Davidson's Amazing "Tinker-Toy" 30BR
Radical Offset, Skeletonized Benchrest Rifle Proves That Innovation Works
Other Guns of the Week
Editor's NOTE: Shelley Davidson passed away in 2008 after a courageous battle with cancer. He was one of the great innovators in benchrest rifle design. This article, written before Shelley , showcases Shelley's creative talents at their best. His "Tinker Toy" design will always be a tribute to Shelley's fabricating skills and imagination.
Shelley Davidson's shooting partners call his radical rifle the "Tinker Toy" gun. We call it revolutionary. This innovative, skeleton design could be to Benchrest what the rear engine configuration was to Top Fuel drag racing. Davidson's new rig throws conventional wisdom to the winds. Shelley readily concedes he "broke the rules" of benchrest rifle building. But this was inspired rule-breaking, because Davidson's rifle shoots like a house on fire. The Tinker Toy gun won its first matches, both for Score AND for Group. And this rifle has also shot more "zeros" in Gene Begg's Texas Tunnel than has any other rifle to date. Hats off to Shelley for inventing a whole new kind of accurate rifle.
Tinker Toy 30BR -- Radical as it Gets
by Shelley Davidson
Although I'm not big on naming rifles, my shooting buddies have christened the gun "Tinker Toy." I can live with that as it does kind of look as if it was made with a Tinker Toy set.
Origins of the Project
This project began with some wild ideas I had in the fall of 2006 about using magnets to tune a barrel. My idea was to use one magnet on the barrel and another on the stock so they pushed against each other to counter gravity-induced barrel sag (and possibly) tame barrel vibration in a beneficial manner. The only way to test these ideas was to build the device and mount it on a gun. That meant I had to build a new rifle because there was no place to mount a magnet on the stock of a conventional benchrest rig. I had a Kelbly-stocked heavy varmint stock with a Michael Kavanaugh paint job on it. I didn't think Kav would ever forgive me if I started drilling holes in one of his works of art. My light varmint was in a carbon fiber Scoville stock that costs about a grand. Drilling into the Scoville for an experiment just smacked of bad judgment. So, the magnet thing was my first motivation for designing a new stock. As long as I was building from scratch I decided to offset the barrel and action 0.75" to the right to counteract the spin/torque from the bullet.
Although there's nothing new here, my second motivation was to build a 30BR that could shoot in the 10.5 lb light varmint class in NBRSA. The magnetic tuner will automatically make this gun illegal in the IBS. The IBS has declared all barrel attachments un-safe and have outlawed them. I personally feel that the IBS really outlawed all barrel attachments to prevent experimentation and innovation. But at least we have NBRSA matches.
|TINKER TOY SPECIFICATIONS
Action: Stiller SS Diamondback Drop-Port (1/2" short), with .308 Bolt Face.
Barrel: Shilen .308 caliber, 17-twist, HV Contour.
Chambering: 30BR, .330" neck, Pacific Tool & Gauge Robinett Reamer
Stock: Davidson Custom Tubular Stock with 0.75" Offset Barreled Action.
Tube Construction: 6061 Aluminum, 1" diameter (left), 5/8" diameter (right).
Load: H4198 powder and 118gr Ronnie Cheek bullets. Loaded to 2980 fps.
Trigger: Jewell, 2 ounce BR.
Tuner: Custom, Adjustable with Opposing Magnets.
Optics: Leupold 36X (locked by J. Schmidt).
Rings: Jewell Foster External Adjusting Rings.
Designing the New Gun--Thinking "Outside the Box"
Once I'd decided to build a lightweight stock that could support experimental devices out near the muzzle, I started drawing up some rough plans. I also took a trip to Jerry Stiller's shop in Wylie, Texas for a brainstorming session with Jerry, the maker of Viper and other Benchrest actions. Jerry is a school-trained mechanical engineer and thinks differently than I do. I came away from Stiller's shop with my design roughed out and sketched on paper. The design violated several covenants of conventional wisdom for building competition BR rifles. For instance, two-piece stocks stress the action. Stress reduction is why most BR rifles are glued into the stock. Another myth is that metal stocks vibrate too much so wood or foam-filled fiberglass or carbon fiber are used.
I had wanted to use an aluminum Stiller Cobra drop port with a 6mmBR bolt face but Jerry had none in stock and he estimated it would be a year before one was available. Although I've waited for up to a year for an action in the past, I wanted to build this rifle during the fall of 2006 while the weather was pleasant enough to work in my unheated and un-air-conditioned garage shop. Jerry did have a 1/2" short stainless steel Diamondback in stock so I purchased it even though it would add 3 ounces to the gun compared to the aluminum Cobra. Three ounces is a lot of weight when you're working with a 10.5-lb limit. I had a heavy varmint contour Shilen 17-twist barrel that would work nicely and I had a Jewell trigger on a rifle that I wasn't using at the time. I also decided to use my Leupold 36X (locked-up by Jackie Schmidt) with the Jewell/Foster adjustable rings.
Building the Tube Fore-Arm and Brackets
I took a wild guess as to tubing thickness and settled on .035" for the 1" left fore-arm tube and .058" for the 5/8" right fore-arm tube. All of the flat stock and tubes are 6061 Aluminum. I did the lathe work and the mill work and every evening I'd put the parts together and think about the proper way to proceed.
When the parts were mostly made, I started thinking that this was a truly ugly rifle. I thought about painting it but that wasn't a good option as many of the parts are designed to slide over others and glue together. Anodizing was the best answer so while looking on the Internet for local anodizing shops I Googled "Home Anodizing". Sure enough there were a few sites that told about how to anodize at home. I picked up some battery acid from NAPA Auto Supply, some Rit Clothes Dye from Wal-Mart, and a bunch of distilled water from the grocery store. Using an old battery charger as my dc power supply I started anodizing and dying the eighteen parts that went into the stock. Although I had to strip and re-anodize some of the parts, the work turned out acceptable.
Putting it All Together--Lug-Mounting the Fore-Arm and Lots of Epoxy
The barrel contour had to be modified to work with the stock which attaches by way of a rear plate which mounts like a recoil lug and a plate that ties the barrel and the stock tubes together 4" forward of the bolt face. The four-inch maximum distance is a NBRSA rule concerning barrel blocks.
The recoil lug-style stock mount is probably the only truly innovative thing I did other than the opposing-magnet tuner. Basically, the rear bracket is sandwiched between the receiver face and the barrel shoulder--positioned where a conventional recoil lug would go. I also added a brass ring (visible in photo) between the anodized bracket and the barrel. This was done to distribute loads over a wider surface area. (I was concerned that the bracket material was fairly soft and I didn't want to crush it as I torqued the barrel in place.) After fitting the barrel and plates I glued the entire gun together using epoxy and various LocTite adhesives. The rest of the parts were assembled but I did not Loctite the scope bases since I thought I'd be disassembling the rifle for re-work after the first trials. That came back to bite me during later testing when the gun started shooting erratically and I went down a couple of blind alleys before finding the loose bases.
Range Testing--Results Are Very Positive
The first range session was a real shocker. Even though the wind was up to 10mph and twitchy, the rifle showed promise from the very first shot. I really didn't expect that kind of performance without, at least, some rework. After sighting in, I shot five, 5-shot groups that, when averaged together, measured .223". That's good enough to win some benchrest group matches. But I wasn't finished with the gun yet--I still wanted to try out my magnetic tuner concept.
The Magnetic Tuner
Next, I built the magnetic tuner. The tuner consists of one rare earth magnet attached to the stock and another attached to a barrel sleeve with the magnets oriented so the magnetic forces repel each other. In order to test the magnets and to determine if the rifle really shot as well as it seemed to, I took it to Gene Beggs' shooting tunnel in Odessa, Texas. I spent two days at the tunnel testing loads and then installed the magnetic tuner. The gun shoots well with the magnets and shoots well without them. I suppose I can't make any claims as to how much, if any, improvement the magnets make. Gene said that my gun was the most accurate rifle to be tested at his one-year-old shooting facility: "Shelley Davidson brought one of the most unusual rifles I had ever seen; he called it his 'Tube Gun.' And boy, did it ever shoot! It still holds the record in the tunnel as the rifle that shot more zeros than any other to date." I definitely recommend Gene's facility for testing and refining shooting techniques and loads. For more info, call Gene at (432) 631-5124.
Shelly Davidson's Rifle is so innovative, that almost every feature except the bare action is very different than you'll find on most Benchrest rigs. Accordingly we felt it would be useful to isolate and describe the key design features, from stem to stern. Click thumbnails to view FULL-SIZE PHOTOS.
Front Bracket with Magnetic Tuner
The tuner consists of one rare earth magnet attached to the stock and another attached to a barrel sleeve with the magnets oriented so as to make the magnetic force repel each other. The purpose is to counter "barrel droop" and, hopefully, dampen barrel vibration. The lower magnet is carried on a threaded shaft (with lock ring), allowing the magnet to be raised up and down to adjust the "up push" on the barrel.
Tubular Fore-Arm Supported by Brackets
Three brackets support two tubes, one on either side of the barrel. The rear-most bracket is sandwiched between the barrel and the action. Four inches forward (max distance allowed for barrel blocks) a second bracket grips the barrel. Near the muzzle a third bracket secures the ends of the tubes and holds the magnetic tuner. To allow barrel offset, Davidson uses a 1" diameter tube on the left and a 5/8" diameter tube on the right.
The rifle rests on a 3" wide plate attached to the underside of the two fore-end tubes. With the plate centered in the front sandbag, the barreled action is actually offset 0.75" to the right (looking forward from the breech). The purpose of this offset is to keep more weight on the right side to counter the tendency of the rifle to torque counter-clockwise. Davidson employed two different diameter tubes to allow for the built-in offset.
Floating Action without Sub-Support or Bedding
On the Tinker Toy gun, the action serves as a load-bearing assembly, holding the barrel in the front, and the skeleton buttstock (or "keel") in the rear. Shelley was told that accuracy would suffer if you stressed a benchrest action in this manner, but so far, that has proven untrue. It is a very simple solution to building a rifle, and it eliminates the need to bed the action. The forearm attaches to the action via a bracket installed like a recoil lug.
Skeleton Rear "Keel" Affixed Directly to Action
Davidson's Tinker Toy does not have a conventional rear buttstock. Instead there is low-profile,
v-shaped metal "keel", as Davidson calls it, that rides the rear bag. The keel is supported by a tubular backbone that attaches at the rear of the Diamondback action. At the butt end is an aluminum plate covered with bubble wrap that serves as a butt pad. The skeletonized rear section helps the rifle maintain a very low center of gravity.
Locked Scope with External Windage and Elevation Adjustment
Shelley runs an older Leupold 36X Benchrest Scope with front-adjusting objective. To eliminate slop or loose tolerances in the erector mechanism that could cause changes in point of impact, the internals have been locked up by Jackie Schmidt. To move the cross-hairs relative to the bore axis, Shelley has a special Jewell/Foster rear ring that allows a limited amount of lateral and vertical movement of the entire scope body.
|Competition--Tinker Toy Wins Both Score and Group Matches
Finally the big day arrived when I’d shoot the first match with my new gun. The North Texas Shooters Association was holding its first club match of the 2007 season. At the Denton, Texas matches we shoot a Score Match in the morning and a Group Match in the afternoon. The March event was at 100 yards and the April match will be at 200 yards and so on alternating throughout the benchrest season.
Match One--Tinker Toy Wins Score with a 250 - 17X
Since the gun is chambered in 30BR and that chambering is almost immune to tuning woes, I preloaded 130 rounds with H4198 powder and 118gr Cheek bullets. I used my SEB front rest and rear bag which are made by Sebastian Lambang in Indonesia. Everything came together, and Tinker Toy demonstrated that the accuracy it showed in the tunnel was no fluke. The gun shot great and I won the morning match with a 250, 17X. The day was quite windy and the next best shooter scored a 250, 15X. So I’d chalked up my first win.
|Score Shooting vs. Group Shooting--The RulesIn a score match, the shooter shoots one bullet at each of five record targets, which are clustered on one target sheet. The aggregate score of five of these targets determines the winner. If the shooter touches the 10 ring on all of his 25 targets he can score a "clean" 250 score. Usually there’ll be more than one shooter who scores a 250 so the winner is determined by the X-count. The 1/2” 10-point ring has a 1/16" dot in its center. Touching the X dot adds to the shooters X count. On my target at right, shot with Tinker Toy, I posted a 50 score with four Xs.
In short-range group matches, the shooter must try to put five bullets through the same hole. At each distance (100 or 200), five, 5-shot matches are scored, the group sizes are added together (moa equivalent at 200) and the total is divided by five to arrive at an Aggregate score.
Match Two--Tinker Toy Wins Group with a .2282" Agg
Tinker Toy won the afternoon group match I entered with a five-group Aggregate of .2282". (The second place score was .2568".) My groups were .149", .197", .243", .302" (oops), and .250". You know how some folks say a 30BR can't be competitive with a PPC? Well that .2282" Agg won't break any records, but it is good enough to win some regional registered BR matches. So this rifle has demonstrated an ability to win in both Score and Group matches. Obviously I have a very good Shilen barrel, great Cheek bullets and the rest of the components are doing their jobs as well. But, the stock is also working well.
Next Challenge--A Long-Range Tinker Toy
My next project is to build a similar rifle to be used in long range and F-Class matches. The long-range stock will be longer to accommodate a longer, heavier barrel and the fore stock tubes will probably have thicker walls as the weight limits are much higher. Will I build another short-range Benchrest rifle, and will the design be updated or modified? Yes there will be another short-range Tinker Toy gun. And you may be surprised, but right now, I can’t think of anything I’ll change on the next 100-300 yard gun. The magnetic tuner is interesting enough that I'll keep experimenting with it, and perhaps include one on the next rifle.
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Topics: Shelley Davidson, 30BR, 30 BR, Skeleton, Tinkertoy, Tinker Toy, Tube stock, Epoxy, Offset, Assymetrical, Tubegun, Heavy Varmint, Light Varmint, Stiller, Drop port, Drop-port, Diamondback, Actions, Shilen Barrels, Ronnie Cheek, Cheek Bullets, Varmint for Score, IBS, NBRSA, 100 yards, 200 yards, 300 yards, Varmint, Varminting, Jewell trigger, Benchrest, BR, Bench Rest, Single-shot, competition, rifle accuracy, Hodgdon, H4198, Dave Kiff, Pacific Tool & Gauge, JGS, Lathe, stocks, Weaver, Wilson Die.