October 28, 2013
During my college years I worked during summers for an electrical contracting company. We often used powder-actuated tools, or stud guns as we called them. Powerful enough to imbed hardened threaded pins into concrete floors and steel beams, they were used to fasten electrical conduit, light fixture hangars and a number of other things in place.
Those types of tools are still used in the construction industry, and the cartridges for them come in several calibers and power ratings. They have the appearance of blanks, which, technically speaking, I suppose they are since the projectile is separate and loaded in the gun first.
To understand what my years in another life have to do with the new .17 Win. Super Mag. we must first take a look at the smaller .17 HMR. Basically the .22 WMR case necked down, the .17 HMR pushes a 17-grain bullet at 2,530 fps at a maximum chamber pressure of 26,000 psi. The guys at Winchester wanted more speed, and in order to safely reach 3,000 fps with a 20-grain bullet, they needed a case strong enough to contain 33,000 psi — or about 25 percent more pressure than generated by the .17 HMR.
The high pressures to which cartridges used in powder-actuated tools are loaded require cases made stronger by considerably greater thickness in the wall of the rim than in the .17 HMR and .22 WMR cases. Since Winchester has for many years supplied that type of ammunition to the construction industry, necking down one of those cases to .17 caliber and loading it with enough slow-burning powder to reach the desired velocities produced the new .17 Win. Super Mag.
In comparing gross water capacities of the two cases, I came up with 9.1 grains for the .17 HMR and 12.7 grains for the .17 WSM. That's about a 40 percent increase in capacity for the bigger case. The .17 WSM case is noticeably bigger, but what gets your attention is a rim diameter of .330 inch — which is almost as large as the .17 Hornet.
A number of rifles became available in .17 HMR soon after it was introduced because quite a few were already being built in .22 WMR. The same magazine works with both cartridges, so modifying existing rifles to handle the .17 HMR was no more difficult than installing a barrel bored and chambered for it.
Such was not the case with the .17 WSM. In addition to being too long for magazines designed for cartridges on the .22 WMR case, the larger head diameter combined with the higher pressures to which the cartridge is loaded delivers a higher level of back-thrust to the bolt of a rifle during firing. That requires breech-locking strength much like a centerfire rifle. The larger rim diameter also requires more offset of the firing pin from the center of the bolt than for the .17 WMR and .22 WMR.
Savage is prone to offer rifles chambered for cartridges other manufacturers ignore — the 6.5-284 Norma and 6mm Norma BR are just two examples — so it comes as no surprise to see Winchester turn to that company for a new rifle capable of housing the .17 WSM.
Called the B.MAG (short for Bolt.Magnum), it comes with a number of interesting features. For one, the firing pin is cocked when the bolt is closed rather than when it's opened. The thicker rim wall of the .17 WSM case requires a harder firing pin strike for ignition than is required for other rimfire cartridges. Using a heavier firing pin spring took care of that, but as the B.MAG action was being developed, it was found that cock-on-closing took less effort on the part of the shooter than cock-on-opening.
It does differ from past cock-on-closing designs. The firing pin in the old military actions, for instance, is usually cocked as the bolt is pushed forward, but the bolt of the Savage travels all the way forward freely, and the firing pin is cammed into its cocked position as the bolt is rotated to lockup. Even then, cocking takes a bit of effort. I found the bolt easiest to close with my thumb, wrapping my fingers around the bottom of the trigger guard for additional leverage. Keeping the surface of the cocking cam ramp of the bolt lightly coated with grease also helps.
The .17 WSM case is so darned sturdy I would be quite surprised to experience a blown rim, but in the event that the unlikely should happen, Savage engineers seem to have done an excellent job of handling escaping propellant gas. Blocking off the rear of the receiver with both the root of the bolt handle and the bolt shroud should direct any gas traveling back through the bolt and receiver away from the shooter.
A counterbore wall at the top of the bolt face would deflect gas and debris downward into the well of the magazine. Some of the gas would also escape through small vents in both sides of the receiver ring positioned adjacent to the face of the bolt.
The front, non-rotating section of the two-piece bolt has a single extractor as well as a spring-loaded, plunger-style ejector. Dual opposed locking lugs at the rear rotate to engagement with shoulders machined on the inside of the receiver, making for extremely strong lockup.
Operating the bolt does take a bit of getting used to. When pushing it forward, the locking lugs can be rotated behind the receiver bridge rather than inside it as should be done. There is absolutely no danger of the rifle firing, but it could be inconvenient were it to happen just as the determined charge of an enraged groundhog needs to be stopped. After absentmindedly doing it a few times I remembered to push the bolt all the way forward before applying any downward pressure on its handle.
The chrome-moly barrel is buttoned rifled at a 1:9 twist rate. During assembly the barrel is screwed into the receiver until it stops against a minimum-dimension headspace gauge and then fastened in place by a post, the threaded end of which extends up through the bottom of the receiver.
The post also serves as the recoil lug and as an anchor point for the latch of the stock bottom plate. It is hollow and inside-threaded for acceptance of the front action bolt. Never have I seen a single part in any firearm serve as many purposes.
The eight-round detachable magazine is the rotary type with a release tab at the front. When loading the magazine you've got to play rough right from the start. The first five rounds go in fairly easily, but coaxing in the final three requires additional muscle.
The magazine can be disassembled by removing two screws, but Savage doesn't recommend it. Occasionally flushing its innards with an aerosol cleaner/degreaser should keep everything rotating smoothly. Unlike some rimfires, the B.MAG is fairly easy to single-load a cartridge directly into the chamber.
The AccuTrigger had a slight trace of creep with a barely noticeable hesitation just prior to breaking. Five consecutive pulls measured with a Lyman digital scale averaged 43 ounces.
The trigger is adjustable with no tools required. Removing the bottom plate of the stock exposes an adjustment wheel just behind the finger lever. Using thumb and finger to turn it adjusts pull weight within a range of 40 to 72 ounces. Sear engagement remains constant.
The two-position safety slide on the receiver tang blocks the trigger while the sear is blocked by a lever inside the AccuTrigger. Pulling the slide to its Safe position does not prevent the bolt from rotating, which allows a cartridge to be removed from the chamber with the safety engaged.
The stock is of fiberglass-reinforced composite with a thin rubber buttpad. Molded-in roughened areas at grip and fore-end make for a nonslip grip. Posts for quick-detach sling swivels are there with the front one perfectly located for use with a sling or my Harris bipod.
Removal of the barreled action from the stock differs a bit. With the bolt and magazine removed, a small screwdriver is inserted into the magazine well and pressed against the latch of the bottom plate, allowing it to be lifted from the stock. With the front and rear action bolts now in view, a 5/32-inch Allen wrench is used to turn them out, freeing the stock for removal.
Don't make the mistake of expecting a .17 WSM to run as long between bore and chamber cleanings. Rimfire cartridges employ smaller charges of quicker-burning powder, and the powder leaves a lot of residue behind; an excessive buildup in the chamber will cause sticky case extraction.
I did not have enough ammo on hand to determine how often the chamber and bore should be cleaned with solvent, but every 200 to 300 rounds should probably keep the rifle running smoothly.
Polymer-tipped bullets have proven to be ideal for varmint cartridges, so for the .17 WSM Winchester initially chose a couple weighing 20 and 25 grains. Respective muzzle velocity ratings are 3,000 and 2,600 fps. Overall average cartridge length is 1.580 inches. The B.MAG action was designed around the cartridge loaded with those two bullets, and they feed from the magazine like grease on glass.
After the design of the action was finalized, Winchester decided to add a third option loaded with a 20-grain hollowpoint, but it has an overall length of 1.515 inches, and as a result that load simply will not feed. It can be manually loaded into the chamber, but it refuses to feed properly from the magazine. The Browning 1885, which is slated for availability in .17 WSM, has no magazine, so all three loads will work in it.
The B.MAG weighs 4.5 pounds, and it comes with a two-piece, Weaver-style base attached to its receiver. Using Warne rings to attach a 6-20X Weaver Grand Slam scope brought its range-ready heft to six pounds. Clocking respective averages of 3,013 and 2,597 fps, the 20- and 25-grain polymer-tip loads lived up to their velocity billings.
A feathery heft makes the rifle just the ticket for hiking over hill and dale and stalking to within 50 paces of a wise old silvertip groundhog, but an extremely light barrel with a muzzle diameter of only .510 inch makes it less than ideal for high-volume shooting in a prairie dog town. Fifteen rounds warms it up, and at twice that it is almost too hot to touch.
Rapid heat-up most definitely affected accuracy. The B.MAG is obviously intended for varmint shooting, so I ran it hot for five five-shot groups before allowing it to cool down. On several occasions I had three bullets snuggled inside an inch and another squeeze or two on the trigger would more than double group size. The 20-grain polymer-tip load had a slight accuracy edge over the 25-grainer, but there was not a great difference between the two.
There is a good chance we will see considerable line extension of the B.MAG. From my perspective, a heavier barrel option is at the top of the possibilities list.
The shape of the bolt handle is a bit unusual but feels fine when cycling the action. The shroud is slanted for plenty of access to the tang-mounted safety.
During lockup, dual opposed lugs at the rear of the two-piece bolt engage shoulders machined inside the bridge of the receiver. A bit of grease helps things work more smoothly.
The .17 Win. Super Mag. (c.) is faster than the .17 HMR (r.) but slower than .17 Hornet (l.)