[Shootingstarr]

The hollow point boat tails in most rifle bullets will be more accurate at long range. The reason for this has nothing to do with the tip of the bullet being more aerodynamic or being more slipery through the air. HPBT's such as the sierra match king has the same tip diameter as the FMJ's. As long as the tip of the bullet is around 1/15 of the total diameter of the round, the aerodynamics of the bullet will not make a difference in flight. Understanding exactly why HPBT's are more accurate at long range is usually hard to understand if you don't know much about the physics of bullet flight. A bullet will be more accurate at long range if the center of gravity is less then the center of pressure. A simple way to put it is that you want a bullet that is tail heavy. A perfectly manufactured HPBT will be more reliable in long range applications, which is why bullets like the sierra match king and the burger bullets are designed with a hollow tip. Balistic tip bullets, also being tail heavy, are very accurate at long range as well. On another note the Boat Tail part of the bullet only brings more accuracy at long range if there is a cross wind. No cross wind, a flat tail hollow point has proven to be more accurate.

[SightNSqueeze]

I know this sounds somewhat elementary, but wouldn't the perceived negatives concerning hollow points be negated by the supersonic speed of a high powered rifle bullet in flight. In other words, wouldn't a hollow point have to have a pretty big cavity to be effected by atmospheric flight, weight, diameter, and center of gravity being the same?

I remember during the Clinton/Reno ban on FMJ 7.62x39mm ammunition back in the 90’s, that same ammo was marketed with an HP feature that still had FMJ performance.

[Bart B.]

Stumbled across this thread from another one. Anyway, some info on hollow point match bullets and their origin.

Back in the 1950's a bullet making company whose products were being used by competitors to win matches and set records with their full metal jacket boat tailed match bullets learned something very interesting. Their head ballistics person checking quality control data noticed the difference in base (junction at boat tail and body) runout between some lots of their 30 caliber 180 and 200 grain spitzer boat tail soft point hunting bullets compared to the same place on their full metal jacket match bullets of the same shape and weight. The hunting bullets were more uniform by only a couple ten-thousandths of an inch. Top marksmen who shot those hunting bullets reported better accuracy than the match bullets.

So, this bullet making company did some experimenting and eventually introduce a new match bullet that shot the smallest test groups they had ever seen. 'Twas a 168-grain hollow point boat tail that had the most uniform base runout numbers they ever produced. It was first used in the late 1950's for some intenational free rifle matches at 300 meters. Scores produced were the best ever.

That company quickly changed their match bullets to hollow points from full metal jackets and the rest is history. Many other bullet making folks made the same move with similar results.

The big difference between full metal jacket and hollow point bullets for competition is that base diameter uniformity. It's the last part of the bullet to touch the barrel when it leaves. If gas doesn't escape all around that point very uniformily, the uneven gas escape will push one side of the bullet off the bore axis upsetting that bullet and degrading its accuracy. It won't fly the same path to the target as those leaving with even gas squirting by all around it.

Thanks to Sierra Bullets for figuring this out.

[Bart B.]

That scenerio on how hollow point match bullets came about reminded me of what full metal jacket match bullets can do....if they're good enough.

Back in 1971, a friend of mine in company with a tool maker at Sierra Bullets did some experiments. A collet was made to hold a bullet and fit in a Dremel Moto Tool. The tool that milled the collet to perfectly match the bullet's ogive was a bit hard design. Checking a particular lot of Lapua 185-gr. full metal jacket rebated base boat tail 30 caliber match bullets showed something of interest on a 30x optical comparitor. Four different ogive curves were observed; proof that four different pointing dies were used to make the bullets. Each pointing die had a slightly different curve to the ogive. This may have explained why 2 or 3 from each box of 100 didn't shoot to the same point of impact.

Sorting out one batch of bullets with equal ogive shapes, they were put in the collet then spun at 30,000 rpm. An amp meter in line with the tool's power cord stayed steady when perfectly balanced bullets were spun. Those with even a slight unbalance caused the amp meter's needle to wiggle a little. When it wiggled a lot, evidence the spinning bullet was quite out of balance, sometimes that bullet would fly out of the collet an bounce off the ceiling, walls and sometimes the operator. After getting a hundred of the perfectly balanced ones in a box, they loaded them in .308 Win. cases then took them out to a range for accuracy tests.

The rifle's pre-'64 Win. M70 action had a Hart barrel in it and was epoxy bedded in a solid wood stock. With the rifle clamped in a machine rest, a few rounds were fired to center on the target. Then several 10-shot groups with those well-balanced bullets were fired. Largest one was about 1.5 inches; smallest around .75 inch, average about an inch. Did I mention the target was at 600 yards?

One of those smaller groups appeared in a Lapua Bullets ad, late 1971 in the American Rifleman.

[Bart B.]

Quote:       Originally Posted by Shootingstarr

On another note the Boat Tail part of the bullet only brings more accuracy at long range if there is a cross wind. No cross wind, a flat tail hollow point has proven to be more accurate.

Please explain the physics behind this. I wouldn't think accuracy would be any different if the wind is constant. But the boattail bullet would have less drag and drift less for a given cross wind speed.

[samuel]

Quote:       Originally Posted by Bart B.

Please explain the physics behind this. I wouldn't think accuracy would be any different if the wind is constant. But the boattail bullet would have less drag and drift less for a given cross wind speed.

At identical mv,s the BT with higher bc would maintain speed better,shortening flight time and give cross wind less time to deflect the bullet. sam.

[Bart B.]

Quote:       Originally Posted by samuel

At identical mv,s the BT with higher bc would maintain speed better,shortening flight time and give cross wind less time to deflect the bullet. sam.

If you're implying BT bullets are more accurate in the wind, I'd don't believe that.

How could a constant cross wind make accuracy better, or worse for that matter? All it does is move the center of the group; less for a boat tail bullet and more for a flat base one.

[Shootingstarr]

The only thing a BT bullet is really good for is long range with a cross wind. No cross wind BT and FT rounds shoot pretty much the same. The BT rounds will be less affected by a cross wind durring flight.

[Shootingstarr]

I posted ealier that a Boat Tail will be more accurate in a crosswind. Let me expain. As most people know with a boat tail you will increase the bullets ballistic coefficient which will creat less wind drift durring flight along with higher down range velocities. It decreases drag not just laterally but horizontally as well. With a crosswind there is more air the bullet must travel through before it reaches it target regardles of range. (More Air = More Drag = More Friction) Quite a few long range competitive shooters are somewhat aware about the harmonics in your barrel and bullet. These are the small vibrations created as the bullet slides through the barrel. This has alot to do with competitive shooters useing Bull Barrels, the barrel is closer to the same diameter from breech to muzzle. This lets the barrel heat and cool more evenly. If a barrel does not heat or cool evenly the harmonics will be different for every shot, throwing your accuracy off the hotter your barrel gets. In a Bull Barrel your vibrations remain consistant along with your accuracy. This applies not just to the barrel but to the the bullet itself as it travels downrange. The boat tail is desined to creat less friction. Now with this you might better understand that durring bullet flight, the more air your round travels through the more friction is apllied to the bullet. Friction is not a friend. The more friction a bullet creates the less stable it will be due to inconsistant harmonics, affecting accuracy. With a flat tail there will be more friction during flight in a crosswind, affecting harmoncs, decreasing accuracy, blah blah blah. So with a boat tail your round with a crosswind will remain stable longer and create less friction, keeping the harmoncis as consistant as possible along with your accuracy.