Converting the Ruger 77/22 Magnum to .17 HMR

There’s no doubt that the .22rimfire cartridge is at the top of the heap when it comes to recreational plinking, small-game hunting, and both formal and informal target shooting. One early attempt to challenge the .22 rimfire’s status came about 35 years ago when Remington strayed from tradition with the introduction of the 5mm rimfire cartridge. This was the .22-rimfire Magnum necked down to accept a .20-caliber bullet.

Remington was the only manufacturer that produced rifles for the 5mm rimfire round, and interest faded overtime. A much more successful attempt came a few years ago when the .17 HMR (Hornady, Marlin, Ruger) cartridge exploded onto the scene; rifles chambered for this new round are now being produced by most of the major manufacturers. Barrel Choice Aftermarket .17 HMR barrels are beginning to show up on a regular basis to safely convert .22 Magnum rimfire bolt-action rifles to this caliber.

All of these barrels that I’ve examined are quite good, and the .17 HMR is proving to be an excellent cartridge with a flat trajectory and superb accuracy. The conversion I’m detailing here involves the Ruger M77/22 VMBZ in stainless steel with a laminated stock and a Green Mountain .17 HMR barrel in blued chrome-moly steel. The idea is to produce a switch-barrel rifle that’s easily converted from .22 Magnum to .17 HMR and back, using the same rotary magazine.

The Green Mountain barrel I ordered arrived contoured, rifled, chambered, and blued, with the barrel shank turned to size. The extractor cuts were in place, along with the half-dovetail cut for the clamp that attaches the barrel to the receiver. Stainless-steel barrels are also available. The only obvious difference in the Green Mountain barrel is its length’around 2 inches shorter than the original factory barrel of 24 inches.

Incidentally, the same style of aftermarket trigger and sear were installed on this rifle as I described in the article "Improving the Ruger 77/ 22" in the April 2006 issue. Removing the Barrel Start by removing the magazine and pulling the bolt out of the receiver. Verify that the chamber is empty and clamp the barrel tightly in your padded-jaw vise. If the rifle has a scope mounted, it would be a good idea to remove it.

The action screws can be removed with a #340-6 Magna-tip bit, and the barreled action is then removed from the stock. This would be a good time to look over the inside of the stock area where the receiver fits for any signs of a mismatch in fit. Epoxy bedding will fill these voids and give the action a stable platform to work from. Scrape away any obvious spots of barrel contact in the barrel channel and seal these with two coats of finish.

There are two #12-24 socket-head 22 Magnum (left) is the parent case for the .17 HMR (right). The cartridge in the middle is the 5mm Remington. cap screws that hold the clamp maintaining the barrel to the receiver. These are removed with a 5/32-inch ballend hex wrench. The ball-end-style wrench will pivot a bit preventing any marring of the barrel finish. Once the barrel clamp has been removed, clean the barrel receptacle in the action and the barrel shank, removing any crud that has gotten into these areas.

Before installing the Green Mountain .17 HMR barrel, you must take a couple of important measurements. The first measurement will require that the bolt be reinserted into the receiver and turned down as though chambering around. Using a 0-1-inch

depth micrometer, measure from the front face of the receiver down to the bolt face (where the cartridge face rests against it) and record this dimension. You will notice that there is a slight recess turned into the front face of the receiver where the original barrel-shank shoulder rests. Measure from the front face of the receiver down to this recess, and subtract this dimension from the previous dimension recorded. You now have a target dimension for fitting the Green Mountain barrel.

Insert a dummy .17 HMR round (a "go" headspace gauge would be better) into the chamber of the Green Mountain barrel and measure from the back face of the dummy round down to the shoulder at the front face of the barrel shank. Record this dimension. As a comparison, you can record the factory-set headspace on the .22 Magnum barrel using the same process. Setting Headspace The consistency of the rim thickness on rimfire cartridges has a lot to do with how well a rifle groups its shots.

SAAMI specifications for .22 WMR rimfire ammunition is .050 inch nominal, and can go minus by .008 inch. Because the .17 HMR uses the .22 WMR case as its parent, there’s no reason not to use these same specs. I measured the rim thickness on a dozen CCI .17 HMR cartridges loaded with the Speer TNT bullet on an optical comparator, and found those case rims ranged from .048 to .051 inch. Headspace specifications for .22 WMR-chambered rifles are .050 inch minimum and .056 maximum.

Minimum headspace would be the goal for maximum accuracy potential’up to a point. We don’t want to set headspace so close that the bolt face could crush the rim of the case. With this combination of rifle receiver and Green Mountain barrel, I came up with a measurement of .920 inch from the front receiver-face recess to the front face of the bolt where the firing pin exits. From the face of a dummy round to the face at the barrel shoulder on the Green Mountain barrel, I measured .910 inch.

Green Mountain provides a shim kit including five .002-inch washer-style shims that slip over the barrel shank and against the receiver in the event the barrel shank proves to be too long. Nevertheless, there’s no provision for extending the barrel-shank dimension should it prove to be too short. Green Mountain has no control over the tolerances used at the Ruger factory; therefore, if the shoulder-face to receiver distance is too short, some lathe work will be required.

I set up the Green Mountain barrel in the lathe along with a dial indicator to record how much metal was being removed from the barrel shoulder face. In this case, the barrel shank measured .010-inch short, so I removed .008 inch from the shoulder face that rests against the front receiver recess. If the barrel shank was left as delivered, most likely the rear face of the .17 HMR cartridges would set back as they were fired, and contact the boltface.

This would not be a good situation as the case will stretch somewhat and could split or rupture. We don’t reload the fired cases, of course, but we don’t want our good work pocked with tiny brass fragments either. Before installing the barrel, make sure the bore is cleaned with a solvent to remove any preservative oil. While installing this barrel, I found it best to secure the receiver in my padded-jaw vise with the bottom facing up and the bolt out of the receiver.

Insert the barrel into the front of the receiver and attach the barrel clamp, leaving the two screws a little loose so that the barrel can be rotated slightly from side to side. Install the bolt into the receiver, carefully pushing it forward to see if the extractor slot lines up with the bolt properly. Rotate the barrel so that it does, and then tighten up the barrel clamp.

Work the bolt forward with the dummy round in the chamber and rotate the bolt closed, striving to feel for any drag at the front of the bolt face and back face of the barrel shank. It may help to blacken the front face of the bolt with a black (continued on page 17)

felt-tip pen to observe any rubbing. If rubbing is detected, one or more of the .002-inch shims may need to be added to the shoulder on the barrel shank. In the unlikely event that you need more shims, they will be sent free of charge from Green Mountain. Conclusions The stock on this rifle was epoxy bedded only in the areas that support the receiver. Because I wanted the convenience of switching from caliber to caliber, no epoxy was used in the barrel channel.

This leaves a considerable gap around the Green Mountain barrel in the forearm channel. Both barrels should be stiff enough to overcome much of the harmonic vibration caused when firing a .17 HMR round. Further testing of this combination will take place when the weather permits. If I detect excessive barrel vibration, my plan is to convert the front swivel stud into an adjustable barrel-dampening pad. I