Perplexed about bullet behavior

[Picknlittle]

Okay. There is this thing that nags at me concerning bullet velocities and bullet behavior.

I understand there are a number of factors that affect accuracy.

Pressure, barrel harmonics, powder burn rate, oal, seating depth and all those sorts of things. But,....why?

Remove a number of variables by making more things equal.

Take, for instance, a 30-06 and a 300 win mag bolt guns. Give both identical barrels with all the standard acurizing treatments, bedding, bushing, etc. The only thing that is really different now is receivers. Now select three different match grade bullets, and load them to "most accurate load" for each rifle.

Why is it that the mag can maintain accuracy at much higher velocities than the 30-06, or any other same cal. chambering? For that matter add to the mix the Weatherby's, H&H mags or whatever. Why is a 165 gr bullet from an 06 limited to say, 2800 fps, well below 60k cup, while the same bullet from a mag can be driven several hundred fps faster at pressures closer to 70k or more.

In my mind thermal expansion is thermal expansion. The things that make an 06 group begin to open up at a given pressure or velocity, should carry over to the mags. What am I missing?

Of course you can take my questions, put a $1.29 with them and buy a coffee in some places.

 

[schmidtbender]

You can load any new brass case for 30-06 to the same pressures as a 300 magnum of any stripe, IF the action is up to it. The only independent variable, if all others have been removed, is how much powder is burning behind the bullet. A 30-378 will shoot a stout bullet (lets says Barnes 180 TSX) faster than a 30-06 because there is a lot more burning gas pushing it along. The Magnum will not be anymore accurate at its best velocity (not necessarily the fastest) than the 30-06 but it will be going faster. Just as the 06 will go faster than a 308 win. I can probably push a 35 Whelen faster in my 700 Classic than your 03A3 because the action will take more pressure with safety. Some rifle shoot MAX loads most accuratly, some do not, but groups don't open up as a function of pressure or velocity as anyone with a BOSS equipped Browning A-Bolt can prove to you. Since you have the ability with a BOSS to tune the bbl to the load, almost any load can be made a tack driver. My wife's semi-auto BAR BOSS 30-06 will shoot every load I have tried from 130-220 grains into a 3 shot 1" or less group @ 100 yds. The A-Bolts shoot like benchrest guns. On the other hand my Benelli R1 300 WinMag is at best a 3 shot 1.5" rifle, and that with selected loads. The fact that it handles like a fine 20 ga shotgun whereas the BAR is a bit of a club, keeps it in my safe. The bOSS was a major breakthru but Browning screwed it up by venting it which will destroy your ears (and those around you). With the non vented end it works just as well, kicks like a regular rifle and doesn't get you thrown off the range.Hope I have helped your quest.

 

[DogBonz]

well...

Although it seems that you are asking two different questions, I’ll try to answer both. While bullet drop may not affect accuracy per say, it can affect how easy it is to shoot accurately. For instance, that 30-06 at 2800fps will drop 54.4” (Serria Match King) at 500 yards, while a 300 win at 3200 will only drop 39.3”. Also, the wind drift of a 30-06 will be 20.6” (steady 10 mph wind) and the 300 would be 16.5”.

Now, the 168 gr bullet fired from a 30-06 and a 300 win mag will hit the ground at the exact same time (all else being equal). The difference is that the 300 will travel farther in that time, or conversely, and more importantly, it will reach the target faster. The faster a bullet gets from your muzzle to the target, the less that “other factors” (such as wind) can affect the bullet. The fewer things affecting the bullet, the more accurate your shot can be, or the easier it will be to replicate that shot, because that is what accuracy is, being able to repeat the same bullet placement over and over again.

The reason that a 300 can push the same bullet faster is that there is more case capacity. The more case capacity, the more powder it can hold. The more powder it can hold the more powder that you can burn. The more powder burned, the more gas created. The more gas created, the more force created to push the bullet. Because you have the same weight projectile, the one that generates the most force will push that same weight faster.

Now this all comes with the caveat “all else being equal”.

Think of a lawnmower engine and a big block chevy. The bigger the cylinder, the more gas and air that you can burn. The more fuel and air burned, the more power the engine can make. Put thoes engines in two exact cars with the same weight and which one will go faster?

 

[Picknlittle]

Still puzzled. If more powder just means more velocity with no impact on accuracy or grouping, then why is it necessary to dial in loads. If a .1 grain powder change can affect a group size by .030 to .050, then there has to be some correlation to pressure,...no?

I know I 'm not the bright bulb in the box, but something has to be responsible for the effect that small changes in charge weight has over groups closing as optimum is reached, and the gradual opening up at optimum is passed.

I'm probably over thinking this but the results from my attempt to load 125 gr sp in 30-06 really blew me away. As my charge changes by .3 gr, these groups moved all over the place. The best groups, which were not "varmint hunting" good, were from the lightest/slowest loads. I expected small changes, I got dramatic changes not only in group size, but in POI. Over a 3 grain range, POI was literally all over the place and group size varied by as much as four inches.

I abandoned the project in favor if maybe selling the 06 and buying a 22-250

 

[Art Eatman]

"Why is a 165 gr bullet from an 06 limited to say, 2800 fps, well below 60k cup, while the same bullet from a mag can be driven several hundred fps faster at pressures closer to 70k or more."

I'm dubious about these pressures being factory loadings. Those pressures are getting into the area of doing Bad Things to primers and primer pockets. Generally, 55,000 psi is the general upper practical limit.

In a claw-extractor rifle, the case head is not as fully supported as in a push-feed rifle--which is another reason for pressure limits.

"If more powder just means more velocity with no impact on accuracy or grouping, then why is it necessary to dial in loads. If a .1 grain powder change can affect a group size by .030 to .050, then there has to be some correlation to pressure,...no?"

It's less an issue of pressure than of barrel harmonics. I don't think anybody knows WHY a tenth of a grain will affect the harmonics; we just know that it happens.

Art

 

[RecoilRob]

The true question should have been titled: Perplexed about RIFLE behavior

So long as a projectile is spun fast enough to stabilize, it doesn't matter a whit exactly how fast it is going (so long as it remains supersonic) with regards to accuracy.

Now, how any individual rifle shoots said bullet is dependant on all the variables inherant in the mechanism. It isn't the bullet that is changing when you dial in a load, but rather tuning the load to what the RIFLE likes the best.

Doesn't matter about caliber or whether it is a 'magnum' or whatever. The projectile follows the laws of momentum and physics once it leaves the barrel. What it does as it travels down the barrel determines (to a large extent) how accurately it will be launched.

Some rifles are very sensitive to their ammo...some are not. If you had a 200lb test fixture firing the bullets, the only difference you would notice...accuracy wise....would be in the Standard Deviation changes that happen when working up a load. Find the load (of whatever powder) that burns the most consistanly for the lowest SD and you would likely have your most accurate load as well.

Also remember that a human fired rifle has the variable of the shooters resistance to recoil to take into account. The lightweight rifle will move whilst the bullet is still travelling down the barrel, and this accounts for a large deviation in shot placement if the resistance and hold are not consistant.

 

[alucard0822]

an observation I have had is that although max pressure may vary a slight amount between different 30cals, the case capacity and pressure wave vary greatly. A 300 win mag has a physically larger volume, that would create lower pressure from a slower burn with the same charge as a 30-06, but the larger capacity allows the rifle to maintain a higher average pressure, with a longer burn time. If you could maintain 55,000 psi from leade to muzzle, you could probably push a 168gr bullet well past 4000fps. There is a tradeoff between more flex (and more room for annomalies) from a longer burn time in a larger case, and higher velocity allowing less interference from other forces, higher temperatures can also vary pressures. The individual dimentions of a cartridge, can also detract from consistency, just as dog boz said a SBC can make more power than a lawnmower engine, but power and velocity have little to do with accuracy. consistency does, if you look at the setups used in bracket racing, there is plenty more power that can be had from the cars, but more consistency is preffered.

just my .02