You probably think having a perfect crown is important to accuracy. I think so too … although this article will make you wonder.
Background: I was told that one of the major chronograph manufacturers checked the velocities of the rifles of three officers so they could determine their come-ups for various ranges. The rifles were described as being similar, as was the ammo. What was interesting was that the bullet’s Ballistic Coefficient (BC) when fired from Rifle 1 was different than the bullet’s BC from Rifle 2 and from Rifle 3. In fact, each rifle could be distinguished by the BC it produced. (As an aside – Was the rifle with the highest BC also the most accurate?) So the question is - What caused the difference? It wasn’t the ammo, since the same ammo was used in all three rifles, and it wasn’t the shooter. It must have been the rifle, but what part of the rifle? My first thought was that maybe the crowns were a little different, perhaps by cleaning rod wear or a less than perfect crown to begin with. It made sense to me that a perfect crown would allow the bullet to start its flight with very little yaw, resulting in the highest BC possible for that particular bullet. On the other hand, a crown that is off center or has a ding in it would allow gas to exit non-uniformly and thus kick the base of the bullet a little to one side as it left the muzzle, resulting in a wobbling bullet with greater drag and a lower BC.
To see if the crown was the problem we borrowed a retired .308 Winchester Model 70, with a 24" heavy barrel and a muzzle diameter of .78". We figured that if we started with a good crown, recorded the BC and then damaged the crown and recorded a lower BC, that would confirm that at least with one rifle the change in BC could be attributable to crown condition. I looked at the crown under the stereo microscope (10X) and found that the edge was slightly rounded over instead of having a nice crisp edge. That would have to be touched up in order to participate in our experiment. We had John recrown the barrel using the Brownells’ Manson muzzle crown refacing kit. This kit didn’t require taking the barreled action out of the McMillan stock, and the 11° crown that he cut looked great under the stereo microscope. We set up the Oehler M83 system, the Oehler 57 Chronograph Sky Screens (set at 15 feet), the Oehler acoustic target (set at 100 yards) and the Phantom v7.3 high speed video. The acoustic target measures the down range velocity, time of flight, BC and the group size. The rifle had a 3.5-10X Leupold scope. I set the rifle on a Sinclair front rest and a Protector rear bag. The ammo was Federal .308 Gold Medal Match 175 grain Sierra Matchking, GMM308M2, all from the same lot. The ammo was not sorted by weight, concentricity or anything else. It was straight from the box. I started with a five shot group from the ‘perfect’ crown. At this point I’ll throw out an excuse regarding my group sizes. I like shooting orange dots. Unfortunately, the dot I put out was the right size for a 20X scope but too small to quarter with the 10X. So I just lined up the crosshairs until the dot pretty much disappeared under the intersection of the crosshairs and pulled the trigger. The groups might have been a little bit smaller if I hadn’t been too lazy to dig out a bigger dot, but to be honest I expected the accuracy to go to pot after the first group so I wasn’t too concerned with wringing out the last bit of accuracy. I expected .75 MOA accuracy with the good crown and four or five MOA accuracy by the time we were done destroying the crown.
The first five round group, with a perfect crown, measured 1.020". The velocity recorded fifteen feet in front of the muzzle averaged 2620 fps, with a Standard Deviation (SD) of 10 (.4%). The Ballistic Coefficient (BC) was .497, SD of .008 (1.6%). Just a comment on the BC – the velocity varies from shot to shot and the BC varies even more! For these five shots the BC varied from .486 to .509. It makes you wonder why BCs are listed to the thousandths place when they vary at the hundredths place. I used to be under the impression that the BC was a set number and only the muzzle velocity varied shot to shot, but as you can see, the BC varied more as a percentage than did the muzzle velocity. Further, I didn’t always believe the published BC for some bullets (thinking that the bullet maker might be guilty of a wee bit of BC exaggeration), but I believed that whatever the BC was, it would be the same for each shot unless there was obvious damage to the bullet nose. I’d be interested to know, and will try this when I have a chance, if polygon barrels will yield higher BCs than conventionally rifled barrels since the bullet’s bearing surface should be smoother than it would have been if cut into by the lands of a conventionally rifled barrel. Regarding group sizes listed here, the Oehler Acoustic Target measures groups by using 4 microphones set in a 24" square. As the bullet passes through the interior of the square, each microphone hears the crack of the bullet’s mach cone at slightly different times. The machine uses this data to determine the bullet’s horizontal and vertical position and plots it on a graph on the computer screen. I didn’t trust it at first, and always compared the computer target to the paper target, but eventually the computer won me over. The system works and it works quite well.
As another example of BC variability I compared a Berger VLD to a Hornady A-Max on the same day, using the same rifle and testing equipment, and at the same altitude, temperature, pressure and humidity. Any set-up errors were the same for both sets of bullets. The Berger’s BCs for three shots were .523, .526 and .540. The Hornady’s BCs were .552, .545 and .541. You might conclude that the plastic tipped bullets are more consistent than those without but I’ve seen some very consistent TSX bullets, for example. Three rounds of the .308 110 grain Barnes TSX had BCs of .268, .269 and .267, but the group was 1.29". On the other hand the .308 168 grain Matchking had BCs of .450, .459 and .469 but had a group of .41". The most widely varying BCs I have observed were with some jacketed soft point .22 caliber centerfire bullets. In one ten shot string, the BCs varied from .141 to .200. Apparently some tips were flattened more than others during manufacturing or loading and this change in meplat diameter (measuring .097" to .114" ̎) affected the BC. This meplat diameter variability causes the BC to vary more with small calibers than with large(r) calibers. Having said all that, a uniform meplat doesn’t guarantee a uniform BC.
Following group one I used a MotoTool cut-off wheel to cut a small notch in a groove in the crown at twelve o’clock. (See Photo 1) We expected to see some gases venting up and perhaps the bullet nose tipping down as viewed in the Phantom, but it wasn’t visible.
Photo 1 – small notch located in a groove at 12 o’clock