6.5-284 MAK TubeGun Target Rifle

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Completed 3/6/05

This rifle was a multiple week project, most of which was spent building the necessary fixturing and tooling necessary to do the action blueprinting and the barrel fitting. All machining was performed on a Grizzly 13X40 lathe, and a Grizzly 9X42 mill, by the author.

The rifle was chambered for Lapua brass. I expect to be using Lapua Scenar bullets, in the 123 and 139 grain weights.

The specifications are as follows:

Performance at 200 yards:

14 shots at 600 yards, windy conditions, also 142 SMK/H4350 load as above:

Fitting the Bolt

As part of the action truing process, I decided to ensure a tight fit between the bolt and receiver by adding and machining four small weld beads, instead of the traditional sleeving process. If the welds are made at the right places, the bolt can move freely when retracting the bolt, but will lock up with a near perfect fit when in battery.

I started by very carefully measuring the bolt raceway with a Starrett snap gage. In particular, I was sure to measure the raceway at both bearing points: near the bolt head, and back near the bolt handle. I found nearly a 0.002” difference between the diameters in the action. Once these were measured, checked, and recorded, I proceeded to the welding process.

I welded a very light bead on the bolt in the four places that will contact the bearing surfaces on the action. I used my Miller TIG welder and a standard S type steel TIG filler rod to make these beads. My research indicated that welding CrMo (4140) type steel with S TIG rod was OK. I was very careful not to touch the bolt lugs, or to over penetrate. I just wanted to add a slight amount of metal to allow the machining process to generate that perfect fit.

After welding, I mounted the bolt in the four jaw chuck using a spud threaded into the back of the bolt (1/2” X 13 TPI), and a live center in the firing pin hole in the bolt face. The bolt was trued in the same way that is used when squaring the bolt lug contact points and truing the bolt nose and face. I then carefully turned each pair of beads, front and back, to the measured bolt raceway diameter minus 0.001”. A very easy process, and it fits perfectly. The final touch was to put a little 800 grit lapping compound from Brownells on the back of the lugs and the bolt lug ears, and gently lap the lugs to the receiver and the bolt raceway. Just a few strokes did it, being careful not to get the lapping compound on the round part of the raceway, just the lug recesses and bearing areas.

I jeweled the bolt on my mill using a rotary table in the horizontal position, and a Damascening tool from Brownells. Be sure to polish the bolt when you are on the lathe, before the jewelling.

The pictures below show the weld beads after machining:

Pinning the Sight Rail

I had a real problem with the sight rail on the receiver sleeve shifting around under recoil given the weight of that Nightforce scope. The flat head screws are not enough to hold it tight, so I added some 3/16” dowel pins to take up the load under recoil. I can now remove the scope rail and replace it without any change in zero.

Here are a couple shots of the pinned rail:

Tapered Sight Rail Wedges for Long Distance Shooting

The tubegun is primarily meant for use with iron sights, and has zero offset angle in the sight rail. For long distance shooting, I needed to add between 20 and 60 MOA in order to keep the scope within its elevation adjustment range.

I designed and machined these rail wedges, meant to go between the rail and the receiver sleeve. It is machined with the required angle, and is held captive by the rear dowel pin, between the rail and the receiver sleeve. I machined it from 6061 aluminum. I actually made three, one each at 20, 40, and 60 MOA. They can be added or changed without loss of windage zero, due to the dowel pins. Here is the 60 MOA rail:

Front and Rear Bag Rails

The tubegun is designed for hand held shooting using iron sights. My shooting at live varmints on off the bench requires the use of front and rear bags. I designed and built a set front and rear bag rests to accomplish this purpose.

First Model for Front Rail- Aluminum Plate Design

The front rest is a simple aluminum plate, made from 0.250” stock. It is about 3” wide by 7” long. It is machined with a narrow ridge along the centerline that fits into the hand stop slot in the tubegun front hand guard tube. A set of T nuts made to fit the slot were made from Delrin, and tapped 10-32. I have found that the rifle likes to be supported as near the receiver as possible, so I have slid the front rest plate as far back along this slot as possible. This works perfectly in my Sinclair benchrest top.

Second Model for Front Rail - Acetal (Delrin) Box Design (4/15/06)

I was not completely happy with the tracking performance of the aluminum plate design, so I built up a three inch wide box using some slippery acetal (trade name Delrin) plastic and some 10-32 capscrews. I used the same T nuts to hold it to the TubeGun front hand grip tube. This design tracks much better in the front rest, and is very slippery, providing a smoother recoil characteristic and making it much easier to return to battery after each shot.

Rear Bag Rail

The rear bag rail is added to the Ironstone competition butt stock using the weight rail as the attachment point. It is a two piece affair made from aluminum, with a 5/8” rod that tracks in the rear bag perfectly.

New and Stiffer Front Tube Assembly 4/26/07

I have never been satisfied with the stock front hand guard tube assembly. While perfect for offhand irons prone shooting, it was not stiff enough for F-Class shooting from a rest, especially with heavy contour barrels. There was quite a bit of bounce and flex, making it a difficult to get consistent group performance. It worked best with a very hard hold on the handgrip, and with the butt stock hard into the shoulder. Free recoil did not work well at all. I have been pondering how to make this assembly stiffer so that the rifle will be able to perform well with a light hold or free recoil technique.

While fitting the rifle with a new Bartlien 5R, 8.5" twist, 1.25" straight cylinder contour 6.5mm barrel, chambered in 6.5-284, I decided to make a new hand guard and front stock assembly. The goal was to make it as stiff as possible, and still keep the overall rifle weight under the 10kg F-Class Open weight limit.

 I settled on a composite assembly, consisting of two aluminum tubes, 0.125" in wall thickness, bonded together with epoxy, and then machined to fit the receiver sleeve. The inner tube had an ID of 1.75", to fit the Tube Gun receiver sleeve. The outer tube as a piece of aluminum pipe, also 0.125" wall, that just fit over the smaller tube, with about 0.030" clearance. Devcon 10110 steel filled epoxy was used to bond the two together. After fitting to the receiver, and providing clearance slots for the front scope Picatinny rail, and the attachment groove for the Delrin front rest support, a (very) large number of 0.75" holes were milled through using an end mill. This did not weaken the assembly, and took off nearly half the assembly weight.

The assembled rifle now weighs 9.860kg. The front rest is very solid, with no flex or bounce. I got a little fancy with the bead blasting of the barrel, just for fun. Break-in will commence this weekend, and results will be posted back here..