The larger the bore, the faster the bullet?

[Newtosavage]

I'm trying to figure something out here and I could use some help.

The .338 Federal velocity numbers got my attention. I was wondering how such a heavy bullet out of a .308 based case can move so fast. Then I started to wonder, maybe there is a relationship between bore size and velocity - i.e. for a given bullet weight, the bullet with the larger diameter will have a faster MV.

Is there something to this?

Looking at reloading data, it would appear there is. IOW, a 100 grain .243 bullet usually isn't going as fast as a 100 grain .260 bullet. Likewise, a 120 grain .260 bullet won't be going as fast as a 120-grain 7mm-08 bullet, and so on.

I'm sure someone here can explain this phenomenon.

 

[troy fairweather]

i think its only mostly true with bottle neck cartridges. i think the shoulders builds higher psi then straight walled cases. and the big bullets make it easier for the gases to leave the case. the 338 fed is about 200 fps faster then the 308 for the same bullet weight.

 

[silicosys4]

I'm not physicist but I think it has something to do with the larger surface area at the base of the larger caliber bullets gives more area to push against as compared to the mass of the projectile. The larger the caliber of the bullet, the larger the area of the base, and the more area that is subject to the forces of pressure/thrust per unit of mass.

Also, I think all things being equal bullet weight wise, the larger the caliber the shorter the bullet so the less surface of the bullet is making contact with the bore, hence less friction. But thats dependent on bullet design and material.

Or something

 

[sarduy]

A lot have to do with powder and case volume as well,

 

[Goosey]

If you look at many neck downs and ups you'll find the change in energy is often roughly the same as the change in bore. Eg, a .270 is 90% as powerful as a .30-06.

 

[LoonWulf]

i think its only mostly true with bottle neck cartridges. i think the shoulders builds higher psi then straight walled cases. and the big bullets make it easier for the gases to leave the case. the 338 fed is about 200 fps faster then the 308 for the same bullet weight.

I'm not physicist but I think it has something to do with the larger surface area at the base of the larger caliber bullets gives more area to push against as compared to the mass of the projectile. The larger the caliber of the bullet, the larger the area of the base, and the more area that is subject to the forces of pressure/thrust per unit of mass.

Also, I think all things being equal bullet weight wise, the larger the caliber the shorter the bullet so the less surface of the bullet is making contact with the bore, hence less friction. But thats dependent on bullet design and material.

Or something

From my understanding, that's pretty much the jist of it.

Pretty much any cartridge bassed on the same case with a larger dia bullet, will deliver higher velocity with the same weights as a smaller caliber cartridge, at the same pressure.

 

[Varminterror]

Force = mass * acceleration. Pressure = Force / area. Increased area for equivalent pressure also means increased Force. Increased force with equivalent bullet mass means greater acceleration, greater acceleration for the same barrel length means greater muzzle velocity.

A larger bore will also mean an equivalent weight bullet is shorter, displacing less powder volume in the case. So a greater powder charge can be used (when using the same powder remains to make sense for both calibers at least) under the larger bore bullet than the smaller. No replacement for displacement - powder means speed when everything else is equal.

Now, there ARE cartridges which do “develop pressure better” by shrinking the bore, which really is a factor of improved powder burn efficiency. Necking down some pistol cartridges, for example, can promote greater velocity with equivalent bullet weights. But in general, a larger bore will allow slightly greater velocity than a smaller bore.

 

[TN Outlaw]

Yesir, it's a piston/hydraulic affect.

More surface area to apply the same amount of pressure to.

I'm curious how much recoil and surface area go hand and hand? With the exact same charge, pressure, and weight (let's say 180gr bullet) from a .270 Win and a 35 Wheelan, that added surface area must add velocity AND recoil to the 35W I would think or can you "get something for nothing" in this case?

 

[Varminterror]

Recoil doesn’t care about bore or pressure in a literal sense, but secondarily so. Same charge, same bullet weight, then the faster of the two will have the greater net momentum, and such impart greater recoil momentum to the rifle. Equal but opposite, and all that... If the case has sufficient capacity to capitalize on the larger bore and produce greater velocity, then the larger bore will have greater recoil.

What you do get, however, by running a 270 and 35 whelan at the same charge and bullet weight is an abysmally poor Sectional Density on the 35cal bullet.

 

[Varminterror]

it's a piston/hydraulic affect.

To expand a bit on this - a hydraulic piston is supplied with (relatively) constant pressure. A rifle bore is not. So the case capacity has to be sufficiently large to suit the bore diameter, otherwise the above generalization I described won’t be true. Rifle cartridges all reach their peak pressure within a couple inches of bullet travel down the bore. Once that peak is reached, if a bore is too large for the case, it won’t have sufficient expansion ratio to sustain the pressure curve as high in the larger bore.

In the case of a large bore, small case, it’s like a hydraulic piston with a low oil condition - the smaller cylinder would stroke fully and at full speed, whereas a larger cylinder wouldn’t have sufficient volume to fully stroke, and power would be reduced during travel.

 

[TN Outlaw]

Recoil doesn’t care about bore or pressure in a literal sense, but secondarily so. Same charge, same bullet weight, then the faster of the two will have the greater net momentum, and such impart greater recoil momentum to the rifle. Equal but opposite, and all that... If the case has sufficient capacity to capitalize on the larger bore and produce greater velocity, then the larger bore will have greater recoil.

What you do get, however, by running a 270 and 35 whelan at the same charge and bullet weight is an abysmally poor Sectional Density on the 35cal bullet.

Agree, greater velocity is greater velocity so regardless of how the extra velocity is added to the equation, the equal and opposite reaction will still be added accordingly. I felt such was the case, but have not tested a situation like this before.



Thanks for the info

 

[Newtosavage]

Thanks for all the thoughtful replies. So what I thought I was seeing, is actually the case. That's very interesting. Makes sense now about the .338 Fed. I was like, how the heck are they getting those speeds out of a .308 case with that heavy of a bullet??? And now I know.

 

[.308 Norma]

Likewise, a 120 grain .260 bullet won't be going as fast as a 120-grain 7mm-08 bullet, and so on.

As others have pointed out, it has to do with the amount of pressure applied to the larger diameter base of the 120-grain 7mm-08 bullet compared to the amount of pressure applied to the smaller diameter base of the .260 bullet. However, we're only talking velocity at the muzzle here. Don't forget that a 120 grain .260 bullet typically has a higher ballistic coefficient than a 120 grain 7mm-08 bullet. So even though the 7mm-08 bullet left the muzzle going faster, it's going to slow down sooner than the .260 bullet - probably not enough to make any practical difference at normal hunting ranges (as far as I'm concerned) but the lower ballistic coefficient bullet does slow down sooner.

 

[Newtosavage]

Yea, I'm pretty OCD about knowing my BC's and calculating drop, and all my rifles are hunting rifles.

Having said that, I managed to miss a coyote at 70 yards this morning

 

[.308 Norma]

Having said that, I managed to miss a coyote at 70 yards this morning

Good job!

 

[Newtosavage]

Good job!

Got greedy and waited too long. I was trying to line two up with one shot. Once I realized that wasn't going to work out, they were halfway into the brush and I threw a hail mary out there. LOL Oh well. That will teach me.

 

[LoonWulf]

Got greedy and waited too long. I was trying to line two up with one shot. Once I realized that wasn't going to work out, they were halfway into the brush and I threw a hail mary out there. LOL Oh well. That will teach me.

Betcha woulda had him with that old turtle-turtle you got

 

[748]

I'm not physicist but I think it has something to do with the larger surface area at the base of the larger caliber bullets gives more area to push against as compared to the mass of the projectile. The larger the caliber of the bullet, the larger the area of the base, and the more area that is subject to the forces of pressure/thrust per unit of mass.

Also, I think all things being equal bullet weight wise, the larger the caliber the shorter the bullet so the less surface of the bullet is making contact with the bore, hence less friction. But thats dependent on bullet design and material.

Or something

Yes, surface area.
The propellant is doing work like a fluid in a hydraulic or pneumatic system and the bullet is like a piston. The bigger the piston the more work the working fluid can do when constrained to a certain pressure limit.

 

[Newtosavage]

Yes, surface area.
The propellant is doing work like a fluid in a hydraulic or pneumatic system and the bullet is like a piston. The bigger the piston the more work the working fluid can do when constrained to a certain pressure limit.

That is fascinating. I know (just) enough about hydraulics that I can't believe I never made that connection. Thanks!

 

[Newtosavage]

So I'm sure some bright person out there has kinda sorta figured out what the most efficient bullet diameter to case capacity ratio is by now...?

Cuz what good is it to push a fatter bullet of the same weight faster, only to generate more recoil and have the lower BC slow it down prematurely anyway?

 

[Dr T]

Back to the OPs original question, long ago I spent a LOT of time pouring over reloading data. And I noted that with the same basic case (308 sized, 30-06 sized, *x57 sized) that the same amount of the same powder would push a larger bore diameter bullet of the same weight and construction a bit faster. And it is very nice to have the role of the cross sectional area base and fluid dynamics noted by 748 and silicosys4 nicely highlighted...Without giving it a lot of thought, I had assumed that the coefficient of friction was lower, not that the piston was more efficient. Thanks for the explainations.

 

[Milt1]

A lot have to do with powder and case volume as well,

I think it mostly has to do with powder and case volume. For example take a look at the velocity of the 22.250 compared to the .338 Federal. No way in the world that the Federal is going to come anywhere close to the velocity of the 22.250 without a much larger case and much more powder.

 

[jmr40]

There is no free lunch. With the same powder capacity a 200 gr .338 bullet will leave the muzzle about 100 fps faster than a 200 gr .308 bullet. But because the .308 bullet has much better sectional density it will out penetrate the .338 bullet at all ranges. And because it has much better ballistic coefficient it will be moving faster at about 150-175 yards.

You get the same results when you compare 338Fed to 308, or 338-06 to 30-06 or 338 WM to 300 WM. The 30 calibers always win on penetration and they always hit with more velocity and energy at some point down range. Usually around 200 yards. And the 30's do it with about 20% less recoil. The difference between a .308 caliber hole and a .338 caliber hole in game is insignificant. Wrap a piece of tape one time around a .308 bullet and it now measures about .338. It's about the thickness of your fingernail.

Every way I try to look at it it becomes clear why the 338's have never caught on.

 

[748]

That is fascinating. I know (just) enough about hydraulics that I can't believe I never made that connection. Thanks!

I was doing some figuring one day and found that sammi pressure times the are of the bore was directly proportional to muzzel energy, at least for higher pressure bottle necked rifle rounds.

 

[MCMXI]

This is a fun thread. I shoot a 250gr TTSX (G1=.424) out of my .375 H&H at 2,800 fps and I shoot a 250gr Lapua Scenar (G1=.521) out of my .338 Lapua Magnum at 2,800 fps. One uses 75gr of powder for 62 ksi and the other about 90gr for 65 ksi.

Every way I try to look at it it becomes clear why the 338's have never caught on.

Not counting the .338 Lapua Mag of course.