Stubby--Lapua

[Pdvdh]

Kirby,

I didn't mean to imply I disagreed with your findings. To the contrary I find them informative, since I'll never complete all the loading, shooting, and testing that you have done to reach this observation on my own. By empirical testing, I was implying nothing more than the actual testing you have performed and the information learned (observed) from that testing. Loading bullets of similar sectional density in different calibers in this instance, and chronographing the velocities in cartridges of similar case capacity. Also comparing penetration results on game animals or in penetration testing media and noting the results. So empirical testing is really where the rubber meets the road. It's what's done to test the truthfulness of predictions based on theory.

I think the explanation for your findings is based on two primary factors. The 1st being what I tried to explain. Using proper powders, if we create a load producing similar bore pressures and those pressures are maintained for a similar duration of time down the bore, that the force exerted on a larger caliber bullet will always be greater than the force exerted on the base of a smaller caliber bullet. This explains why larger caliber bullets can always be shot faster, given bullets of equal weight.

But I think the second, offsetting, factor is that to obtain bullets of similar sectional density in two different calibers, the larger caliber bullet will weigh more. There will still be more force acting against the base of the larger caliber bullet, but since the bullet weighs more than the smaller caliber bullet, more force is required to accelerate the heavier bullet equally to the lighter (smaller caliber) bullet.

A larger caliber bullet always has the benefit of a greater force applied to its base, given similar bore pressures. This is a major velocity advantage for the larger caliber. But given similar sectional densities, the larger caliber bullet will be the heavier bullet. This reduces the velocity advantage for the larger caliber bullet. Combine these two factors together and I think they explain why you see the results you have observed time and again. That two different caliber bullets of similar sectional density can be driven to similar velocities in cartridges of similar case capacity. That's the rule of thumb I was referring to.

I think we knew that a larger caliber would send bullets of equal weight to faster muzzle velocities. My post was an effort to explain the reason why the larger caliber bullet has the velocity advantage with bullets of equal weight.

I appreciate your informative posts. Please continue.

 

[Fiftydriver]

Dave and Phorwath,

It seems we are arguing the same point here in the end, maybe just different theories as to why we get the same outcome.

We all agree that larger diameter bullets of same weight will produce more velocity potential then smaller caliber bullets of the same weight. No discussion there.

Out discussion is WHY this happens.

I am sure bullet surface area exposed to the pressure may have something to do with it.

I also feel that it has as much to do with the ability to use faster buring powders as well. I am sure we are all right to some degree or another.

The science of "why" it happens is interesting but to be honest, I am not a good enough thinker to explain it. I see what happens and then in general terms try to explain why it happens. I may be wrong, you may be right or vice versa.

Point is, In the end, we all agree what happens.

I think we have talked this one to death!!

Kirby Allen(50)

 

[jwp475]

The larger the base is the more force that is exerted.The same applies to hydrolics.Take a hydrolic pump that puts out 1000 psi of pressure and attach it to a jack with a 1" diameter piston and it can lift X amount of wieght,now attach the same pump to a jack with a 2" piston and now the same amount of pressure can lift a lot more wieght,but the speed at which the piston is moved will be slower unless the volume of the pump is increased,this is why a fasfer burn rate of powder is needed to increase the volume of pressure for a faster rate of presure not a higher peak pressure.........

 

[Fiftydriver]

+1!!!

Kirby Allen(50)

 

[davewilson]

we all understand why a faster burn rate of powder is needed.to keep the pressure curve the same.my opinion of the faster powder is it probably HURTS the speed a little.with the pressure curve the same,and the bullet exiting a same length barrel a little faster, the barrel time will have to be a little less.so the quicker powder will be pushing on the bullet the same amount, but for a shorter time. is this how you're explaining the bullet to go faster?

again, MOST of the gains in speed are due to extra surface area being pushed on. not less friction or faster burning powders.

 

[jwp475]

[ QUOTE ]

MOST of the gains in speed are due to extra surface area being pushed on. not less friction or faster burning powders.

[/ QUOTE ]

The larger base of the bullet allows more force to be applied at the same peak pressure. The larger base and bore require that pressure be made at a faster rate inorder to maintain the pressure curve.........

 

[davewilson]

yes, we've established that...what's your point?

 

[jwp475]

I thought that you wanted clarification to my eairlier post,I guess I missunderstood........

 

[J E Custom]

Bearless

The 2003 Remington catalog posted these velocity's
as an average.

MV 2000 to 2500fps changed 10fps per 1"

MV 2500 to 3000fps changed 20fps per 1"

MV 3000 to 3500fps changed 30fps per 1"

MV 3500 to 4000fps changed 40fps per 1"

So you can look for a known load and barrel
length and use this information to get close.

It is a shame because a big case like that can
use a longer barrel.

The shorter the barrel gets the more muzzle flash
you will get.

But cheer up with a little faster burning powder
you may be able to get the performance you want
with good accuracy.

I hope this will help
J E CUSTOM

 

[jb1000br]

A thought...actually a result...

Subject: 6mmbr/diamondback/broughton 5c

Bullets: 106CR(actually 105.5gn), 105(actually 104.9 gn)gn Scenar
Chrono: 4ft. Oehler 35

Same load of 30.8 R15, same seating in relation to the lands, same chrono/bench/light setup 80thou difference in bearing surface

5-shot group average velocities and ES figures were the same +-a couple fps.

That is about as large a difference I can imagine regarding bearing surface measurements on similar weight bullets, and it didn't matter.

That is only a single "data point" but I agree, BS/bore contact area is not near as important as bore diameter in this case...

YMMV,
JB

 

[Fiftydriver]

JB1000BR,

Just for curiousity sake, what is the jacket thickness of those bullets in relation to baring surface length?

Here is an example to show just the opposite,

338 Allen Magnum
40" Lilja 1-10 twist
Bullets tested and top velocity potential with each:

265 gr Al RBBT.............3575 fps
300 gr SMK.................3410 fps
300 gr ULD RBBT............3375 fps
300 gr Al Tip..............3275 fps
311 gr Al Tip..............3210 fps
350 gr ULD RBBT............3225 fps
375 gr ULD RBBT............3150 fps

The 265 has a baring surface slightly shorter then the 300 gr bullets even though its OAL is significantly longer, 300 gr SMK and 300 gr ULD RBBT have baring surfaces nearly identical in length and area. The 265 has a much higher velocity potential obviously because it is 35 grains lighter but also because of its shorter baring surface.

If you look at the 300 gr bullets, the 300 gr SMK has the higher velocity over the 300 gr ULD RBBT because the jackets are much thinner then the 30 thou thick ULD RBBTs with the same baring surface. Its hard to push a heavier jacket down the bore and as such, the ULD RBBTs pressure out faster and produce less velocity on the top end average.

The 300 gr Al RBBT has a much longer baring surface then the other two 300 gr bullets. Same bullet weight but longer baring surface and velocity potential is limited by 100 fps slower then the ULD RBBT and 135 fps less then the SMK. Interestingly enough, the jackets on these bullets were thinner then even the SMK but the baring surface is significantly longer which results in more friction and less velocity potential because pressure spikes sooner.

If we look at the 311 gr Al RBBT, this bullet has a VERY long baring surface length. The baring surface length is roughly 0.980" in length compared to the SMK of around 0.680" in length.

With the bullet weight only being 11 grains difference which when dealing with a bullet of this diameter and weight is really not much at all, why such a drop in velocity potential if baring surface has nothing to do with velocity potential??????

What is interesting is that the 350 gr ULD RBBT is very similiar to the 300 gr Al Tip in baring surface length and as you can see velocity potential is extremely similiar even though one bullet is 50 grains heavier then the other, only 50 fps difference in velocity potential.

Also, the 375 gr ULD RBBT is extremely similiar to the 311 gr Al RBBT bullet in baring surface length and their velocities are much closer then you would ever expect with their difference in bullet weights, only around 60 fps difference. Why is that? Because baring surface lengths are the same in my opinion.

Maybe in the smaller calibers it does not make a difference but in the larger calibers with larger powder charges it most definately makes a big difference even with same weight bullets.

I even tried to make the 300 gr Al Tipped bullet reach the velocity potential of the 300 gr SMK. I never could I did get them up to match what the 300 gr ULD RBBT topped out at and I got one firing per case because the primers were falling out.

All bullets were tested by increasing powder charge one grain at a time until primer pockets just showed the slightest sign of loosening with a one grain increase in powder charge.

Now I am not saying bullet jacket thickness does not have something to do with this as well but even similiar thickness in jackets held the same pattern with the longer baring surface bullets being limited in velocity to same weight, shorter baring surface bullet designs.

I am not saying the AL tipped bullets are not well worth the slight reduction in velocity. With the huge BC advantage they will have, I will never miss 100 or even 200 fps less velocity. They really make the 300 gr SMK look pretty doggy ballistically!!! More solid numbers on that coming soon.

Perhaps in the small bores this does not make much difference and that would even make sense. As bore diameter increases, surface area increased expodentially so a larger bore diameter bullet will have a dramatically larger increase in surface area with only 80 thou increase in baring surface length compared to a 6mm bullet with the same length increase.

Again, I think we have beaten this dog stupid, obviously there will be no agreeing that it is more then likely a combination of all the things we have talked about that contribute to the higher velocity. To each their own. I am fully able to admit it is likely a combination of everything mentioned but others are not able to do that I guess.

Anyway, time to talk about something else I think.

Kirby Allen(50)

 

[davewilson]

i guess it would be nice if we could just get others to admit when they're wrong.

 

[Fiftydriver]

My point exactly!!! Only one highway to drive on here I supose. /ubbthreads/images/graemlins/grin.gif

Kirby Allen(50)

 

[Pdvdh]

Now that's some good stuff in your prior post Kirby. That's what I meant by empirical testing/data. It's hard to beat this type of field-proven data. Even though the velocity data is out of your 40" barreled 338 super-magnum (which I'll probably never own the equal of), it's still good information to digest. And it's data I might never generate in my lifetime. Gives me an idea how the 265 Al-tipped Wildcats might perform in my 338 Edge compared to the 300 SMK, of course at reduced velocities. Thanks for sharing your velocity data. It does speak to some facet of bullet construction affecting velocity in addition to variable bullet weights within the same caliber bullets. Whether that factor is bearing surface or toughness of the jackets, or a combination of them both... hard to know. But I see where you're coming from based on these types of chronographed velocities for the various 338 bullets.