40-90 BN Sharps Question

[bruce m]

xtr,
b.c is everything.
the above post addresses that to some degree.
2 nose shapes now seem to predominate, those being elliptical and money.
the former is very similar to the american old dead guys bullet, and the latter to metford's bullet used in britain.
these shapes were proven by shooting a lot.
one place you can lose b.c. in in marginal stability.
if that bullet does a little wobble, it temporarily has an increased frontal area, more drag, and will shoot lower.
this is a great cause of vertical, and the nature of this sport is that you can blame yourself for that.
simply by hardening alloy i have had bullets shoot 2 moa higher at 1000 yds.
reduced drag?
2 moa on the elevation staff is neither here nor there, but what is the difference in wind deflection for that same bullet.
if our windcalls are 90% right, they are 10% wrong.
10% of a smaller ammount can mean the bullet on one side of the line or the other.
bruce.

[XTR]

What do you recon the BC to be on a 430gr 40?

[XTR]

What do you recon the BC to be on a 430gr 40?

[Distant Thunder]

bruce,
I watched another video dealing with subsonic, transonic and supersonic flight of airplanes. They used an airfoil and showed that during transonic flight the supersonic air flow on the top and bottom of the airfoil moved at different speeds similar to what you mentioned happens on the top and bottom of our bullets caused by the arced path of the bullet, which is rather curved for our bullets and would in effect be similar to the airfoil.

This was presented as the airplane was taking off, flying subsonic and increasing in speed through the transonic zone and into supersonic speed, just the opposite of what our bullets do while slowing down.

So if in my mind I can run this idea in the reverse of how it was presented it would look like the following.

As our bullets leave the muzzle near the upper end of the transonic zone, 1346 fps, and are flying somewhat less curved than they will further out when the velocity drops the supersonic air flow on the top and bottom are probably pretty much in line with each other, equal in size and near the base of the bullet. As the bullet continues on and drops down through the transonic zone the two air flows would move forward but at different speeds, with the upper one moving first and faster to a point. The bottom supersonic airflow would eventually more slowly move forward to nearly the same if not the same point on the bullet and would slowly be reduced in size as it does.

As the bullet approaches the lower end of the transonic zone, 900 fps, the two supersonic air flows would continue forward but the bottom one would get smaller and may even eventually disappear as it does on an airfoil entering subsonic speed and dropping below 900 fps. If the bullet slowed enough the upper air flow would eventually disappear as well. This whole phenomenon has to has an effect on the bullet.

In the video I linked earlier the two supersonic air flows were very even in location and size, but this is on a bullet starting out below the speed of sound. With our type of bullets starting out at 1300 to 1350 or even as high as 1400 fps with the .40-90 SBN, the air flow on top and bottom would be all supersonic. Moving through the transonic zone to subsonic is when the location and size of the supersonic flow would be changing. Somewhere near the midpoint of the zone it would look similar to the bullet in the video that I linked.

One thing I don't know is how much the nose shape itself affects the air flow in either supersonic or subsonic. As you said, it would be nice to see some of our designs run through that process.

It's all very interesting.

[Distant Thunder]

BC is important, however in the transonic zone the laws of diminishing returns can quickly catch up with you.

You have to keep in mind that we are forced to shoot is a speed zone no high power shooter in their right mind wants to be in. If you try applying the BC principles of supersonic jacketed bullets to transonic cast lead bullets you will quickly get into stability troubles. You can only spin a lead bullet so fast unlike the jacketed kind. Also the torque of spinning these big bullets quickly becomes a problem.

We are forced to do a very delicate balancing act if we want to be successful. I think the limitations are the part of BPTR I enjoy the most. We are forced to let go of so much that smokeless powder and jacketed bullets have taught us and embrace limitations that make this probably the most challenging shooting sport there is. Ain't it great!

[Distant Thunder]

bruce,

Your "caliber X 1.5" for nose length works out to about 47% of a 1.440" long .45 caliber bullet. That's the overall length I find works so very well in an 18-twist .45s. That is a bit longer percentage wise than I like based on my experience. I'm much happier with 37 to 45% (max). I will admit my .45 caliber paper patch bullet exceeds my standards by some good bit. Go figure!

Just one of those things I am struggling with and haven't been able to set in stone yet. It just seems when you are approaching 50% nose it's getting too long.

[Don McDowell]

I can across this video looking for some answers to just what the heck is going on with the whole supersonic, transonic, subsonic bullet flight thing. It's the best explanation I've seen on this stuff, but brings to mind even more questions. Have a look.

I would love to see this done with some real BPCR bullets, a lot of different ones. I also noted what a good friend of mine once said, "Our bullets are actually flying backwards with regard to air flow in the first seconds of flight!". Yep!

https://www.youtube.com/watch?v=BPwdlEg ... e=emb_logo

That's a pretty neat video.

[XTR]

I’ve seen some shadow images of subsonics that show little shock waves caused by SS flow around the curvature of the ogive. That adds a whole other aspect to the model.

I’d really like to see Bryan Litz explore this but he’s too busy working on ELR and govt contracts. No money down this road. I know Bryan and some of the folks who work for him. They can and have done some really good work. The have some awesome tools for this stuff....like a Doppler that records velocity every foot till the bullet hits the steel at ~1000 yds. More info that I compulsion process though. Kind of like the guy who said he went and watched it snow on the range a Oak Ridge once.... I don’t want to know what that looks like, I’d never pull the trigger.

[XTR]

Don, that is the video I saw

[bruce m]

one thing where we are lucky with our alloy bullets is that they are larger caliber.
larger calibre requires less spin rate for the same stability.
i am confident at this stage that a minimum miller stability of around 2.75 is the number to try for.
it is always said that you only spin a bullet as hard as you have to, and in our case that is it.
i would for fun like to fry our bullets in a 458 lott like don says.
they have a twist more like 1:12, so really there are more important things to try.
the lott and 458 win mag have that fast twist not for expanding bullets as they get shorter and wider as they expand.
full metal jackets on the other hand might be called upon to go through feet of bone in an elephant's head.
they should retain their length, and when they hit bone the overturning moment will be lengthened a LOT.
yet to penetrate they must stay nose on.
so they must be spun faster to remain stable in denser material.
our bullets only go through air and paper.
i have questioned litz on transonic flight and he pretended to give an answer, but it contained less than i know. (and that is not much)
bruce.

[bruce m]

jim, i am like you on nose length.
i believe brent's bullet is 50% nose length, and who can argue with his results.
i wonder how long his nose is as the bullet leaves the barrel?
myself i would be too scared to go that long.
arnies super hard dd bullets that do not bump up might just solve that problem.
if we can get longer ogives we can get higher b.c.
then we come to airflow over the bullets?
by the time, if ever, this is worked out, we will probably be old dead guys too.
bruce.

[bruce m]

someone here said they could not get a 405 win to shoot.
well mine can shoot, and do it along with a 45.
i will defend the cartridge's potential, and suggest that there are questions the maker of the satement should be asking themselves.
usually if you do the work you reap a reward, or find the problem.
it is so easy for these sort of statements to create urban myths which are actually the opposite of the truth.
bruce.

[Don McDowell]

I said I couldn't get mine to shoot very well with 400 gr bullets and blackpowder. Never said a word about what sort of small groups it shoots with 300 grain bullets at 2350 fps.
5 shots 100 yards

[bruce m]

so what is the reason for this don?
there must be one.
bruce.

[bruce m]

What do you recon the BC to be on a 430gr 40?

if it is 1.5" long and with a reasonable shaped nose and cast as hard as will work, comeups combined with muzzle velocity suggest about 0.5.
bruce.