Home Forums Hunting & Shooting Competitive Shooting Slow motion why crowns & bullet types matter

Yep

The 11 degree target crown was developed in an attempt to match the crown with the boat tail angle of the original Sierra 168 Match King.

Flat based bullets offer a lot of advantages. It's been proven that an indicated crown with a 60 degree cut back from the grooves to the top of the lands will give considerably better life....despite the type of crown shape used.

Good shootin' -Al

Great post...thanks. We knew that...but we didn't know why. It also provokes some thoughts, in my "mind" at least, maybe we should experiment with some convex crowns instead of concave with a little flat annular ring or boss to avoid sharp edge gas erosion. Intriguing.

Post a picture of the one you do.

Nothin' to it but to do it. -Al

I was pretty excited there for a few minutes, til I checked into the cost of the camera that would shoot 25,000 frames per second...150 grand. If I had your money though...I'd throw mine away, and do it. Wonder if we could get Big Stick to finance this, I'm guessing he has the money.
But seriously, think about it...if you were to shape a muzzle to keep that violent cloud of turbulence behind the bullet, regardless of the bullets posterior shape, maybe you could improve accuracy/stability at all ranges. Has this been done already in the form of muzzle porting? Has the effect of lowered exit pressure been measured?

If you reduce the metal on a button rifled barrel at the exit of the bore it will bell out some, but a cut rifling barrel would be fine.

I have the time to make the cuts on a barrel crown, but not to test the thing by doing all the shooting with it afterwords.
I’m putting tuners on all my riffles and building 2 bench / F class rifles this year so I will have WAY too much time testing those loads.


I need a couple of you competition guys closer to me for this kind of baloney.

Reverse crowns have been tested in 100-200-300 Benchrest applications. -Al

Is the old rule of thumb that 300 and in target shooters use flat base bullets and over 300 the boat tail bullets take over? I have not kept up with current times.

Pretty much.

Apparently they didn't improve anything? I would think to gauge results you'd need something very controlled like the Houston Warehouse tests.

Actually, no. Real world testing with flags show a wider range of trends than tunnels. Tunnels...even the best ones like Gene Beggs'...present their own set of issues.

Also, the Houston Warehouse testing is controversial, to say the least. The guns that shot so well there never performed in real world outdoor conditions, even with multiple high level shooters driving them.

I believe a lot of this is in the interpretation. When I watch this video, I don't see burning gases leaving before the bullet. What I see is the air in the barrel under immense pressure being shoved out the muzzle by the bullet behind it. Every bullet in any barrel does this, as the video clearly shows this happening with both bullets. As long as the crown is square to the bore, and there is no damage at the end of the bore, I don't think the cut has anything to do with stability of the bullet.

It's smoke.

All cartridges/barrels leak gas as the brass expands to release the bullet, but before the bullet seals the bore. With oversized bores/undersized bullets the leaking may not ever stop. Tighter chambers can lessen thisphenomena.

Back in the days of iron sights & service rifles, there were times when I could actually see the gas come out the muzzle first and the bullet pierce the cloud, creating a smoke ring. I guess those are relatively loose chambers, because the gas volume was much greater than in those videos.

LOL Where did you come up with that little gem of wisdom. The 11 degree crown was a result of an attempt to mimic the angle that escaping gas takes when exiting the barrel.

this is a first for me, Red Cornelison came up with the 11 degree crown, it was picked as that was
supposed to be the angle that gas exited the bore when the bullet cleared the bore

Without a doubt, Red Cornelison was one of Benchrest's true innovators and experimenters. And likely the first to bring that style of crown to 100-300 Benchrest. He was a true pioneer in the world of extreme accuracy rifles.

According to Kevin Thomas, chief ballistician at Sierra from 1987 to 2007, AMU armorers began using an 11 degree crown to match the angle of the original version of the .30 cal. 168 Matchking.

There's more than the angle involved. Google won't help there.

The boat tail angle on a bullet is off the X axis. The 11 degree crown is off the Y axis. If you want to compare apples to apples the crown angle would be 79 degrees. It's nothing similar to any boat tail bullet that Sierra ever made. No Google needed.

If we started referring to those crowns as 79 degree...which is correct...people would really get confused. The 'reverse crown' I worked with was a true 11 degree. It's on a f-form barrel now....should dig it out for a few pics.

Good shootin' -Al

P.S. FWIW, the Sierra .30 cal 155 Palma Match King bullets I have here are a 9 degree boat tail.

Al, I was fortunate to be able to clean out Red's big shop and his walnut blanks from the loft of his barn. You cannot imagine all the brass, bullets, powder, primers, and loaded rounds that Red had. He must have had a relationship with Federal. I had 5,000 rounds of RF primed brass that he chambered for 17 cal many years before the 17HMR appeared. I have a few of his loaded rounds as well as 250 Savage brass without flash holes, BR brass without flash holes, 30-30 and others. I still have a few of his Herters 30 cal. wasp waisted bullets, and long neck 223 Fed brass. Early on Red shot what he called a 22 Red Russian. It is approx .125 short BR case, rebated the rims to 222 size to fit his bolt, and a step machined in the neck for the bullet to sit on. I don't think Red ever shot a factory cartridge and all that brass that I got, maybe 85% was already in wildcat form.
Red owned a large engine rebuild shop in Seminole, Ok. His specialty was oil field stationary motors, but he had a small 1 cyl inboard boat motor that he gave me. I could write pages on his shop and its contents. He was a quiet and a very kind man. He was the first in the first class of shooters in the BR Hall of Fame.

Butch, I know you and Red were good pals and you helped his wife tremendously after his passing.

Thanks for being one of the good ones. -Al

PS: I have a bunch of paperwork on the 11 degree 168 MK/crown thing, including articles from early P.S. Magazine as well as a written work order from. Sierra for 3 barrels where the 11 degree crown was specified "....as our standard to match the 168 MK boat tail angle."

Yep--and Kevin Thomas got to experiment a LOT with various stuff, because it was his job, not a hobby. Learned many things from him, involving all sorts of stuff from the once-popular moly-coated bullets to other stuff.

Kevin is a smart, smart guy with a huge dose of common sense. If he said something, there was no marketing involved....just the facts. The receivers he sent from Sierra's .30 cal. accuracy testing rigs for rebarreling were a story unto themselves.

FWIW, I've found that any crown shape that's been done in an indicated fashion (and finished with the same final step) will give the same on-target performance. Some crowns have a very short life span, however. But it has nothing to do with the angle. I do prefer an 11 degree crown on my accuracy rigs. For a hunting rig, a recessed or floor board style crown gets the nod.

On a factory gun, Remington's rounded crowns were pretty darn good.

Hope all is well with you, John.

Al has been around the turnip patch several times and hasn't fallen off the tractor.

No falls off a tractor, Butch....but I did have a couple of unintentional dismounts off a couple of my HD's in my early days. One of them, a real deal XR750 factory HD flat track bike tried to kill me a couple times. I modified a small front disc brake from a Bultaco to fit, added small lights and a wider rear tire. Even though the bikes were sold without a serial number and weren't intended to be titled, the engines had a job/serial number on them when they left the HD Race Shop. Using the engine number, I was able to get a title and plate. They were right at 300 lbs. dry weight...my mods added about 15 lbs. The engines made right at 100 hp with the slide valve Mikuni carbs.

Questionably street legal, it was like no other 'street' HD around at that time.

I'll bet that it was fun. I had a Pursang in the olden days.

I always wanted to do that with a XR. Nowadays they are getting expensive!

Brownells sells a 79* chamfer tool. It's commonly referred to as an 11* tool but they market it as a 79*.
kwg

79 degree chamfer tool

I cut a few 11 degree crowns, at the request of customers. but never bought into the idea that it did anything. I used the same recessed flat crown, with a 60 degree chamfer, from the day I started to shoot BR to the present.
Regarding the performance of flat base vs BT bullets for short range, it is difficult for me to give any definite opinion. I can only say that my best BR rifles shot better with boat tails. I have to admit, my aggregates, back in the day, were not nearly as small as are being shot today. Might be that BT bullets could not compete today. GD