The Over-Rated Crown
The rifles were described as being similar. Maybe they were all Remington 700s in .308. Maybe there was a 20" LTR with three flutes and a 1:12 twist, a 24" SPS with a 1:10 twist and a 700P with a 26" 1:12 twist heavy barrel. If the LTR had a ‘slow’ barrel that had been shot a lot and the 700P had a new ‘fast’ barrel, the muzzle velocities could be far enough apart to affect the BC. If the ammo utilized was Remington loaded 168 grain Sierra Matchkings at 2680 fps and the BC was determined over 100 yards, the BC for the first half of its flight would be in the upper range (>2600 fps = .462 BC, < 2600 fps = .447 BC) for the long barrel. The LTR velocity would start below 2600 fps and would thus be in the .447 BC range for the entire 100 yards. The barrel length difference alone could account for the BC differences noted between two of these three rifles. But what if all three rifles had the same barrel length?
I attended an NDIA Conference where Jeff Siewert of Arrow Tech Associates (of PRODAS(1) fame) had a booth. He sent me a presentation titled “Factors Affecting Small Caliber Dispersion”(2). He pointed out that “small caliber bullets in high pressure systems operate at stress levels above projectile material yield stress”. So at 60,000 psi that bullet starts acting like putty in the barrel. “Deformed projectile shape may not be symmetric” and “orientation of in-bore angle and CG (center of gravity) offset varies shot-to-shot”. I think there are a couple of implications here. If the throat is oval and/or non-concentric with the bore and/or perfectly cylindrical but oversized, as the bullet turns to putty, it is going to conform to the throat’s diameter and shape right before it gets swaged back down to bore size. During these transitions the bullet is deformed from its original perfect (?) shape and this deformation may not be symmetric. As an example, if the reamer cuts the chamber off center to the bore by .003" and the loaded ammo concentricity varies by .003", then you could have errors that add to or cancel each other out. Also, if the reamer is off center then the lands will be cut further down the barrel on one side compared to the other. (So your jam length would be measured to the first land, not to all the lands.) When that happens, the land closest to the chamber will become the first speed bump the bullet hits, will slow the bullet on that side and try to make it tip. Strong ejector tension can tip the cartridge and its bullet in the chamber (more in a sloppy chamber than a tight chamber). Depending on the orientation of the closest land to the direction of tip, the closest land can increase or decrease the tendency to tip. It’s possible that the shooters that believe in barrel indexing are experimenting, at least partially, with this effect when testing factory barrels.
Don’t expect the bore to correct chamber and throat problems. I have looked at recovered bullets that had land impressions that are longer on one side of the bullet than on the other. That means that the bullet was tipped (the in-bore angle was not zero) as it went down the barrel and that the bullet nose was tracing a helix as it moved down the barrel. The longer the boattail and the longer the nose, the shorter the bearing surface. The shorter the bearing surface, the greater the likelihood that the bullet can tip inside the barrel. (A VLD can tip more in the barrel than a wadcutter bullet). The implication here is that I may have been looking at the wrong end of the barrel to explain the BC change among the three officer’s rifles. Maybe the crowns were all good but the throats or chambers were different. Or maybe the throat differences plus the crown differences combined to make the BC changes so noticeable. If the throat was oversized then the bullet could tip as it left the cartridge case and the in-bore yaw could continue the length of the barrel. Of course if throat condition is the great question and crown condition is the good question, that’s going to be a real pain in the neck since it’s relatively easy to look at the crown but looking at the throat is not as easy.
Jeff discussed some Internal Dispersion Factors that could have had an influence in the three barrel test. Let me say up front that I don’t assume that all the factors that contribute to changes in BC also contribute to inaccuracy, but I think that some of the factors overlap. In any case, they’re interesting to consider. The bullet’s geometry, mass properties and radial stiffness are assumed to be the same since the ammo was the same. Bullet run-out could be a problem, but again it should have been the same across the board. So gun-to-gun differences are prime considerations. These would include, although some don’t apply:
“Barrel flexural properties (bending & hoop stiffness)” - “The gun barrel diameter grows elastically in response to internal pressurization with the OD influencing ID growth”. “The projectile tips in the bore due to the ID growth”. In other words, the max heavy contour will ‘grow’ less, both externally and internally, as the bullet goes down the barrel compared to a sporter contour barrel, and it will also bend less. As the barrel ID expands, the bullet has a greater chance to tip. Add to this that some factory bores have tight and loose spots in them which allow the bullet to tip inside the oversized sections and once the tipping (yaw) has begun it can continue the length of the barrel. “The projectile tilt, CG offset and spin during early in-bore travel drives barrel transverse motion.”(3)
“Bore Straightness” - I can confirm, having shortened a few barrels, that some bores are shaped like a banana. I say that based on the fact that the crown was centered originally but after cutting the barrel shorter the new muzzle crown was no longer centered within the OD of the barrel. I think this would affect the accuracy more than the BC, especially as the barrel heats up.
“Engraving variations” - I know that many shooters have issues with bullet coatings, but Jeff had an interesting Engraving Force Variation graph. Comparing Push Force Standard Deviation in kN versus InBore Travel in mm, the coated Barnes 168 BT showed much less variation than the uncoated Barnes 168 BT.(4)
“Muzzle Blast / Base pressure at Muzzle Exit”(5) - Lower pressure on the base of the bullet is considered a good thing, so longer barrels are better than short barrels in this regard. Bullets continue to accelerate for several inches past the muzzle as the high pressure muzzle blast gas continues to push on the base of the bullet without the friction of the bore to slow it (the bullet) down. High pressure on the base of the bullet, especially if the bullet was canted in the bore and therefore canted at exit, won’t help accuracy and could also increase bullet wobble and decrease its BC. As an aside, I chronographed a suppressed rifle the other day and found that with the suppressor attached, the velocity dropped about 10 fps compared to its velocity without the suppressor. That was unexpected, as suppressors often increase the velocity by 10 or 15 fps. Apparently this last suppressor did a better job of slowing and stripping away the muzzle blast gas, thus robbing the bullet of its normal muzzle blast acceleration.
“In-bore clearance”(6) - The “projectile body is nominal interference fit with lands, but elastic deflection of bore due to internal pressurization allows the projectile to tip in-bore relative to bore centerline.” This effect can be more pronounced on looser barrels. I slugged one of my Remington .308 barrels and the slug measured .3085 ̎across the grooves. Compare that to a Krieger barrel that measure .3065 ̎ across the grooves. The tighter barrel should reduce in-bore clearance. Random orientation of projectile in-bore angle applies loads to the barrel which affect barrel pointing and cross velocity at muzzle exit.”(7)
Solid bullets shoot smallest dispersion with .050"- .080" free run while conventional drawn copper jacket / lead core bullets shoot smallest dispersion with .015"- .030" free run.(8) My take on this is that if you are limited to magazine length ammo but have a factory barrel with a long throat, solid bullets, such as the Barnes TSX, Hornady GMX and Nosler E-tip, could provide better accuracy since they tolerate being further from the lands. Example – if your magazine is 2.85" long and your jam length is 2.92" long, the .070" difference is quite a jump for conventional cup and core bullets but is just right for the all copper / gilding metal bullets.
Getting back to the crown and the three rifle BC variation - I’m afraid I ended up with more questions than answers. If I had to guess, I think that the throat dimensions would be worth checking out if the barrel lengths and contours were all similar.
Conclusion: Having a perfect crown is an absolute requirement for pinpoint accuracy ... unless it isn’t.
(1) PRODAS stands for PROjectile Design and Analysis System. Arrow Tech Associates are located at 1233 Shelburne Road, Suite D-8, S. Burlington, VT 05403
(2) Jeff Siewert, Arrow Tech Associates Inc., May 2010, National Defense Industrial Association Joint Armament Powerpoint presentation, page 3
(3) Ibid, page 18
(4) Ibid, page 17
(5) Ibid, page 4
(6) Ibid, page 6
(7) Ibid, page 7
(8) Ibid, page 23
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