Barrel twist 40-65 for silhouette

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beltfed
Posts: 1962
Joined: Sun Jun 03, 2007 10:07 am
Location: Central Wi

Re: Barrel twist 40-65 for silhouette

Post by beltfed »

The SD does not tell all, at long range.
Yes, a Lyman Snover 410663 at 1.37 or so length and around 410 gr will
perform well at out to 600yds or so from my 16 twist 40-65.
It will shoot "ok at 800. It will then start to "open up" at 900.
At 1000yds It becomes "Spray and Pray"
Tell me how I know.
OTOH, a 1.325" long Elliptical ogive-- gg or Paper patch , tapered to seat out
IS proven stabile out to 1000yds. 99-3X at 800, still a 88 at 1000yds. And round holes in the paper.

That being said, I would Still go with a 14.5 twist barrel if I built another 40-65. Or a 40-70,etc
beltfed/arnie
SchuetzenDave
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Joined: Mon Jan 07, 2013 9:07 am
Location: St. Albert, Alberta

Re: Barrel twist 40-65 for silhouette

Post by SchuetzenDave »

For a 14:1 twist barrel the Sg for the .40 caliber BACO bullets are:
400 grain Money 2.54
415 grain Postell 2.80
420 grain Money tapered first band 2.41
420 grain Spitzer 2.83
430 grain Credmoor 2.81
430 grain Creedmoor tapered 1st band 2.81
430 grain NASA 2.87
444 grain NASA 2.74
460 grain Money tapered 1st band 2.40
460 grain Creedmoor 2.40

It is known that accuracy starts to degrade if you overspin the bullet (Sg> 2.0) too much.
Many suggest the point is if you exceed an Sg of 2.5 and more specifically if the temperature is high or the atmospheric pressure is low.
Thee Sg also increases by higher velocities of the bullet.
My Sg calculations were made with a velocity of 1,200 fps.
beltfed
Posts: 1962
Joined: Sun Jun 03, 2007 10:07 am
Location: Central Wi

Re: Barrel twist 40-65 for silhouette

Post by beltfed »

SDave,
Could you please add this bullet to the SG calculations:
380 grain, 1.325" long .403 base diameter/0.395" body dia./Elliptical ogive Paper patched blended into hemi meplat,
Chrono'd at ave. 1402fps in 16 twist barrel.
This shoots well from 100 out to 1000yds if I do my part.
beltfed
Posts: 1962
Joined: Sun Jun 03, 2007 10:07 am
Location: Central Wi

Re: Barrel twist 40-65 for silhouette

Post by beltfed »

Dave, it would also be good if you listed lengths of all those bullets.
beltfed/arnie
SchuetzenDave
Posts: 864
Joined: Mon Jan 07, 2013 9:07 am
Location: St. Albert, Alberta

Re: Barrel twist 40-65 for silhouette

Post by SchuetzenDave »

Bullet length:

400 grain Money 1.400"
415 grain Postell 1.370"
420 grain Money tapered first band 1.450"
420 grain Spitzer 1.370"
430 grain Credmoor 1.385"
430 grain Creedmoor tapered 1st band 1.385"
430 grain NASA 1.375"
444 grain NASA 1.413"
460 grain Money tapered 1st band 1.510"
460 grain Creedmoor 1.500"
SchuetzenDave
Posts: 864
Joined: Mon Jan 07, 2013 9:07 am
Location: St. Albert, Alberta

Re: Barrel twist 40-65 for silhouette

Post by SchuetzenDave »

Caliber 0.403 Inches
Bullet Weight 380 Grains
Bullet Length 1.325 Inches
Barrel Twist 16 Inches/turn
muzzle velocity 1402 fps
Temperature 50 degrees Fahrenheit (59 is standard)
Pressure 29.92 inches of mercury (29.92 is standard)

Sg = 2.22

Without grease grooves there is less need to overspin the bullet.
We try to overspin the bullet to compensate for the drag on the grease grooves over longer distances (Dan T.'s last investigations).
And the Elliptical nose provides good central balance and good dynamic stability (less yaw) over long range.
bruce m
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Location: australia

Re: Barrel twist 40-65 for silhouette

Post by bruce m »

dave,
not sure what you mean by yaw.
if you refer to yaw of repose, which is the angle of the bullet in flight after precession and nutation have happened, this is a function of spin rate.
longer bullets that are stable will have a greater yaw of repose simply due to requiring a faster twist to be stable.
overall their angle of attack will be more nose up in relation to the tangent of trajectory, and more to the right (for a right twist) than shorter slower spinning bullets.
the longer faster spinning bullets also take a greater distance of flight to completely precess and nutate than shorter slower spinning bullets.
this is why for shorter ranges slower twists and shorter bullets can be marginally more accurate.
at longer ranges we look for some forgiveness in the wind by using longer higher b.c. bullets.
the increased yaw of the longer bullet causes greater spindrift.

digressing I will mention another twist experiment here.
looking for max optimum bullet length for a 1:13" twist I started at 1.55".
at first glance this bullet was stable out to 1200 yds.
however closer observance of targets revealed that at around 700 meters holes in the target were not quite round.
before and after 700 they were round.
reducing the bullet length to 1.5" solved this problem.
if shooting at sil ranges the problem would of course not have existed.
bruce.
ventum est amicus meus
SchuetzenDave
Posts: 864
Joined: Mon Jan 07, 2013 9:07 am
Location: St. Albert, Alberta

Re: Barrel twist 40-65 for silhouette

Post by SchuetzenDave »

I am discussing the increased angle of yaw that occurs from the movement of the center of gravity backwards.
CG moves backwards depending on the amount of material removed from the nose.
CG can be moved back forward by removing enough material from the grease grooves.

This dynamic shift in the CG with an increased angle of yaw is most noted with Spitzer bullets; which have more material removed from the nose; moving the center of gravity further backwards.
High speed photography indicates these bullets have more angle of yaw than flatter nosed bullets like the Creedmoor or Elliptical.

With higher velocities the yaw is easily stabilized.
Unfortunately with black powder and cast bullets we cannot stabilize this yaw without stripping the bullets or get enough powder in to achieve the velocities to stabilize bullets with more angle of yaw.
These bullets tend to loose their dynamic stability at longer distances (go to sleep) resulting in poorer accuracy at longer distances.
Bullets with a more central Center of Gravity do not have as much yaw and are more dynamically stable at longer distances.

Best approach for BP is select a bullet with a flatter nose (center of gravity further forward) and GG bullets with deeper grease grooves which also moves the CG forward.

Works better than trying to drive cast bullets fast as " " to stabilize or reduce the angle of yaw at longer distances.
SchuetzenDave
Posts: 864
Joined: Mon Jan 07, 2013 9:07 am
Location: St. Albert, Alberta

Re: Barrel twist 40-65 for silhouette

Post by SchuetzenDave »

Basic ballistics says to obtain accuracy you FIRST need to have bullets that are gyroscopically stable.
Proper balance of bullet length to rifle twist.
For long range accuracy you need to have bullets that are dynamically stable.
Have the appropriate CG that reduces angle of yaw or speed it up fast enough to stabilize the yaw and eliminate affects of drag over longer distances.
martinibelgian
Posts: 1610
Joined: Thu Oct 26, 2006 1:20 pm

Re: Barrel twist 40-65 for silhouette

Post by martinibelgian »

In other words, use a 457125 govt. or a postell bullet. Not really....
bruce m
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Location: australia

Re: Barrel twist 40-65 for silhouette

Post by bruce m »

dave,
I suspect you are talking about the distance between the centre of mass and the centre of pressure of a bullet in flight.
this distance is called the overturning moment, and the longer it is, the more spin is required to keep the bullet stable.
certainly moving the centre of mass forward reduces the overturning moment.
however, putting more mass forward by bulking up the nose and bluntening it moves the centre of pressure forward too, possibly negating it.
removing mass from the rear 1/2 of the bullet by increasing grease groove dimensions might be the better choice of the two.
however the big loss here in both cases is a reduction in b.c., which while not of much effect out to ram range, makes long range that bit harder due to increased wind deflection.
your suggestion that the elliptical nose is a good compromise could be on the money.
this is close to the nose shape used in latter times of creedmoor, referred to in the day as the pointed bullet.
bruce.
ventum est amicus meus
BFD
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Joined: Mon Jan 21, 2008 7:36 pm

Re: Barrel twist 40-65 for silhouette

Post by BFD »

Which ellipse nose?

An ellipse is family of 2 dimensional shapes that range from a perfect circle to within epsilon of a straight line. You guys say "ellipse" like that says it all.

Bullets are 3 dimensional.
beltfed
Posts: 1962
Joined: Sun Jun 03, 2007 10:07 am
Location: Central Wi

Re: Barrel twist 40-65 for silhouette

Post by beltfed »

Brent,
Absolutely right.
In other words, we can have a longer or shorter elliptical ogive/"nose".
I like about the "nose" to be about 45% of the overall length of the bullet.
But, to get more weight, one could go shorter nose, sacrificing bc for knock down as in Sillywet, the base subject line of this thread.
And some of us do use what we should call a Mod Elliptical "nose" where we blend the ogive into a hemi meplat
to hopefully reduce deleterious effects on the bullet by headwinds.
beltfed/arnie
SchuetzenDave
Posts: 864
Joined: Mon Jan 07, 2013 9:07 am
Location: St. Albert, Alberta

Re: Barrel twist 40-65 for silhouette

Post by SchuetzenDave »

There are many variables that affect a bullet's flight and I initially discussed Gyroscopic Stability and Dynamic Stability.

You are quite right indicating the BC (resistance to air flow - drag) is also influenced by nose shape.

However does a poorer BC result in poorer accuracy or just result in more drop over distance?

This is more important for hunting (flatter trajectory) or having your bullet reach further distances; but does it really affect accuracy?

For BPCR Silhouette we are shooting fixed distances so we have already determined the drop of our bullet so does BC really matter?

Yes there are slightly different shaped noses on various Elliptical or Money bullets but in general categories The Spitzer is pointier and more tapered than the Money, than the Elliptical, and then the Creedmoor and finally Government Round Nose.

However if you are concerned about a better BC for long range shooting you may wish to start differentiating between the different noses on the various Money or Elliptical bullets.
BFD
Posts: 2789
Joined: Mon Jan 21, 2008 7:36 pm

Re: Barrel twist 40-65 for silhouette

Post by BFD »

SchuetzenDave wrote: Fri Jan 17, 2020 8:43 am
However does a poorer BC result in poorer accuracy or just result in more drop over distance?

This is more important for hunting (flatter trajectory) or having your bullet reach further distances; but does it really affect accuracy?

For BPCR Silhouette we are shooting fixed distances so we have already determined the drop of our bullet so does BC really matter?
BC affects wind drift. Higher BCs have less drift and thus if you miss a wind call by say 10%, that 10% will be larger with a low BC than a high BC bullet.
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