November 13th, 2007

Magnum Drop Port from Oz

There was a lot of interest in Jeff Rogers’ Heavy Gun, featured in yesterday’s Daily Bulletin. Jeff is a race-car mechanic and engine builder in Australia, and you’ll find many innovative mechanical features on this rifle.

1. Drop Port for Magnum Case
Jeff reports: “The action is home-made. My shooting buddy Tony Z was the magician here–without his extensive machining experience, the action would still be on the drawing board. As you can see it is a Right Bolt, Left (loading) Port design. What makes it unique is the big gravity port, perhaps the only Drop Port ever made for a case of this size. The port will work will cases up to 2.850″ in length, with a .550″ max rim diameter.” The action is constructed from Swedish 709m chrome-moly steel measuring 2.00″ diameter x 10″ in length. It has roller-assist extraction to help with the high pressure loads Jeff runs. The action is both conventionally bedded and glued into the stock.

2. Removeable Aluminum Side Pods
The two alloy “side pods” were milled from 6061 T6 billet. Given airline weight limit of 32 kg per piece, Jeff made the pods modular (they are through-bolted), so the gun can be air-transported in two sections. Concerning the dual side-pod design, Jeff observed: “[This] is unconventional… but I wanted was something that was true [when bolted together]. Milling the thing up on every surface insured this and the pods are installed with dail indicators on assembly.”

3. Tension Barrel System
The Broughton barrel sits inside a threaded barrel sleeve that holds the barrel in tension. Tensioned barrels have been used on Heavy Guns before, but few have been as successful in competition as Jeff’s rig. Jeff tells us: “The 30-cal barrel is a 1.500″-diameter, 1:13 twist, 32″ Broughton. It is straight for 6″ then tapers to 1.250″ at 32″. The tension tube is 6061 T6 alloy 2.0″ outside diameter with a .187″ wall thickness. At the chamber end, a 431 stainless adaptor is fitted to the barrel, about 1.5″ long, stepped to a slip fit inside the tube for another 1.5 inches. This allows the main heat area of the chamber to sink into the tube. The tension tube is all jointed with a CAT high-temp anti-seize. On the muzzle end is a left hand 1.250x16tpi thread to take the tension nut. We have tried various tensions and found torque settings do not matter to the groups as long as the nut does not come loose. Because I can really rattle down those 10-shot groups with the Drop Port action, remember that the barrel grows in length before it gets a chance to transfer heat to the outer tube. If you shot the gun at pedestrian pace, then only 50 ft/lbs of tension is needed.”

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