In what amount, if any, muzzle velocity is affected by the bullet shape??

[saturno_v]

Let's say we fire the 2 identical powder loads during the same range session, a 30-06, one with your typical spitzer boat tail shape another with a round or a flat nose, same bullet weight.

Same thing, let's say, for a 30-30....2 identical loads both 170 gr, one round nose, the other flat nose.

How affected will be the muzzle velocity beyond the typical normal variance you always get between different shots ?? I did some research on the web but I could not come with any meaningful info on the topic.

Intuition tells me that air resistance should affect the bullet even inside the barrel...a more aerodynamic bullet will have less air resistance for the propelling powder gases during the acceleration phase (other things being equal such as the friction generated from the rifling)....right???

My ballistics books and manuals talk about air density and temperature....less density will result in flatter shooting but if the less density is because of change of altitude your load could perform less because of the potential drop in temperature (colder powder and case affects muzzle velocity) so the 2 things partially offeset each other.

In general hotter days will results in higher muzzle velocity (and higher pressures) and vice versa.

But I don't have any info about how bullet shape affects muzzle velocity.

Thanks!

 

[Ridgerunner665]

Inside the barrel...no aerodynamic effect, but the bearing surfaces will likely be different and that will affect it.

But the difference is noticed very soon after it exits the barrel...even high BC rifle bullets lose about 6 fps between the muzzle and the chrono (about 10 feet away)

 

[saturno_v]

Inside the barrel...no aerodynamic effect, but the bearing surfaces will likely be different and that will affect it.

But the difference is noticed very soon after it exits the barrel...even high BC rifle bullets lose about 6 fps between the muzzle and the chrono (about 10 feet away)

Rather interesting.

How come aerodynamic doesn't play a role inside the barrel?? I'm trying to understand it but I cannot come up with an explanation.

Interesting observation about the bearing surface....a blunt or round nose bullet may have increased surface exposed to the friction of the rifling and then affect muzzle velocity...

 

[Ridgerunner665]

Because the air can't travel around the bullet...there is no drag, which is created by the vacuum at the rear of the bullet when its in flight.

 

[saturno_v]

So the aerodynamic drag is only when the air can flow around and behind the bullet?? I thought the the vacuum drag was only one type of drag that affects bullets (or any moving object)


Thanks...really interesting. Trying to create a mental picture of the phenomena...the air cannot flow around so its pushed out no matter what at the speed of the gases propelling the bullet...I think i got it.

 

[Duke of Doubt]

Shotgun pellets and other shotgun loads would be the exception.

 

[Ridgerunner665]

Look at it this way...

As a bullet travels down the bore...it pushes that air out ahead of it...all of the air, it doesn't make any difference whether its a flat point or a ballistic tip, all the air gets pushed out...the VERY SMALL amount of resistance will be the same either way.

There is aerodynamic drag (effect of the vacuum and the shape of the point) then there is good old friction (resistance of air moving over the bullets surface)

 

[saturno_v]

Yes because they have some air flowing around them in the smoothbore barrel but it get mixed with the hot gases....well it gets complicated...let's stick with bullets!!

 

[Ridgerunner665]

How would shotgun pellets be any different?

 

[Ridgerunner665]

Nope...the wad seals to the bore and pushes all the air out.

 

[saturno_v]

Thanks guys, it make perfect sense....I'm re-reading some articles and my manuals about aerodynamics and air resistance and it is clear...drag is experienced when you flow through a fluid, that's the key.....and I'm almost a pilot!!! What a shame isn't it??

Thanks again

 

[saturno_v]

Nope...the wad seals to the bore and pushes all the air out.

That's right!!!

Ridgerunner made my day!!!

 

[saturno_v]

Ridgerunner

Trying to play devil's advocate...when you shoot a spitzer bullet versus, let's say a completely flat nose, some air inside the barrel flows around the part of the bullet smaller than the caliber up to the bearing surface....of course, that air cannot completely flow around the entire bullet.........no drag there???

 

[Ridgerunner665]

Nope...no drag...it gets pushed out like all the rest.

Like you said...through the liquid (or air)

 

[saturno_v]

Same concept of a Syringe I guess...doesn't matter the shape of the plunger head...the resistance is the same.

What about air density, indipendently from the shape??? It doesn't affect muzzle velocity?? What would be the "muzzle velocity" of a gun firing under water (assumign the water filled up the bore and barrel would not blow up)

If I push a syringe filled with pudding rather than water the resistance is higher.


Thanks again...very interesting conversation.

 

[Ridgerunner665]

Air density matters and does affect MV but the difference would be too small to measure in relation to MV (at the muzzle).

Under water...it would be slow IF it would fire, which it wont in most cases and it would blow the barrel. (some lower pressure rounds may fire under water...but high pressure rounds won't, I don't think...I've never tried it)

 

[saturno_v]

So, in practical term, less air density would generate a minuscole increase in MV hardly measurable, but other factors (powder temperature for example) would have well greater measurable effects.

It would be interesting to know the difference in MV for, let's say, an anti aircraft cannon (for example, a Vulcan 20 mm) between firing at MSL on an "average day" and firing it during aerial combat at 40.000+ feet where air density is extremely low but so is the temperature (and this would affect powder performances).

I don't know if fighter jets have some sort of device to keep the ammo cases "warm" before firing to avoid very cold or frozen powders and primers.

 

[Ridgerunner665]

Some powders are more affected by temps than others...Varget for example is often "hotter" in cold temps (I have seen this in my 308's...30 fps faster at 20 degrees than it was at 70 degrees)

I can't help you with the aircraft rounds...other than to say that you have to add air speed to the muzzle velocity.

 

[saturno_v]

I can't help you with the aircraft rounds...other than to say that you have to add air speed to the muzzle velocity.

Oh yes, I was talking about relative muzzle velocity.

 

[Duke of Doubt]

Depends on the wadding, if any.

 

[Ignition Override]

Ridgerunner- the absence of drag at the rear of a bullet might be the case, but an aircraft wing and all other surfaces experience either form, induced or parasitic drag, maybe a combination.
The different bullet shapes should have various types of form drag, but I'm sort of new at dealing with rifles/bullets.

 

[1858rem]

i was wondering about this too...in my LEE reloading manual there is only one section for all the .308 150 grain bullets....no different sections for spitzer, hollowpoint, or boattail shaped bullets, i recently got some 150 sst boat tail ballistic tip bullets and was thinking ...shouldnt they go faster than the published loads or work as less psi since they have less bearing surface on the barrel than a flat based 150g bullet?

 

[saturno_v]

i was wondering about this too...in my LEE reloading manual there is only one section for all the .308 150 grain bullets....no different sections for spitzer, hollowpoint, or boattail shaped bullets, i recently got some 150 sst boat tail ballistic tip bullets and was thinking ...shouldnt they go faster than the published loads or work as less psi since they have less bearing surface on the barrel than a flat based 150g bullet?

The amount of bearing surface definitely has a role in affecting MV.

 

[1858rem]

im debating on putting down my 200 bucks and getting 500 bullets and 3 lbs of powder, or half that stuff and a chrono...would it be worth it...been thinkin about one for a while but only can put in a order once a month at 6.55/hr lol. if the bullets shoot just as well and i dont plan for more than 500 yds do i even need one...also need to step up from my lee safety scale(which is excellent for powder and my .20 cal bullets) to one that can weigh .308's on i think...

 

[R.W.Dale]

Oh yes, I was talking about relative muzzle velocity.

On an interesting side note the B58 Hustler had a 20mm vulcan mounted in the tail on air force insistence. With a MV of 3500fps +/- and the aircraft traveling forward at just under 2000fps the projectiles fired would practically fall away with less than 1500fps and an enemy aircraft would literally have to fly into them.