40-90 BN Sharps Question

[bruce m]

don
as i am more than a little interested in either the 40/90 sharps or the 40/90 ballard, your bullet etc made me do some numbers.
using the miller stability value, i have calculated some numbers for various bullets.
starting with the standard 540 gn, 1.45" long 458 bullet at 1300 fps from an 18 twist barrel, we get a stability of 2.422.
some guys are now going to 16 twist, and there stability jumps up to 3.066.
some of these guys are however going to 1.5" long bullets, so i will calculate that later.
however i believe guys that use this twist primarily do it to reduce or elimimate wind shear.
dan theodore used to talk about this.
my 440 gn 1.55" long bullet in 408 at 1300 fps from a 13 twist has a stability of 2.529
the same wt at 1.5 long is 2.778.
giving your bullet at 1.4 long a speed of 1400 from a 16 twist has a stability of 2.132.
this form of stability can be calculated.
the other form of stability, dynamic stability cannot to my knowledge be calculated, but must be actually discovered by shooting and hoping you don't find dynamic instability.
the military and most disciplines use the number 1.5 for acceptable stability, but they are not shooting bullets into the transonic zone and out the other side like we do.
my studies of bullet holes at ranges indicate that any wobble seems to occur at about the bottom end of transonic.
i have never had a bullet go so unstable as to leave the scene, but have had wobbles then the bullet started making round holes again further out.
these bullets always shot a little low, but still on target.
when i went down to 1.5" length, the whole thing went away, with round holes all the way out.
if i do a 40/90 a 12 twist would be worth trying, just to get the higher b.c. of the longer bullet.
instability will give you vertical so eliminating it will reduce it.
b.c. will reduce wind deflection, so is your friend.
i am coming to the conclusion that at least part of the instability of transonic flight is due to this.
when the bullet is supersonic, it is pushing a pressure wave in front of the nose as we see in high speed photography shadowgraphs.
as the bullet goes down into the transonic, this pressure wave eases back onto the nose while reducing, and then the nose pokes through it.
it then moves back along the bullet until it leaves the bullet at the base.
this might be the time we hear the sonic crack.
that crack suggests an ammount of energy going somewhere.
if it flies off the bullet evenly, probably not a problem.
however, due to the trajectory of the bullet, its topline has further to go than the bottomline, and the shockwave might then leave the bullet on one side before the other with less than ideal results for stability.
throw in a windshear about now and you have double trouble.
seeing wobbly holes in targets at the end of transonic distances would tend to support the above suggestion.
for long range shooting, with the above in mind, for all the investment and time it takes to get a long range rifle working, i will be looking at a miller stability of 2.778 min, because i know that works.
bruce.

[bruce m]

here are further figures.
a 45 cal 540 gn 1.5 long bullet at 1300 fps from a 16" twist has a stability of 2.786
there are guys around now that swear by this formula.
my 40 cal 440gn 1.5 long bullet at 1300 from a 13 twist has a stability of 2.778.
close enough to be the same.
i used to think my 1.55 long bullet was it because i never had a sideways hole in the target.
until i tried the 1.5 long bullet and got better results.
this shooting and testing took about 2 years, but was interesting and worth it.
bruce.

[bruce m]

the above stability numbers assume no spin decay.
yes there is an old wives tale that says it does not exist.
however i have read of u.s. military testing that proves it does, and they have been able to measure it.
obviously spin decay would happen more for a longer time of flight, and guess what.
our low muzzle velocities offer long times of flight.
what this might mean to our real stability at distance god only knows, but we must assume our stability is decreasing as range increases.
bruce.

[Don McDowell]

Bruce I am pretty well convinced from experience in hands on shooting, that the 40-90 bn may very well be the only 40 caliber that is truly capable of handling long range shooting in all conditions. I do believe it is because of the velocity attained. But the attendant recoil of the thing shoving that much burning powder thru that 40 caliber funnel would dictate a have rifle, 12 pounds would certainly be better than this 10 lb rifle, and a 14-16 lb rifle would be better yet.
As to twists. I have come to the conclusion again based on shooting experience that matching the bullet length to the twist rate is really critical across the spectrum. I have a 40-70 with what is apparently an 18 twist and it shoots bullets just a touch short of 1.3 inches real well, but bullets over that length will give you fits in crosswinds over 10 mph even as close as 300 yards. I did do some experimenting with my 405 winchester and 400 gr bullets, but found the velocity attainable with blackpowder left the consistent accuracy a crap shoot in the windy conditions we experience here in the west.
In the 45 caliber, 45-90 to be specific, I have rifles with 18 and 16 twist, and find little difference in long range ability between the them in the typical "535" gr bullet weight, regardless if grooved or patched. The 16 twist will do a spectacular job with the 525's out to 600 and shoot ok to 1000 but the just a bit longer 535's hold much tighter groups at 1000, in ideal group shooting conditions ( a rare occasion) to that distance. All of my 45-70's and both my 110's have 18 twist so I have really can't comment on how the difference between the twists would be. I can say I would really like to find someone with a 458 lott and a spirit of adventure to be able to test out the really fast twist with heavy bullets and black powder.
I have not given the 44's with a 17 twist with bullets longer than 1.35 a good test as that length that shoots so very well,will offer sporadic results from a 19 twist. Keeping the bullet length at 1.3 max regardless of nose shape with the 19 twist will keep it quite competitive to the 1000 yard line.

[Distant Thunder]

bruce and Don,

Now you guys are getting to some of the most interesting parts of stability in the transonic zone, outstanding posts. DON'T STOP NOW!

If more shooters would follow this line of thinking even if they don't fully understand it (that would be me) we would have a lot more people being competitive in long range matches, especially when the winds are doing all they can to defeat their efforts. It took me something like 10 years to realize that there seems to be a specific length of bullet that continues to preform very well form my 18-twist .45 caliber rifles in some pretty tricky winds. I stumbled into it much quicker in my 18-twist .40 caliber rifle.

I also believe that there are nose shape that are superior in the transonic zone. They ODG knew this about the nose shapes, I'm not sure they had the length to twist thing figured out. If they did I haven't seen that.

Some many shooters seem to be stuck on bullet weight as if there is something magic about a certain weight when the length and nose shape are far more critical to success in the transonic zone.

I happy to see you guys have done the testing and are able to put into words the thoughts I've had for a while now. Thank you for your efforts!

[Don McDowell]

I don't know Jim it may be more of a sign of a wasted youth?
I am fortunate in that here on my ranch I have the ability to go out and shoot at any distance from powder burn close, to a bit over a mile. It is interesting on this steel target to be able to shoot a string, and if there's bullet strikes on steel and off, with the off being unexplained at the time, and no rhyme nor reason to the pattern of the hits, to be able to examine the target and see sometimes bullet profiles, sometimes nice even round bullet strikes and some times odd shaped bullet strikes. It can really give a person time to reflect on things. The Caldwell target camera has been a big help.
Someday I need to get around to putting up wind flags of some sort, to assist my lovely assistant in helping with wind calls, altho she's getting really good at mirage reading.

[Distant Thunder]

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

[XTR]

I’ve read that folks make 40-90 Sharps BN cases from 45 2.6 cases, do you need to stretch the parent case or will it be long enough after forming?

[Michael Johnson]

XTR, I have a Shiloh 45-100 with a 40-90bn nearing completion. When I took annealed 45-100 Starline cases that had been shot 22 times and ran them thru a RCBS 40-90bn die they average 2.626 in length. RMC brass for 40-90bn averages 2.642 in length. I also have Buffalo Arms reformed (from 45-2.6) that averages 2.630. I never trimmed my original Starline cases. So they have probably stretched a small bit over the 18 years I have shot my 45-100 Shiloh. I imagine BACO stretches their 2.6 brass when they reform it as well. My 40-90bn will have a 1/14 twist with a heavy 32 inch round barrel.Planning on shooting PP with it.I hope this helps you out.

[XTR]

Here is another question that goes straight to BC.

What are those shooting the 40-90bn needing for a come up from 100 ( or 200) to 1000, and how does that compare with a similar bullet shape in a 45?

A hundred FPS is nice, but MV is temporary, BC is king beyond 600 yds in most applications.

I know that elevations are greatly bullet nose shape, so the idea is comparable shapes compared.

[Don McDowell]

Making 40-90 bn cases from 45 2.6 is pretty straight forward, run them into a .40-65 die, load with powder and bullet and fire form.

[PacRATT]

I've made sample rounds of 40-90 BN from new 45-100 Starline brass and some 45 HDS basic , it seems to match the reamer.

[PacRATT]

I have a set of CH4D 44X77 BN Sharps dies.

[XTR]

It sounds like making the brass isn’t bad then.

[bruce m]

jim,
length and nose shape and twist are what it is all about.
the bullet that fits that will have a weight as a result.
we have been talking about length and twist so far, but nose shape also matters.
a bullet , still or in flight, has a centre of mass.
hopefully for accuracy this is at some point on the axis of the bullet.
once in flight, the bullet also has a centre of pressure.
the higher the drag on the nose, the further forward is the centre of pressure.
the distance between these 2 points is known as the overturning moment.
the longer the overturning moment, the faster the bullet needs to spin to remain stable.
there are 3 ways to shorten the moment for increased stability.
first is to shorten the bullet.
however for long range shooting we want longer bullets, as they are heavier and as such have a higher b.c. to reduce wind deflection.
second is to put more mass in the nose and less in the base, by having a blunter nose and a hollow base.
this i suspect was how the old dead guys got away with slower twists.
third is to reduce drag on the nose.
nose drag is reduced caused by meplat diameter and ogive length.
here is the area modern bp shooters are working on these days.
of the 2 things, length of ogive has the most to offer assuming a sensible meplat .
modern vld jacketed bullets are pushing the envelope here, but cast bullets have their limitations due to setback of the nose.
we can all get into big arguments over ideal ogive lengths.
in my opinion, 1.5 calibres is a realistic max length up to and including 40 cal, and a bit less in 45 cal.
however i have an open mind.
harder alloys are your friend here, and the work arnie is doing with antimonial alloys and dual diameter bullets looks interesting.
of course as you lengthen the ogive you reduce the weight of the bullet.
drag and bullet wt both come into the b.c. equation, so somewhere in there will be a balancing act.
the video you linked is very interesting, but as you say did not include our shaped bullets.
there are many videos and pictures like that of pistol bullets which might be more like ours.
i have seen one where a pistol bullet was hit in mid flight by a shotgun pellet, wobbled, and then went stable again.
the shockwave in front of our bullets seems to have more of a curve than like a point as in the video.
those little shock waves coming off the side of the subsonic bullet demonstrate our transonic problem.
airflow on top of the bullet has higher speed than on the bottom due to trajectory, so the shock waves will be different.
ultimately we need enough gyroscopic stability to overcome dynamic instability and other issues like windshear to have equipment at the pointy end of competition.
for the ammount of work it takes to get to a competition, you want to give yourself the best chance.
if you hope to give brent, zack, jim, kenny, and a few others a pizzling, you need something that does not put you behind the starting point.
if you want to go for beer and chips have a good time anyway.
bruce.