September 23, 2010
By Layne Simpson
Today's benchresters go to extremes to ensure incredible consistency.
By Layne Simpson
Modern benchrest shooting as we know it today got its start during World War II when a group of Washington riflemen called the Seattle Sniper's Congress started holding matches. The sport eventually made its way to the opposite coast, and in 1947 the Eastern Benchrest Shooters Association headquartered in Johnstown, New York, was formed by Harvey Donaldson who, among other things, shot several thousand woodchucks, drove a Chevy Corvette quite late in life and developed a wildcat cartridge called the .219 Dondaldson Wasp.
In the beginning, most bench guns were built around military surplus actions such as the 1903 Springfield, 1898 Mauser and 1917 Enfield, and the sky was the limit on rifle weight and size. It was eventually decided that the competition would be divided into different classifications with unrestricted rifles of original design used in what was first called Benchrest Rifle Class and then later changed to Unlimited Class.
That class continues to exist in benchrest shooting today, as do the Light Varmint, Heavy Varmint, Sporter and Hunter classifications. Certain weight and dimensional restrictions apply to rifles used in all except the Unlimited class, and, in the case of Sporter and Hunter, to caliber as well.
Except for its extremely heavy weight, the Unlimited Class rifle of the 1940s was quite similar to those used in other classes, and all were shot with their stocks and fore-ends resting atop sandbags. Then some bright soul decided that while weight was important, it was not the total answer to shooting the smallest groups possible on paper.
What was needed was a way of more uniformly returning the rifle to battery between shots. The first part of the final answer came with the development of the mechanical rear rest.
One of the first was called the Ferguson rest after its builder; in addition to precision windage and elevation adjustments, it had a steel bearing that mated with a V-way inletted into the bottom of the wooden buttstock of a rifle. The mechanical rear rest became a big success, and competitors soon decided that if replacing the rear sandbag with such a device was good, replacing both sandbags with two adjustable rests would be even better.
Even though a rifle capable of being mechanically returned to battery in preparation for each shot was more precise than the use of sandbags, it still looked much like the rifles used in the other classes. But since there were no restrictions whatsoever on firearms used in Unlimited class, there was no reason for it to even closely resemble a rifle.
I have no idea who came with the first rail gun, as it is now commonly called, but Bill Hall, who built mine during the 1970s, may have been one of the first. It was later updated by Kenny Jarrett, who used it in competition for several years.
One of the smallest 10-shot groups fired by him with that gun at 100 yards was officially measured at 0.154 inch, which was good enough to take home the small-group award in that particular Florida match. After Hall's retirement, Jarrett built a rail gun for himself from scratch, and I inherited the one you see in this report.
Like all rail guns, the only things mine has in common with a shoulder-fired rifle are a turn-bolt action and a rifled barrel. It consists of two major parts: a base and the upper section, both made up of various aluminum, steel and bronze parts.
Shaped like a short ironing board, the aluminum base is one inch thick, measures 10 inches wide at the rear, five inches wide at the front and is 26 inches long.
The large knob on the rear rest (l.) adjusts for windage. The lever just forward of it shifts point of impact between record and sighter areas of the target. The front rest (r.) uses a mariner's wheel for elevation adjustment. The Delrin bearing on top mates with a V-way in the upper unit. Look, ma-no stock! In benchrest competition, contact with the rifle itself is limited to the trigger. Competitors don't even look through their scopes while firing, paying attention to wind flags instead.
Each of the leveling bolts of the base was fabricated by drilling a hole in the end of a half-inch bolt, inserting the tungsten carbide core from a military-surplus armor-piercing bullet and securing it in place with epoxy glue. This gives the leveling screws extremely hard points capable of digging into the concrete top of a benchrest. During a match it is not uncommon to see a shooter use a plastic mallet to hammer the tips of the bolts a bit into the concrete and then mark those spots so the base can be placed back on the bench in the exact same spot in preparation for the next relay.
Secured to the top surface of the base are front and rear adjustable rests. Rather simple in design, the front rest begins with a 3.5-inch aluminum cylinder standing upright with one end bolted to the base. Inside the cylinder is a bronze bushing, and resting inside the bushing is a steel elevation post replete with mariner's wheel.
Imbedded into the top of the leveling post is a bearing machined from Delrin, a low-friction, wear-resistant polymer developed by DuPont during the 1950s and still used in some commercial applications today as a rust-free substitute for metal. The bearing mates with a precision-machined V-way attached at the front of the bottom of the upper unit. A rubber bumper at the rear of the V-way arrests forward travel of the upper unit when it is pushed forward into battery in preparation for firing.
Since the rear mechanical rest is designed to perform more functions, it is a bit more complicated. A vertically mounted elevation knob supports the forward edge of a hinged aluminum plate that has left and right Delrin bearings attached to its top surface. The two bearings mate with V-ways running from front to rear on the upper unit.
When the gun is used in competition, the rear rest is used for windage corrections required to compensate for changes in range conditions while the wheel of the front rest is used to make changes in elevation.
The rear of the support plate of the rear rest is supported by a steel axle that is attached to the base by a couple of aluminum brackets. A horizontal threaded shaft with a knurled knob reaches through the left-hand bracket to make contact with the left side of the bearing support, and since it is free to slide left and right on its axle, turning the knob moves it
for windage adjustment.
Twisting the knob clockwise shifts bullet point of impact to the left by moving the rear of the upper unit to the right, and in doing so it compresses a spring located on the axle between the edge of the plate and the inside of the right-hand bracket. Turning the knob counterclockwise allows the spring to push the rear of the upper unit to the left for a shift in bullet point of impact the right.
A small lever on the rear rest overrides the windage adjustment knob and cams the bearing plate to higher or lower elevation. Doing so shifts bullet point of impact up and down between the record and sighter groups on the target.
Here is how it works. Let's say that after firing four shots of a 10-shot group for record, wind flags positioned downrange indicate a shift in wind speed or direction. To see what effect the change is having on bullet placement on the target, a push on the lever instantly shifts point of aim from the record group to the sighter group. Then after a shot is placed there to determine how much windage and/or elevation adjustment must be dialed in to compensate for the change, the lever is used to switch the point of aim back to the record group where the shooting proceeds.
Or if, after firing a test shot, the shooter decides that no windage or elevation correction is necessary, the lever will return the gun precisely from sighter target back to record target with no shift in bullet point of impact.
Simpson's rail gun has a sleeved Remington XP-100 action with a two-ounce 40X trigger glued inside a massive aluminum block, which is bolted to the upper assembly. The base (foreground) includes the front and rear adjustable rests. Behind that is a bottom view of the upper assembly with its three bearing V-ways.
It might be of interest to note that a top-ranked competitor seldom has to shoot a sighter shot after beginning a 10-shot string for record. Most often he watches his wind flags until a particular repetitive condition is spotted, and the next time that condition comes around he machine-guns all of his shots as rapidly as possible. A really good shooter who knows how to make a rail gun sing its sweet song can get off 10 shots inside half a minute with seconds to spare.
Getting back to the construction of my rail gun, the primary section of the upper unit is a 26-inch long section of five-inch aluminum I-beam with a web thickness of close to half an inch. Attached front and center on its bottom side is the V-way with its rubber bumper.
About 10 inches farther back, a Delrin bumper prevents the upper section from recoiling completely off the base during firing. This would not likely happen with a mild-mannered cartridge such as the 6mm PPC, but it could with a harder-kicking cartridge such as the .308 Winchester, which was once mildly popular in benchrest competition. Attached to both sides of the rail at the rear is a pair of V-ways that mate with the bearings atop the rear adjustable rest.
The method used to attach the barreled action to the rail differs depending on the action chosen. When a solid-bottom receiver of cylindrical shape such as the Remington 40X or XP-100 is used, it is first turned in a lathe to perfect concentricity and then glued into an aluminum sleeve.
My gun has an XP-100 action with a two-ounce 40X trigger, and its sleeve is two inches in diameter. At 10.25 inches, the sleeve is considerably longer than the receiver. The front section of the sleeve is glued into a massive block of aluminum measuring six inches long, 3.5 inches wide and three inches tall. The block is attached to the rail with four large bolts.
Most rail guns built today utilize actions made specifically for benchrest competition, with the Hall and Stolle a couple of examples. When a flat-bottomed receiver such as the Stolle is used, the attachment block is different, but its principle is the same. Drilling and tapping the top of the block for the mounting of a scope completes the rail gun.
Since there is no weight limitation, the rail gun usually wears a barrel measuring 1.5 inches or larger in diameter. I retired my gun from competition quite a few years ago, but since I continue to use it for accuracy testing, it has three interchangeable bolts and barrels chambered for a number of cartridges including the .223 Remington, 6mm PPC, .270 Winchester and .308 Winchester. Weight will vary slightly depending on the barrel it wears, but it's around 65 pounds.
The first step in setting up the rail gun for shooting is to shift it around on the bench until the muzzle of the barrel is aligned with the target. The upper unit is then removed and the base is perfectly leveled by using its leveling bolts along with a small circular level.
After a dab of lubricant is applied to the Delrin bearings (Pledge spray wax works great), the upper unit is placed on the base, and the front adjustable rest is used to bring the crosshairs of the scope to the record group area of the target. The elevation cam lever is then flipped to confirm that the crosshairs of the scope move from the record-group area to the sighter-shot area.
When shooting a rail gun, it is important to develop a smooth and repetitive routine. A cartridge is inserted and the bolt closed while the gun is at rest in its rear position. It is then pushed forward into battery with the same amount of pressure against the rubber stop for each shot.
I should mention at this point that since the gun is free to recoil as much as five inches to the rear, the shooter is not looking through the scope when firing; instead, his eyes focused on wind flags positioned downrange. The scope is used only for initial alignment of the gun on the target and to check to see what effect a change in range conditions has on bullet flight in the event that a shot has to be fired on the sighter target. Anytime the gun is loaded, the shooter's head is kept a safe distance away from the scope.
When shooting my gun with a barrel chambered for a mild-mannered cartridges such as the .223 Remington or 6mm PPC, I allow it to recoil freely with my trigger finger the only part of my body making contact. But when the gun is wearing a barrel chambered for a cartridge such as the .308 Winchester, I allow the rear of the rail to recoil into the palm of my left hand to avoid excessive battering of its Delrin stop. Due to the weight of the upper unit, recoil is light even when shooting the .308.
Since the only connection between the rail gun and shooter is the trigger finger, and since shooter error in returning the gun to battery between shots is virtually eliminated, one might logically that only one-hole groups are ever fired.
This might be true if the shooting took place in a vacuum, but in a game where the ability make the proper corrections during changes in range conditions before squ
eezing off each shot is as important as having a super-accurate rifle, consistent bug-hole groups come only to those shooters who know how to read the wind and understand its effect on bullet flight.