This report is going to start a controversy, because it dares to question the things that are currently held dear among airgunners and firearms shooters, alike. Sorry, but here it goes.
What is a crown?
The crown is the end of the barrel, or the place at the muzzle that has the final influence upon the bullet as it transitions to ballistic flight. One popular belief is that if the crown is not perfectly symmetrical, then one side of the pellet or bullet can exit before the other and allow escaping gas to impart a destabilizing effect on the bullet at the beginning of its path to the target. So, crowns have to be perfect, according to the vast majority of shooters.
The other side
But there have been experiments done that show that escaping gasses have zero effect on a bullet in flight. The most well-documented of these were done by Dr. F.W. Mann, who wrote about them in his book The Bullet’s Flight, From Powder to Target. Dr. Mann did numerous experiments until finally he demonstrated that a plank six inches long placed within 1/16 inch of the muzzle blast has absolutely no effect on the accuracy of a bullet.
You see, in Dr. Mann’s day riflemen believed that the muzzle blast had a deleterious effect on the flight of the bullet, and they warned shooters to keep the muzzle clear of any and all obstructions.
But is what Dr. Mann tested the same as an inaccurate crown? Maybe not. The question seems to be what, exactly, does the crown do?
The end of the rifling and the face of the muzzle bore must be as square as possible to the bore for the crown to be perfect. The reason for this is as I stated earlier — that the base of the bullet/pellet leaves the muzzle at exactly the same point around its circumference, rather than one part coming out before the rest. But there are all kinds of crowns, including some that don’t look like a crown at all.
Let’s look at some crowns now.
The crown of this Ballard target rifle is flat and polished like a mirror. The old-time shooters felt it was easier to see the distribution of the bullet lube — as it made a pattern on the face of the muzzle. There’s almost no break between the bore and the muzzle on this rifle — which is one of the more accurate ones I own. In the 135 years since this rifle was made, there has been no damage to this crown.
This Butler Creek bull barrel for a Ruger 10/22 has a recessed crown that’s similar to the Ballard crown except for the recess. However, on this one, it’s possible to see a tiny break (chamfer) at the muzzle. With the right ammunition, this rifle can hold 10 shots close to one-half inch at 50 yards. The recess supposedly protects the actual crown from inadvertent damage.
No doubt that this crown on an FWB 300 target rifle will look more familiar to most shooters. It’s the traditional rounded or radiused crown with a protected chamfer at the true muzzle. It’s on my most accurate ten-meter target rifle. Doesn’t look so pretty up close, does it?
The crown on this HW55 SF air rifle is similar to the one on the FWB 300, but up close it looks pretty disgusting. The rifle is one of the more accurate 10-meter target rifles I own. So, looks can be deceiving, and a “perfect” crown may not be all that it’s cracked up to be.
Not looking like your typical crown, this Swedish Mauser M1938 crown is a lot like the “redneck” crown job that hobbyists do on their guns. This is on a very accurate rifle. The lighting makes the bore seem to have a shoulder around the inside of the muzzle, but it doesn’t.
The redneck crown
Since the 1960s, there has been a hobbyist approach to crowning a barrel. It consists of a round-headed brass screw and a grinding compound — like automotive valve grinding compound. Chuck the screw in a hand drill and coat the domed screw head with grinding compound. Then, run the drill motor slowly while allowing the axis of the drill to oscillate to avoid making an oval cut. The result will look something like the crown on the Swedish Mauser M1938 shown above.
The crown on a custom barrel for a .17 HM2 rifle. Though brand-new and not even broken in yet, this rifle has already shot a five-shot 50-yard group that measured 3/8 inches across the centers of the widest shots. Note the powder burn pattern around the muzzle. This is the same thing that old-timers analyzed on the mirror surface of the Ballard muzzle when it was bullet lubricant that spread out instead of carbon fouling. This is another deadly accurate rifle that has no noticeable “crown” to the muzzle. The transition is very close to 90 degrees.
The crown on an AirForce Condor is very similar to the recessed target crowns shown before, except that this one has a definite chamfer or break at the muzzle. This rifle shoots half-inch five-shot groups and three-quarter inch 10-shot groups at 50 yards. And, yes, I did notice that it is time to clean this barrel!
So, what’s the verdict?
I’m not sure. That’s where I am on the whole crown issue. The reasoning makes some sense, and I can see why a PCP or a CO2 gun would then need a good crown, but a springer barely has any compressed air exiting the muzzle, so where’s the advantage there?
Don’t say anything about crowns removing burrs at the muzzle, because Dr. Mann did an extensive test in which he screwed blunt-tipped screws into the side of his Pope barrel at the muzzle to see if burrs at the muzzle that deformed bullets affected accuracy. They did not. He set his blunt-tipped screws to plough to the bottom of the grease groove of the exiting bullet, and no change was noticed in its accuracy at 100 yards.
Are crowns placebos?
I’m still undecided on the importance of crowning a barrel. I’ve read what everyone says, which is that the crown is of paramount importance to the accuracy of the barrel, yet I’m not convinced that it is. I’m also not convinced that it isn’t. I just don’t know.
I think there’s something more that has not yet been discussed about crowns and their importance to accuracy, but I’ll be darned if I know what it is. Do shooters shoot better after receiving (or doing) a crown job on a particular barrel? If you read what they write, they seem to. And most shooters believe that the barrel’s crown is of great importance to the performance of the barrel.
I wish I knew for sure, but I don’t.