To Bed A Rifle
September 23, 2010
For more than a century, riflemakers have known that the interface between a rifle's stock and its action is one of the biggest determining factors of accuracy. The evolution of technology and high-speed cameras have paved the way for the next generation in riflestocks.
An integral rail of the Savage AccuRail system extends forward to stiffen and reinforce the forearm of the stock.
It would be interesting to know at what point in time American rifle builders, gunsmiths, and riflemen began to understand the importance of precise action-to-stock fit in the accuracy equation. When the action bolts of a rifle suffering from an uneven fit between its receiver and stock are tightened, the strain and stress on the receiver can cause accuracy to go to pot. A shoddy bedding job also encourages movement of the receiver inside the stock, and if it does not return to the same position between shots, group size will increase.
The importance of stock fit might have been discovered during the era of single-shot rifles, back when Harry M. Pope was making some of the world's most accurate barrels for competitive shooting. Or it may have started a bit later when, during the early 1900s, military-surplus Krag-Jorgensen rifles first became available to civilian target shooters who replaced their barrels with match-grade barrels made by J. Stevens Arms and Tool Co. of Chicopee Falls, Massachusetts.
In July of 1903, American shooters armed with Krag rifles wearing Stevens barrels beat the British team in 800-, 900-, and 1,000-yard competition at Bisley, England. My guess is the barreled actions of the rifles they used had been carefully bedded into their stocks.
Pillar bedding consists of aluminum pillars that are installed in the stock to support the receiver at the front and back.
Professional stockmakers who really know their stuff are capable of using simple hand tools to perfectly mate the stock of a rifle with its barreled action, and they have been doing so for a very long time. I have a 1903 Springfield sporter in 7x57mm Mauser that was built during the 1920s by R. F. Sedgley, and it is capable of shooting groups inside minute of angle all day long.
It is not uncommon to see a mass-produced rifle perform in the same manner. During the early 1960s, I used a Winchester Model 70 in .225 Winchester to shoot my very first sub-half-inch group. There have been others since, but there have also been rifles with bedding problems.
Stock Bedding Advancements
One of the biggest advancements in riflestock bedding came with the development of epoxy resins. They are easy to apply, they are inexpensive, they are durable, and most important of all, their shrinkage during the curing process ranges from slight to undetectable. Armorers who accurized M1 Garand rifles for various U.S. military rifle teams may have been first to use it, but it did not take long for commercial rifle manufacturers to begin using it in their riflestocks.
At first, the new material was difficult to come by, but then came Acraglas from Brownells, and the practice of glass-bedding barreled actions spread like wildfire among professional gunsmiths and shade-tree hobbyists alike. Other products, such as Microbed, Bisonite, Duro, and Devcon, have been and still are used, but Acraglas is the one that started the wheels rolling. Where it had once taken a gunsmith many long hours to arrive at a near-perfect fit between barreled action and stock, it could now be done in a matter of minutes.
Introduced by Brownells in the late 1950s or early 1960s, the Acraglas bedding kit
can improve the accuracy of a rifle if properly applied.
It is possible that pillar bedding originated with the Germans and their Mauser rifles sometime during the late 1800s, but it did not begin to become popular in the United States until the 1980s. At least that's when I first became aware of it.
Steel pillars were commonly used in the past, but today, most of them are made of aluminum. Installing them in a stock results in a solid metal-to-metal connection between the receiver and the bottom-metal assembly. This accomplishes several things, and high among them in importance is the option of tightening the action bolts of a wood-stocked rifle to a high level of torque without danger of crushing wood fibers. And they work equally well in synthetic stocks.
When installed properly, the pillars also prevent uneven stress on the receiver when the action bolts are tightened. Installing them in a stock is more of a challenge for the home gunsmith than glass bedding, but anyone who has a drill press, a good metal file, and the ability to follow written directions can handle the job. Most gunsmiths offer pillar bedding, and quite often, it is one of several modifications included in an accurizing package. For those who choose to do the job themselves, Virginia gunsmith Richard Franklin (540-890-8649) offers a step-by-step instructional DVD.
Riflestocks have been made entirely of aluminum, but the intricate machining required makes them relatively expensive. The next best thing is to install a bedding block made of the same material in a synthetic or wooden stock. It offers the same advantages as glass and pillar bedding, but since the block runs the full length of the action, it increases the rigidity of the receiver. It also serves to increase the strength and rigidity of a stock at its weakest point. Whether or not this is important depends on the strength of a particular stock to begin with. I'd just as soon have pillar bedding on a top-quality fiberglass stock, but I do believe the bedding block is better on an injection-molded stock. The downside to a bedding block is it adds several ounces to the stock, but this matters only when overall rifle weight is an issue.
Use of an aluminum bedding block in a riflestock is not as popular as glass and pillar bedding due to its higher cost. It is more often seen in either custom rifles or in one of the more expensive factory rifles, such as the T/C Icon. One of the more interesting designs is the full-length bedding block offered by Hogue in its OverMolded synthetic stock. An aluminum rail extends forward from the receiver bedding block out to the end of the forearm. In addition to adding strength and rigidity to the stock, the rail also serves as an attachment point for the fr
ont sling swivel. This is the stock worn by the Remington Model 700 VSSF II varmint rifle. Knoxx also offers a similar option in its recoil-reducing CompStock.
The Latest Trend
The clever fellows at Savage have gone beyond the simple bedding block with a new design capable of having an even more positive effect on accuracy. The new bedding system works as well in an inexpensive injection-molded stock as it does in the most expensive laid-up synthetic stock, and since the former costs considerably less, it should be affordable to one and all. To understand how and why it works, we must first take a look at the synthetic stocks worn by many factory rifles.
The stock of this Remington Model 40-X (l.) has a conventional aluminum bedding block. An aluminum wedge of the Savage AccuRail bedding block (r.) decreases movement of the receiver during firing by applying pressure to the front of the recoil lug.
In the beginning, the manufacture of synthetic stocks was extremely labor intensive, and it still is for top-of-the-line stocks made by McMillan and a few other companies. The Weatherby Mark V Fibermark was the first factory rifle to wear a synthetic stock, and the cost of its stock made it more expensive than a Mark V with a walnut stock.
Then came the riflestock made of injection-molded plastic. This made it possible for a manufacturer to offer a rifle with a synthetic stock at a price equal to and sometimes even lower than the same rifle with a walnut stock.
To cover as much as the market as possible, some companies have chosen to offer the same rifle in two grades--one with an inexpensive injection-molded stock, the other with an expensive stock made by the lay-up process. Remington and Weatherby are examples.
The injection-molded stock has a few things going for it. It is lightweight, it does not absorb moisture, it seldom breaks or splits, and it is not likely to attract beavers or termites. And as I said before, its low manufacturing cost can shrink the price tag on a rifle, making it more affordable to you and me.
But the low-dollar plastic stock is not without its shortcomings, with one of the more serious being a tendency of its forearm to warp when subjected to high temperatures. Unless the clearance between a free-floating barrel and the edges of the barrel channel in the stock are big enough to throw a tomcat through, the forearm can warp into the barrel, causing the rifle to lose its zero. Shooting a rifle with a bipod attached to its forearm can also cause a flimsy forearm to deflect into the side of the barrel. The same thing can happen when the carrying sling is wrapped around an arm for steadying the rifle before taking a shot.
A bit further on, I will take a close look at the method Savage chose to use to overcome those problems, but first I want to give you an idea of what happens to the fit between the stock and the receiver when a rifle is fired.
For as long as I have been shooting, I had assumed that so long as the action bolts of a rifle were sufficiently tight and the recoil lug fit tightly with the stock, both the stock and the barreled action pretty much moved under recoil as if they were a single unit. A video made by Savage technicians with a high-speed camera revealed otherwise.
During firing, the ultraslow-motion video showed that the receiver squirms and wiggles inside the stock like a snake with its tail caught under a leg of Grandma's rocking chair. If the stock bolts are tight and the fit between the stock and the receiver is precise, the barreled action will likely settle down in its same relationship with the stock with no ill effect on accuracy. But if everything is not just right, it may or may not settle in the same between shots, and that can have a detrimental effect on accuracy.
With the development of its new AccuRail bedding system, Savage has addressed those problems and has also elevated the performance capability of an inexpensive injection-molded stock beyond that of the most expensive synthetic stock containing either a conventional bedding block or pillar bedding. The bottom of the machined-aluminum bedding block--or nesting block as it is more appropriately called--has two 0.400-inch-wide rails that support the bottom of the receiver ring and bridge, and its sides wrap around the full lengths of both sides of the receiver. And since the distance between the two sides of the block is slightly less than the diameter of the receiver, they actually deflect outward a bit when the action bolts are tightened. You can feel it by holding the sides of the stock with one hand as you tighten the bolts.
Layne compared the accuracy of a Savage Model 116 in .243 with both a conventional stock and one with the new AccuRail bedding system. Externally, there are no apparent differences between the two.
That kills two birds with one design detail. For one, it creates far more metal-against-metal surface area contact than conventional bedding pillars or a bedding block. Secondly, and equally important, constant tension placed on the action bolts discourages them from loosening up during recoil. And we all know that a rifle achieves its best accuracy when its action bolts are tight.
The great ideas don't stop there. The AccuRail system utilizes three primary action bolts. Two of the bolts secure the stock at the receiver ring and bridge, which is typical of bolt-action rifles. Tightening a third bolt located just forward of the receiver ring pulls on a steel block called a recoil-lug wedge. As the wedge moves downward, angled mating surfaces between it and the front rail cause it to cam against the front surface of the recoil lug. Considerable tension placed between the rear surface of the recoil lug and the front surface of the receiver-ring rail prevents to-and-fro movement of the receiver in the stock during recoil, while the side tension on the receiver that I mentioned earlier discourages receiver rotation and lateral whip during recoil. And it really works.
While watching the high-speed video of the action area of a rifle wearing a stock with standard pillar bedding, I could easily see the receiver squirming around in the stock during recoil. With the same barreled action "nesting" in a stock with the AccuRail, movement was hardly perceptible. I had to watch the video more than once to convince myself that my eyes were not deceiving me.
Adding considerable stiffness to the stock is an aluminum rail that extends forward from action bed out through the full length of the forearm. And since the front sling-swivel stud is attached to the end of the rail, the stock is ideally suited for use with a bipod.
How stiff is the stock? I can easily bend most any injection-molded stock by placing my knee at its action area and pulling on both ends of the stock with my hands. I find it impossible to do that with a stock containing the AccuRail.
Another great thing about this better idea from Savage is the design lends itself to mass production. The person in charge of the molding process simply positions an AccuRail unit inside the mold, and since all outside surfaces of the unit are undercut, it is keyed into the stock as the plastic resin flows around it, making it a permanent part of the stock. Ease of manufacturing is bound to keep its price far below that of bedding systems, the designs of which require highly skilled labor and a lot more of it, to boot.
Of course, the proof of any new system is in the shooting, and I did just that by taking turns shooting the same Savage Model 116 Weather Warrior with two identical injection-molded stocks--one with the AccuRail, the other without it. The AccuRail stock came with installation instructions, and since each rifle wearing it will depart the factory with those instructions, I won't use up a lot of space describing them. I will say that the sequence used to tighten the three action bolts and the amount of torque applied to them seem to be critical.
During his comparative analysis of the AccuRail system, Layne tightens the recoil-lug wedge to the factory-specified torque after swapping stocks. The two Savage stocks are identical except one has the AccuRail bedding system, and the other is a standard injection mold.
The rifle I shot was in .243 Winchester, and its rather thin 22-inch barrel measured 0.58 inch at the muzzle. To kick off my accuracy comparison, I fired five, five-shot groups with each of three factory loads and two handloads at 100 yards with the rifle wearing the standard stock. After firing each five-shot group, I cooled down the barrel completely. Using Shooter's Choice solvent and a brass brush, I cleaned the bore thoroughly between each 25-shot string. After each cleaning, I fired one fouler off target before shooting the next group. I then switched stocks and repeated the routine for another 125 rounds.
As you can see in the accompanying chart, accuracy of the Model 116 was better with four of the five loads when it wore the AccuStock. The improvement ranged from about a tenth of an inch for the Hornady load to half an inch for the Federal load. Average accuracy improvement of the four loads was just under two-tenths of an inch.
While this is impressive enough, I believe the accuracy difference would be even greater for a rifle chambered for a more powerful cartridge simply because the heavier the recoil, the more the barreled action wants to wiggle around in the stock during recoil. The AccuRail system adds about 5.5 ounces to a stock, but I consider the extra weight insignificant when the positive effect it can have on accuracy is taken into consideration.