Accurizing a Precision Rifle on a Budget, by Mike L.

Tuesday, Jan 8, 2013

"Only accurate rifles are interesting." - Col. Townsend Whelen

We all know when we are shooting a super accurate rifle; when everything just "clicks" and the shooter, cartridge, and rifle come together to make great groups. But what are the variables involved in making a rifle accurate? And more importantly, how can we control some of those variables ourselves?

One of the keys to accuracy, perhaps the most important one, is consistency. We know that, as shooters, we need to be very consistent to become good marksmen. We mount the gun the same way every time, control our breathing and even our pulse to release the shot at the same interval within our "wobble area." We press the trigger the same way, and use the same place on our trigger finger to release it. We adjust the parallax out of our scope and adjust optics for best focus of reticle and target. All these things help make our shots more consistent.

Well, the combination of rifle and the cartridge it fires are very dependent on consistency as well. Let's consider just a few of the important items.

Stock bedding
If your rifle action moves around in the stock, it will never shoot consistently. Each time you shoot, the rifle will recoil, and potentially land in a different place in the stock. The most accurate rifles typically are glass bedded within a CNC-machined aluminum bedding block, in a synthetic stock. Synthetics are great, because they aren't affected by temperature or humidity. Classic wood stocks are beautiful, but they can swell or warp. Laminates work well, because they are relatively immune to environmental factors.  There are synthetic or laminated stocks now available for nearly any semi-automatic or bolt action rifle suitable for survival situations. Many of these are available in semi-finished form at a very reasonable price; these require mostly work with a few simple hand tools and the application of a durable finish to make an excellent finished product. Instructions and supplies for glass bedding these stocks are available from suppliers like Midway and Brownell’s.

You might have heard of "pillar bedding." Basically, this involves embedding a couple of aluminum or steel spacers in the stock, through which your action bolts run to bolt the action to the floorplate (aka "bottom metal"). In the olden days, people would cinch down these action bolts tightly enough to start crushing the wood of the stock. Do this long enough and often enough, and the stock starts getting loose on the gun and accuracy suffers. Pillars prevent this crushing. For best accuracy, you can even use a torque wrench to always tighten your action bolts to exactly the same torque (about 65 inch-lbs is often used). Pillar bedding is also a project that is well within the capabilities of a moderately skilled woodworker. You can buy the aluminum pillars pre-made, or if you’re handy with a lathe or drill press, can easily make your own from round aluminum bar stock. Again, instruction is available through gunsmithing suppliers, or you can find detailed instructions by a simple internet search.

Free Floating the Barrel
"Free floating" the barrel is also an accuracy enhancer for most rifles. The action is bedded behind the locking lug to provide a full-contact fit between the action and stock. There is also a small area of the action and barrel glassed in just ahead of the receiver, but most of the barrel is not in contact with the stock. With a properly-floated barrel you can slide a couple sheets of notebook paper between the barrel and stock almost all the way to the receiver.

Now why do this? When you fire a shot, the barrel basically rings like a bell, doing a complex set of oscillations before, during, and after the bullet's departure. If you free float the barrel, nothing will interfere with these oscillations, and they will occur consistently. If a portion of the stock touches, the harmonics may or may not occur consistently. And remember, we're striving for consistency here. Some barrels actually do a better job with a carefully engineered bearing surface near the muzzle, but for most barrels, free float is where it's at.

Barrel Attachment
Most barrels screw into the receiver on bolt action rifles and many centerfire semi-autos. There are some exceptions - AKs (not legendary in the accuracy department), HKs, and a few other battle rifles have pinned barrels. But for the average guy wanting the most accurate rifle at a reasonable price, a bolt action with screwed in barrel is what you'll end up acquiring. It stands to reason that you'd want all the surfaces of the barrel and action to mesh up perfectly when you screw them together at the proper torque. But in fact, that doesn't always happen. If the barrel and action are not in perfect alignment, the barrel might be slightly cocked in the action, and the bore axis won't align with the action. Or, even if they are aligned, if the bearing surfaces don't mate exactly, when a shot is fired (remember that "ringing like a bell"?) the barrel might move minutely with respect to the action.

How do you fix this? By truing all the mating surfaces, much like "blueprinting" a big block Chevy engine. This can be done on a lathe, and/or by using specially made lapping tools to make sure all the surfaces line up, that the threads are true, and that the axes of action and barrel are properly aligned. This work requires a bit more expertise in machining – if you use a lathe.

However, if you use truing tools available for use by hand, the tools basically self-align with the part being worked on and the surfaces are almost guaranteed to be true. These tools are available through outfits like Midway, and though relatively expensive, can be purchased by a group and used to accurize many rifles of the same action type.

Bolt to action fit
Now we have a barrel that fits precisely to the action and the barreled action is securely bedded into a stable stock. The action screws are tightened snuggly and consistently, and the barrel is free-floated. What else can we do?

The bolt is the next thing to consider. If the bolt face isn't aligned to the bore properly, it will hold the cartridge at a slight angle to the bore when the shot breaks. The bullet will actually leave the case at a slight angle to the bore axis, it will engrave the rifling unevenly into its jacket, and it'll never really recover from this indignity. The result will be yet another inconsistency and poor groups. The answer to this problem is to lap the bolt face so that it is exactly perpendicular to the bore axis, and each cartridge will be held precisely in the same place. You can buy a tool for this operation as well, using your power drill and lapping compound (the barrel has to be removed from the action to do it, though). With a lathe, it's a pretty straightforward task to build your own bolt face lapping tool.

Now, the bolt also won't stay properly aligned if it doesn't lock up consistently. This is the result of the engagement of the bolt locking lugs with the matching recesses in the receiver. Take a look at the rear of the lugs on the bolt in your favorite rifle. If all of the lugs show the bluing is evenly worn off, and each lug shows about 80% engagement, you're golden. More likely, none of the lugs show this much engagement, and in a worst case, one lug is taking all the load of firing, with the other lug just hanging free in space. As you might guess, this will allow the bolt to cock with respect to the bore, your poor bullet gets abused again, and inconsistency is the result.

This condition can also be rectified by lapping. You don't even have to have a tool to do it, but a spring loaded tool that presses the lugs against the bolt does make the job easier. Smear some lapping compound on the rear of the lugs (strip the bolt first), insert the bolt, pull back on it to maintain good contact between the bearing surfaces, and just work the bolt multiple times until the surfaces are well matched up.
                 
Where are we now? Let's see - bolt is square to the receiver, holding the cartridge in perfect alignment with a bore that is also aligned to the receiver. The barrel is seated against a trued action; it isn't going to move upon firing. The action is securely held in a stable stock. The barrel is free to vibrate at its harmonic frequency.

Barrel Crown

The very last influence your rifle will have on that speeding bullet is as it exits the muzzle. That's why the muzzle crown is of primary importance. If the crown is dinged or uneven, as the bullet exits, expanding gas behind the bullet will leave the bore unevenly. This can move the bullet out of  alignment, imparting a lopsided spiral motion to it. You want the bullet to leave the bore in perfect symmetry. Why are there so many crown shapes? Mostly to protect that crown by recessing it away from potential dings. A perfectly straight crown, perpendicular to the bore axis, will do just fine, and can be accomplished with a high quality square and a file. But it's easier to do it with lapping tools or a lathe. A freshly cut crown will often do astounding things to improve the accuracy of an old rifle.

Lock time
Military rifles, like the Mausers that are well-suited for accurizing, were designed for reliability under battle conditions. They have a striker that hits the primer with ferocious intensity, driven by a heavy duty spring. Unfortunately, that mechanism is really heavy. Weight equates to inertia - when you press the trigger, it take a while for all that mass to get up to speed. This is called lock time. Ideally, you'd press the trigger and the bullet would exit the bore immediately, with no lock time at all. A long lock time (like in a Mauser action) gives you more time to wiggle around between the time that you press the trigger and the primer ignites the powder. More contemporary commercial actions (Remington, Sako, etc.) have greatly reduced lock time. Remington even invented an electronic trigger and electrically fired primer (Etronix) to virtually eliminate lock time, but it never really caught on. For the Mauser, you can buy a "speedlock" inner bolt assembly that's made of aluminum and titanium to significantly reduce lock time. They are available for other rifles as well, and you can even buy a titanium firing pin for your AR-15 that will cut down its lock time as well. There's a balancing act though - if the firing pin is too light, it might not reliably detonate your primers, so beware.

Optics mounts
This is an easy one. If the optics (scope or iron sights) are loose, they will bounce around from shot to shot. This is more common than you might think. It's pretty common to see a hunter at "sight in days" shooting up an entire box of ammo trying to zero his rifle. He'll be fine for elevation, but a shot will hit to the left. He dials in some right. The next shot is far to the right. He dials in left to correct. Now the shot is far to the left. He scratches his head, shoots again. Next shot is to the right! What? What is probably happening is that he has a loose scope mount, that's just bouncing to the limits of its travel with each shot. Or it could be a scope with loose internal parts. Or it might be that the hunter should hit the range more frequently than once a year. You can't blame the equipment for everything!

The solution here is easy. Buy good quality bases and mounts, install them correctly to the proper torque, and check them periodically!

What else?

That about covers the rifle components and interfaces that contribute to accuracy. Of course, an expensive Shilen or Lilja match barrel will be more accurate than a shot out WWII barrel. The most accurate benchrest rifles have special actions that are super stiff, to remove any hint of flex that might cause inconsistent performance. But, you might be surprised at how a bit of tuning can up the performance of even a modest barrel.

The benefit of this basic tuning is that it improves the accuracy of the rifle without harming the reliability for situations where the rifle simply must function properly, all the time. A survival rifle is no place to try out fancy gimmicks that may fail when the chips are down – simple, reliable, and tested techniques like the ones described here will often turn a reliable clunker into a tack driver. Just refreshing the barrel crown might take a rifle that can't shoot less than a 6 inch group at 100 yards and reduce that group to 2 inches.


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