In terms of accuracy, the critical kiss-off is that last part of your barrel that touches the bullet.
By Craig Boddington Posted: 10-26-07Categories: Gun Notes
With a rounded sporting crown, the easiest way to examine the blast marks for concentricity is to paint the crown with typewriter white-out, then fire about three rounds. What you want to see is concentric, consistent blast marks at the rifling grooves where the hot gases escape as the bullet exits.
Once in a while you'll hear knowledgeable shooters claim that their rifles shot comparatively better groups at 200 and 300 yards than at 100 yards. This clearly doesn't wash because we understand "minute of angle," don't we? The natural progression is that a one-inch group at 100 yards will disperse to a two-inch group at 300 yards and so forth. Ignoring wind and aiming errors as distance increases, this is the best-case scenario, right?
Surprisingly, the phenomenon of better groups at 200 yards than can be obtained at 100 yards does occasionally occur. It would be really rare for the groups to be smaller, but if you shoot a one-inch group at 100 yards and a 1 1/2-inch group at 200 yards, you have beaten the MOA and your 200-yard group is comparatively smaller. This does happen, and as unlikely as it seems, it isn't that rare. Especially when you consider that relatively few of us shoot groups at longer distances.
How can this be? The most likely answer is a muzzle crown that is slightly irregular. As the bullet exits the muzzle, the burning gas exits considerably faster. Most nitrocellulose propellants expand at about 5,200 fps--faster than any speeding bullet. These expanding gases have no ballistic coefficient, so they slow down quickly, but as the bullet exits the muzzle the faster blast squirts past it. The muzzle crown is the last thing the bullet touches. It is essential that the crown be uniform and exactly perpendicular to the bullet's flight or these escaping gases will exert uneven pressure on the bullet's base, causing it to yaw.
Air resistance plus the stabilizing spin from the rifling will overcome this. Serious riflemen call this "going to sleep." Exactly at what distance this occurs depends on many factors, but an irregular muzzle crown is one of the primary culprits in yawing. Absent wind, aiming errors and all the rest, a stable projectile in flight will follow a geometric dispersion pattern. If you're getting one-inch groups at 100 yards, the best you can hope for--without a bit of luck--is a two-inch group at 200 yards. If your dispersion is exponential, as in a four-inch group, look to external factors (like your benchrest technique). If you are actually getting comparatively smaller groups at 200 yards than at 100 yards, the most likely culprit is your muzzle crown. This is because yawing affects accuracy until the bullet goes to sleep.
So how can you tell? Well, I learned a trick at the range with my buddy Geoff Miller, accuracy freak and proprietor of the John Rigby Company. I was shooting a new Remington 770 in .30-06 . It showed promise, but I was getting irregular flyers. It wasn't the bedding. It could have been the loads, but it might have been an off-center crown. Geoff whipped out a bottle of typewriter white-out fluid and painted the crown
A recessed or target crown will clearly show the blast marks from the escaping gases. So, most of the time, will an unblued barrel. A rounded sporting crown on a blued barrel will disguise the evidence. What you hope to see are concentric blast marks where the gas exits at the rifling grooves, and the white-out will tell the tale. I fired three shots, and the white-out clearly showed the blast marks--and showed that they were irregular, obviously kicking the base of the bullets to one side.
No problem. We broke out Brownells' crowning kit, which resembles a handloader's case-chamfering tool except with a spud that fits the muzzle and guides the tool. You have to take your time, and as you turn the handscrew, make sure you stop in a different place with each revolution so as to avoid metal buildup.
My last group prior to recutting the crown was, honestly, worse than the average to that point, but it had no called flyers. It was a bit over 1 1/2 inches, with the average--to that point--maybe a quarter-inch less. At this stage the point of impact was a bit high and right, so I came left eight clicks, more to keep the groups separate than for concern about zero. After crowning, I fired the next group into the same target using the exact same load. The improvement was dramatic. The first three shots after recrowning were almost touching, a very tidy group of less than a half-inch. This meant that the simple process of tidying up the crown reduced group size by more than half.
I wasn't shooting at 200 yards, so I don't honestly know if this rifle would have produced smaller groups at that distance than at the 100-yard distance I was shooting. I suspect it would have. An irregular crown had clearly been impeding the accuracy, and it's likely that the bullets would have eventually gone to sleep in flight and delivered the accuracy the barrel was trying to produce. That's speculation. It is not speculation that recrowning the barrel cut the average group size of this rifle from an acceptable 1 1/4 inches to a spectacular half-inch.
Out-of-the-box muzzle crowns are often imperfect, and it is amazingly easy to damage a crown. You can do it by placing your rifle muzzle-down in a vehicle and grinding the crown on dirt and debris. You can also do it through careless cleaning. Checking the crown, and fixing it when required, is just another tool in the little bag of tricks that accuracy-conscious shooters carry around.