of the barrel thread and the bottom of the action threads. The chamber should be a size to precisely fit the cartridge case.

The shape or angle of the crown makes practically no difference as long as there are no burrs or irregularities in the crown and that one side of the bullet doesn't clear the barrel before the opposite side of the bullet does. In other words, the barrel doesn't need to be longer on one side than on the other.

Why, you may ask does all this concentricity and 90 degree stuff make any difference? The sole purpose is to launch a bullet precisely centered in the bore straight down the center of the bore and out the crown with as little wobble as possible. If the bolt face is at 90 degrees to the chamber and the barrel bore, some of the relationships in the receiver don't have to be perfect and still you will obtain peak accuracy. The thing is that it is far easier to obtain a bolt face perfectly at 90 degrees to the chambered barrel if every other part is in its proper relationship within the receiver.

An oversize chamber can ruin all your best efforts at building a tack-driving rifle. The oversize chamber will allow the cartridge to lie in the bottom of the chamber and throw the bullet out of alignment with the bore. Why then do benchrest shooters get such good groups when they are fire-forming brass? The necks of their cases are turned to such close tolerances that the bullet is still in alignment with the bore as well as the case head is still supported by the chamber.

I haven't mentioned anything about barrel quality. I have used barrels from most of the top makers of benchrest barrels and have had high quality barrels from all of the makers that I have used. If you are going to economize on building a custom rifle, the barrel is not the place to do it. Stick with a top maker and it's hard to go wrong.

Good triggers are critical for the accurate rifle. Not that it is necessary for peak accuracy, but that it makes peak accuracy easier to obtain. Two-ounce trigger have their place in the accurate rifle. That place is on target rifles. With practice a two-ounce trigger can be squeezed and released if wind conditions change, but only if they are being shot from the bench. Two-ounce triggers and field conditions don't mix. I once had a two-ounce trigger mounted on my .280 deer rifle. I was watching a coyote come into a windmill at 300 yards. I placed my crosshairs on him and had a dead coyote. Only thing was, I wasn't ready to shoot yet or at least, so I thought. I replaced that trigger that day with a two-pound trigger and haven't used a two-ounce trigger on a hunting rifle again.

The Stock

The stock and its relationship to the barreled action play a significant role in the accuracy of the rifle. There are three major methods of stock bedding to barreled action. In the majority of benchrest rifles, the predominant method is the glue-in in which the action is glued into the stock with epoxy glue. Another method that is being used is pillar bedding. Conventional glass bedding is also used, but by very few in the benchrest community.

The glue-in is the easiest to use and the easiest method to obtaining an accurate rifle. It does, however, have its drawbacks. For one, it is harder to take the action from the stock in case of mechanical problems. Triggers have to be able to be removed from the bottom of the stock, which is no problem with the actions commonly used in benchrest. These actions usually have trigger brackets that are removable from beneath the action. The Remington actions that are being used in competition have holes through the stock that allow the trigger pins to be driven out and the trigger removed. This is not too practical if the glue-in happens to be on a hunting rifle like a Remington 700. The thumb safety would make it extremely difficult to remove the trigger from the bottom. Secondly, just because an action is glued-in doesn't mean that it is glued-in without stresses on the action. If the action is stressed when it is glued-in, you then have a bedding problem. But, it’s a bedding problem that you can't detect. Lastly, the glue-in is intended to be bedded permanently. But in time a glue-in can become completely or partially unglued by cleaning solvent seeping under the action or any other number of reasons. Murphy's law applies to this really well. When you can least afford the gun coming apart is probably the time it will. A friend of mine had just shot a 0.140 group at 100 yards in the previous match, when his action came unglued before he could shoot the next group. The 0.140 did win him small group for the 100-yard segment, but he was unable to finish the match. That won't happen with a pillar bedded rifle and is also unlikely to happen with a properly glued-in action.

Pillar bedding with aluminum or glass pillars is a method that has started showing up in a large number of benchrest rifles, as well as in high accuracy hunting rifles. As you can guess from my opinion of glue-ins, the rifles that I shoot in competition are pillar bedded. (I now use both methods in competition, pillar beds and glue-ins. I did shoot strictly pillar beds in '96 when this article was published. A properly done glue-in will not cause any problem.) The glue-in is still the most popular bedding method in benchrest competition. But, the number of top competitors, such as L. E. Red Cornelison, Harold Broughton, Speedy Gonzales, Frank Wilson and others, that have shot pillar bedded rifles in competition shows that they believe that they are not losing any accuracy because their rifles are not glued-in. Believe me, if a competitor thought that he was at any disadvantage by shooting a pillar bedded rifle, there is no way that he would be shooting it in competition.

One of the chief advantages with pillar bedding is that the rifle can be taken apart. The bedding can be checked with a dial indicator that measures in ten thousandths by checking the movement between the barrel and forend of the stock as the action screws are loosened and tightened. There should not be any more than 0.002" movement between the barrel and the forend. If there is more than that, say 0.003", you have a bedding problem and your rifle will not shoot to its potential. The action cannot bind in the stock. With a round receiver, you should be able to take the guard screws out of the stock and have the barreled action drop out of the stock. Flat bottom receivers with square sides won't do this. But, even with them, the barreled action should come easily out of the stock. The bedding area and the receiver should be completely clean before they are screwed together. Even the smallest piece of crud between the action and the bedding surface will show up when the dial indicator test is used. All action screws should not touch any part of the stock except at the screw heads. Likewise, the bolt handle must not touch any part of the stock. When the bedding tests out right, don’t' take the action out of the stock any more than absolutely necessary. Every time the action is removed is just taking a chance of nicking the action or messing up the bedding area in the stock. Really tight screws are not necessary; so don't get carried away tightening guard screws.

Conventional glass bedding has all the advantages of pillar bedding except that the bedding can easily be ruined if the guard screws are over-tightened. I personally know of only one competitor that used conventional glass bedding and he may not anymore. (To my knowledge, he doesn't compete anymore.) There may be more that I am unaware, but this is an area that is not listed on competitor equipment lists.

Stock shape makes a difference in accuracy not because it makes the rifle's potential accuracy any better, but because it makes that accuracy easier to obtain. That is why benchrest rifles have three-inch wide flat forends. But, stock shape has to be practical for the purpose that the rifle is to be used. After all, who would want a deer rifle with a three-inch flat forend?

The Scope

An accurate rifle is only as good as the scope mounted on it. Scopes that are good for one use may not be good for other uses. Most of the scopes used in benchrest competition are Leupold, Lyman conversions, Bausch & Lomb, or Weaver 36 power target scopes. They are great for obtaining the most accuracy that you can get from your rifle. But, they are highly impractical for a hunting rifle. They do have a place for use in a hunting rifle, though. A 36-power scope is a great scope for working up the load that you will use with your accurate hunting rifle and then switching to the scope that you will use for hunting. My heavy barreled .280 will shoot significantly smaller groups with a high power target scope than it will with the Kahles 8x56 that it has on it now. The 8x56 will, however, shoot 1-inch groups at 100 yards by full moon light and better under better light. In Texas, as in the rest of the United States, you have to watch your watch when you are hunting with it or you could get in trouble for deer hunting too late. It will allow you to make that shot on a cloudy day at the limit of legal hunting time that a lesser scope wouldn't be able to see.

An accurate scope has to be able to hold its point of aim. The scope is one of the variables over which we have the least control. It is commonly said that benchrest shooters don't look for accurate rifles, they look for accurate scopes. A scope that loses a shot can be enough to make you lose a match or worse. With an unlimited rail gun set on eighteen inch centers, one thousandth of an inch movement will move you approximately 0.2" on the target at 100 yards. With a scope, the inner adjustment tube may only be 3 or 4 inches. That same movement within the scope of 0.001 could move you well over an inch at the same distance. By comparison, a sheet of paper measures approximately .004. It is a wonder that the scopes that are in production are as accurate as they are and it is a marvel to the machining ability of the optical industry.

For the past several years, there have been a number of modifications to target scopes to help limit movement on the adjustments and also, to limit movement in the parallax in either the objective or by other means. These have ranged from setscrews opposite the turrets to coil spring plungers and some other internal modifications as done by Cecil Tucker of Odessa, TX. Dick Thomas at Premier Reticles at one time fixed the objective in place and moved the parallax adjustment to a set of lenses in front of the turrets. He hasn't offered that modification for quite some time. Jim Carstensen, presently (2002), makes a modification that locks the objective front and rear on Leupold 36 BR's and also has a modification to the objective to suppress movement on the older non-locking Leupold 36's. Burris has taken Cecil's modification a step farther on their scopes with the introduction of the Burris Posi-lock that combines a coil spring plunger and locking mechanism on the plunger. This has to be a good idea, especially on a hunting rifle that is sighted in and then usually left alone.

It goes without saying that the accurate rifle cannot be accurate without high quality ammunition, but that’s another story.

No matter what the shape, no matter what the weight, no matter what the caliber or purpose, the same elements that make a benchrest rifle accurate are the same elements that make a hunting rifle accurate. I hope that this discussion has left you with a little more insight into why certain rifles are accurate and why some are not.

 

 

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