Will Someone Please Enligten Me?

[mugsie]

I have been reloading for years with for me is success (still have all my fingers and toes and both eyes still work). I work up a load for my rifles, then because I like to fiddle, I change something, bullet, powder, primer, brass, action screw torque, even barrels at times. I like to fiddle. So, were I need enlightenment is in the statement "boattail bullets need more distance to stabalize". What does that mean? Do they wobble? If they wobbled they would wobble to the point where they tumbled. Does the nose bounce up and down? What does stabalize mean? They're doing 30k some revolutions a minute so I don't see them going off axis. All I know is if I shoot boattails they go where my rifle points. If I shoot flat base they go where my rifle points. What does one mean when they say a bullet takes more time to stabalize?

Respectfully,
grasshopper......

[Blue Avenger]

nutshell...it means at 200 yards you might get a better group then at 100 yards

[Grit #1]

Some of them are actually spinning in excess of 250,000 rpm.
Best regards,
Grit

[Celtic Warrior]

They do wobble about their CG until they become "spin" stablized (at super sonic speeds). Yes, the spin rpm can be very fast http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/ and that is part of the determination of where the bullet stops or slows it's wobble or "goes to sleep".

When a bullet goes sub sonic and also loses some of it's spin rate it becomes unstable (wobbles) again, until it reaches a point where it does tumble.

I read an artical awhile back about why 5 shot groups are better than 3 shot groups. the premis was that because of stability bullets go to sleep during different times during flight (caused by MV, stability, different CG, etc.). These different times cause what appears to be flyers but the flyers are are caused by the bullet hitting the target in a wider arc of the wobble at the nose of the bullet. (The 5 shot group allows from the flyer, or whatever it is to be thrown out of the analysis and still leaving a sample large enough to have validity.) This made sense to me but I would really like to see high speed film showing this happening.

[82boy]

The hole BT bullets are not good at 100 yards thing is an old wifes tale. The proof is that the new fad in short range benchrest is to shoot match grade BT bullets (Hotenstiens, Denoffs, ETC) in a 1 in 13 twist barrel. I have shot many of 52gr SMK's (They are a BT bullet.) at 100 yards, and never seen them have a problem. The claim or theroy comes from the way the gas flows over the boat tail, when exiting the muzzel. I would say that the possible problem would lie in the design of the bullet when produced, when a BT is made it is one more step in the process, that can go south. I would say that the BT bullet can be prone to mistakes. A bullet is either stabilized or not, there is a yaw factor when the bullet spins that may be interped as stabilization.

[fgw_in_fla]

250,000 RPM?!?!?!

Geez... Wht's holding these things together? I've heard stories about bullets exiting the muzzle & turning to dust due to excess spin. I had no idea they spun that fast.

As far as BT vs FB...
I've been using primarily BT's .... religiously. As I recently (within the past 3 months) started using FB & "match" grade bullets, I really don't see a whole lotta difference.
You load 'em
You OCW 'em
You find the good node(s)
Load again with appropriate amounts on propellant.
Shoot, marvel at the accuracy you've discovered.
Repeat as needed.

I see Nosler BT's shooting as tight as Hornady "match" FB and BT. I really would like to see one explode in flight, though.... :o

[mugsie]

Thanks guys. I guess there really isn't much difference. I'm pushing the single hole at 100 yards holy grail quest and didn't know if there was a significant difference in bullets. I shoot Nossler 168g HPBT as well as Hornady 155g A-Max bullets. I have been using Varget, but just switched over to BLC(2) - mostly for ease of metering. The little mouse turds give me fits when metering. As far as difference in accuracy goes, most every deviation from perfect I find is caused by the nut behind the trigger, not the one on the barel!

Thanks again for the replies. I'm going to the range again this evening with 12 more 155 grainers and 46.5, 46.7, 46.9 and 47.1 grains of BLC(2) to see which yields the smallest groups (these numbers are a spread around the node somewhere between 46 and 47 grains tried earlier.

Stay well and good shooting.....

[darkker]

250,000 RPM isn't "fast" either. Here is the formula:
MV * 720 / twist = RPM

So a 223 example shows this: 3400fps * 720 / 9 = 272,000 RPM

This is merely what the manufacturers have told me, NOT Gospel, barrel smoothness dependant, etc.
.224 Cal 55 gr bullets:
Dogtown/Varmint nightmare - 240,000 - 260,000 RPM limit
VMax/SP - 290,000 RPM
BlitzKing - 352,000 RPM
Varminter - 216,000 RPM
TNT(low vel version) - 250,000 RPM

As to "going to sleep" and stability. Gas flow over the boat tail isn't the primary issue.
It has to do with A LOT of factors. A bullet's problem is that the center of pressure(CP) is in front of it's Center of Gravity(CG). There is NO 1 BC for a bullet, it changes with velocity. When a bullet's speed decays and enters the subsonic area the CP gets a radical shift forward and tends to tumble since it is no longer dynamically stable.

The longer the bullet, the more the CG/CP discrepency, spin must be increased to dynamically stabilize it. Bullets spin and their center of Mass, is NEVER exactly aligned, and a yaw is introduced. This slight "side-ways skidding" introduces a High & low pressure variance on the bullet, also causing an attempted "climbing effect". This effect is worse with longer(usually bt bullets) and takes longer to have the forces align, thus causing the bullet to "go to sleep".

[mugsie]

so if there was a method to verify CG for each particular bullet, then by sorting bullets using the same CG we could eliminate one more variable and further increase our chances of perfect accuracy?

Hmmm? I think I'll go invent a whatchamacallit to measure bullet CG's. Then I'll go out and win some national BR competition, have someone do an article on me and ask me "to what do you attribute your perfect accuracy to?" to which I'll answer - well my friend, it's the whatchamacallit I invented! Without this little baby in my arsenal, I could never have done it" I'll make millions on people touting the effectiveness of the whatchamacallit! Ahh yes - that's the ticket......

(OK - off to the range......)

[Eric in NC]

The hole BT bullets are not good at 100 yards thing is an old wifes tale. The proof is that the new fad in short range benchrest is to shoot match grade BT bullets (Hotenstiens, Denoffs, ETC) in a 1 in 13 twist barrel. I have shot many of 52gr SMK's (They are a BT bullet.) at 100 yards, and never seen them have a problem. The claim or theroy comes from the way the gas flows over the boat tail, when exiting the muzzel. I would say that the possible problem would lie in the design of the bullet when produced, when a BT is made it is one more step in the process, that can go south. I would say that the BT bullet can be prone to mistakes. A bullet is either stabilized or not, there is a yaw factor when the bullet spins that may be interped as stabilization.


I don't think it is really "boat tails aren't good at 100 yards" its "REAL long bullets aren't always good at 100 yards - no matter what the base design is". I only see the "less MOA at 300 than at 100" when I am shooting really heavy/long for caliber bullets (not 52 grain 224 SMKs but 77 grain 224's or 240 grain 308's etc.). Most pronounced examples I have ever seen are on heavy black powder cartridge bullets (410 grain 40 cals and 550 grain 45 cals) often see about the same size group at 300 that you saw at 100 - all those are flat based.

[fgw_in_fla]

Geez.... All this technical stuff. You guys are gonna make my brain explode & leak out my left ear. :o

How about we all agree:
All rifles / guns will shoot differently given the same specs, criteria, ammo, etc...
Bullets go very very fast.
Sometimes you shoot one hole groups... Sometimes it looks like a buckshot pattern.
Bottom line - if you're within 1in of your POA, what you were aiming at - it's dead.
Can't we all just get along?

Just a few tid bits of useful info from your 'ol Uncle Frank ;)

[Grit #1]

What the hell is wrong with your right ear?
LOL
Best regards,
Grit

[mugsie]

Just a quick update on the range session. The first group using BLC(2) was virtually one hole. 155g A max 46.5g BLC(2), Federal LR Magnum primers, PPU Brass (what I had at the moment. 46.7 opened up a bit, 46.9 opened up a lot more and I never got to shoot the 47.1's because I got a call from my alarm company saying the house alarm went off and the police were responding. I didn't even get to pick up my target! Alarm was a false alarm - battery was weak in the circuit and triggered by itself. Only good thing is I now know what combination shoots best using the above components.

Shoot well.....

[Celtic Warrior]

so if there was a method to verify CG for each particular bullet, then by sorting bullets using the same CG we could eliminate one more variable and further increase our chances of perfect accuracy?

Hmmm? I think I'll go invent a whatchamacallit to measure bullet CG's. Then I'll go out and win some national BR competition, have someone do an article on me and ask me "to what do you attribute your perfect accuracy to?" to which I'll answer - well my friend, it's the whatchamacallit I invented! Without this little baby in my arsenal, I could never have done it" I'll make millions on people touting the effectiveness of the whatchamacallit! Ahh yes - that's the ticket......

(OK - off to the range......)




It has already been done for us. The back end of Bryan Litz's book Applied Ballistics for the Long-Range Shooter has tables of common bullets that he has compiled the form factor for, which considers everything everyone has been saying and more. The data includes BC at by velocity, Stability Factor by twist rate.
But as Mugsie stated, even knowing all this and loading rnds that consider it my issue is still the loose nut behind the trigger.

[82boy]

I don't think it is really "boat tails aren't good at 100 yards" its "REAL long bullets aren't always good at 100 yards - no matter what the base design is".

It depends on the barrel, some barrels will do well with VLD bullets. My Shilen, and my Criterion barrels will both shoot VLD bullets like they was a light weight FB bullet. I have shot many 5 shot 100 yard groups in the .3's to even the high .1's with these barrels, shooting long bullets, such as 107 SMK's, 108 BT Bergers, and 108 FB BIB's.

[MrMajestic]

Think this one will go to sleep? Distance 23 yards! ::)
http://i1215.photobucket.com/albums/cc505/majesticcnc/StabilityQuestion.jpg

[GaCop]

I don't think it is really "boat tails aren't good at 100 yards" its "REAL long bullets aren't always good at 100 yards - no matter what the base design is".

It depends on the barrel, some barrels will do well with VLD bullets. My Shilen, and my Criterion barrels will both shoot VLD bullets like they was a light weight FB bullet. I have shot many 5 shot 100 yard groups in the .3's to even the high .1's with these barrels, shooting long bullets, such as 107 SMK's, 108 BT Bergers, and 108 FB BIB's.


+1! I recently got a .199" group at 100 yards with the 155gr A-Max and a .686" group at 300 yards using RL-15.

[mugsie]

Mr Majestic - was that fired from a slingshot?! Ouch!

[bigedp51]

I have a few milsurp Enfield rifles with cordite bore erosion and muzzle wear, and I can tell you that a normal boat tail bullet will not upset and fill the bore of the rifle like a flat base bullet will. (Obturate) This can cause poor accuracy to shotgun pattern key-holing, my worn Enfield barrels like long flat base bullets that fill the bore.

What a bullet needs is stability and to be the correct size for the bore for good accuracy.

http://i122.photobucket.com/albums/o254/bigedp51/balancedflight.jpg

http://i122.photobucket.com/albums/o254/bigedp51/boattail.jpg

[mugsie]

BigEd,

Great explaination - thank you - now, where can I get some rabbited base boat tail bullets? I'd like to try them myself.