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.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.
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.