375 CHEY TAC twist rate

XLR

XLR

boss man
Commercial Supporter
Full Member
Minuteman
May 10, 2009
751
228
40
Grand Junction, Colorado
www.xlrindustries.com
#1
I have a question, It seems that either shooters are using a 11.5 or 8 twist barrel in their 375 chey tac rifles. It appears that all bullets made for the chey tac are solids; therefore, there is no core stripping occuring. What is the disadvantage of just going with an 8 twist?
 
#2
Re: 375 CHEY TAC twist rate

If you are going to use the 350 SMK the 11.5 twist is the way to go; however, if you wish to optimize the 375/408 you will want a tighter twist. I have an eight twist that is too slow for the 6.5 and 7 cal bullets Noel is making. It dont really like the 350 SMK, the Rocky Mountian jacketed hollow point came apart. Noel is sending me some 6 cal solids that should work well in the 8 twist, I will post how they work as soon as I know. I have a 13-6.5 twist barrel on the way from Bartlien that should work well with Noels 6.5 and 7 solids. There are some jacketed alum tipped bullets that should be available soon from various sources. To make a long story short, there is much work to be done before this chambering is optimized and perfected. My belief is that the 6.5 and 7 cal bullets fired from 6.5ish gain twist barrels will bubble to the top. I know this probably just muddies the water for you but there are simply more questions than answers right now. Right now the 11.5 twist using the SMK and the Lost River/Hooker solids are the only game in town that has any extensive testing. I dont think the 8 twist has much of a future unless it is with the jacketed alum. tipped bullets or Noels 6 cals. The projectiles with the most potential for high BC are giong to require the 6.5 twist (I THINK) Hope this helps.
 
#4
Re: 375 CHEY TAC twist rate

Yes, overstable is possible and gets more noticable at long range such as the Chey-tac is built for. It makes the bullet not want to nose into the flight as it goes over it's arch of flight. Gyroscopic effect takes over.. Augustus seems to have alot better knowledge than me on this.
Also as noted above, to fast of twist can tear the bullet apart.
 
#5
Re: 375 CHEY TAC twist rate

I have a 1:10,5 in the works for shooting the new GSC 355 grn SP
bullet, this twist will also work with 350 grn SMK.
This is for a repeater action rifle with magazine, no need to get a twist for a bullet that will not fit in the magazine
grin.gif


http://www.gsgroup.co.za/375355SP173.html

Oh I also have a LGT in the works
wink.gif
for the ZA375/7.0 looking forward to seeing those needles fly
grin.gif



Cheers,

Master Diver
 
#6
Re: 375 CHEY TAC twist rate

CMM,

Both excessive, and inadequate, static stability (spin) generate conditions causing dynamic "nose up" instability relative to the trajectory, as traced through the CG, which are exaggerated in high aspect-ratio (6.0+ caliber) projectiles. The actual mechanics between the two situations are quite different however.

The stability issue Augustus experienced, from his 8" twist barrel, has to do with the destabilizing, but ever present, "overturning moment" induced by the center-of-pressure located forward of the center-of-gravity, overcoming the counter-force of gyroscopic resistance to axis change about the CG. All spin stabilized projectiles balance these two forces in a finely tuned dynamic tension which "tracks" the trajectory path in conjunction with the tail configuration's ability to act as the aero-ballistic equivalent of a horizontal stabilizer.

When the gyroscopic force decays more rapidly than the overturning moment, the aerodynamic "lift" is immediately translated into yaw (side angle off-set) as a consequence of ninety degree out-of-phase shift in pertubation force. This is the natural reaction of any gyroscopic axis altered by an outside force. A "coning" precession is initiated at this point affecting projectile accuracy initially, and culminating as a "tumbling" motion in which the bullet flies tail forward in a reverse "coning" orientation. This can take place at any location in a projectile's trajectory depending on the transitional balance of counter-forces.

The dynamic instability induced by "over-revving" occurs only at the far end of a projectile's trajectory. In this situation, static overstability prevents "tracking" resulting in a drag generating, nose-up, attitude which culminates in an abrupt lateral roll in the direction of spin and, once more, a reverse coning orientation.

... and this is the simplistic explanation of what happens
wink.gif
. Other forces are also at play. That is the reason Augustus (correctly) stated that perfecting the 375-408 cartridge is still in process.

Best,
Noel





 
#7
Re: 375 CHEY TAC twist rate

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Augustus</div><div class="ubbcode-body"> If you are going to use the 350 SMK the 11.5 twist is the way to go; however, if you wish to optimize the 375/408 you will want a tighter twist. I have an eight twist that is too slow for the 6.5 and 7 cal bullets Noel is making. It dont really like the 350 SMK, the Rocky Mountian jacketed hollow point came apart. Noel is sending me some 6 cal solids that should work well in the 8 twist, I will post how they work as soon as I know. I have a 13-6.5 twist barrel on the way from Bartlien that should work well with Noels 6.5 and 7 solids. There are some jacketed alum tipped bullets that should be available soon from various sources. To make a long story short, there is much work to be done before this chambering is optimized and perfected. My belief is that the 6.5 and 7 cal bullets fired from 6.5ish gain twist barrels will bubble to the top. I know this probably just muddies the water for you but there are simply more questions than answers right now. Right now the 11.5 twist using the SMK and the Lost River/Hooker solids are the only game in town that has any extensive testing. I dont think the 8 twist has much of a future unless it is with the jacketed alum. tipped bullets or Noels 6 cals. The projectiles with the most potential for high BC are giong to require the 6.5 twist (I THINK) Hope this helps. </div></div>

The Rocky Mountain bullets i sent to you were a test bullet that we were playing around with. I didnt expect good results out of them. The latest one I sent you should work good.
 
#8
Re: 375 CHEY TAC twist rate

Thanks, also the first ones may work fine out of a slower twist. I would like to add that The Rocky Mtn Bullets were sent for testing purposes only and like others are a work in progress. One should not form any opinions about any of the folks involved in development of the 375 projos based on the testing done so far. All of them are trying very hard to come up with a wide range of viable 375 bullets and all should be applauded for their efforts. In the near future I think there will be a wide range of bullets for barrels with twist rates from 11.5 to the fast gain twist.
 
#9
Re: 375 CHEY TAC twist rate

Should I be reconsidering the 1:10 twist barrel I was going to use on the 350 Gr SMK bullets?

I have been considering getting a second barrel down the road for solids but hoped to be able to get good performance out of the 350 SMK's out to 2000 yds. What would be considered the best twist rate for the 350 SMK's?

Thanks
 
#13
Re: 375 CHEY TAC twist rate

Noel,

I understand the COG vs COP, this seems logical. Knowing that this is a function of the profile of the bullet dictating the greatest mass to be at the rear, has anyone considered a way to eliminate/minimize this?

This had previously occurred to me and it seems that, theoretically, if the rear of the bullet were constructed of the copper alloy which has a much lower relative density than lead, with the non-contact nose portion constructed of lead, it would help shift the COG. You could further this by hollowing the rear of the projectile leaving enough wall thickness to resist crushing but providing a projectile shape with the required aerodynamic profile.

Ignoring the constructability issues, would this improve the spin rate balancing act in theory? Just curious.
 
#16
Re: 375 CHEY TAC twist rate

375 Mojave,

In a spin stabilized projectile, the CP must be forward of the CG. Only projectiles like arrows, bolts, and ring airfoils are designed with CP behind the CG.

The real challenge high aspect-ratio bullet design faces is countering lateral, and vertical, CP shift as sidewind and angle of attack vary. Projectile shape has the greatest effect here, but I would consider mechanical adjustment of CG as a viable design tool.
 
#18
Re: 375 CHEY TAC twist rate

Noel,

I understood that the CP would always be forward of the CG in this equation but studying your bullets made this occur to me a while back and I figured this was a good opportunity to ask the question.

It is pure theory but I just wondered if shifting the CG by mechanical means would improve the physics of the equation. I knew you would know the answer.

 
#21
Re: 375 CHEY TAC twist rate

375 Mojave,

The option of relocating the CG within a given bullet contour, and mass specification, is a useful tool in dealing with the CP shift... it is also very feasable.

The issue becomes one of cost. The six-caliber 338 solid, which will be released soon, will retail for ~ $1.30 each. A ballistically identical hunting counterpart will be coming in at around 50% more. Some of the same machining operations would be necessary to do a modified CG projectile. There may be a market for it... we will see.

Bobby,

You can not stop progress, and I can not understand why anyone would want to. ;-)

Best,
Noel
 
#22
Re: 375 CHEY TAC twist rate

ROAH,

The majority of mass need's to be behind the CP for dynamic stability, but we are focusing upon more than the simple relationship between CG/CP. In a built-up projectile, density distribution is very important.

Greater stability can be achieved, at equal rpm and mass, if material with a higher specific gravity is on the outsiide of the projectile body, relative to a less dense core, compared to an equal monolithic mass of the same configuration.

- Noel
 
#25
Re: 375 CHEY TAC twist rate

Man ! Finally somebody is thinking out of the box....You might be on to something here. We can make the gas cap out of Beryllium. Thats a very light metal for those who never herd of it.So this way you can add the weight where you want it.

Atomic Number: 4

Atomic Weight: 9.012182

Melting Point: 1560 K (1287°C or 2349°F)

Boiling Point: 2744 K (2471°C or 4480°F)

Density: 1.85 grams per cubic centimeter

Phase at Room Temperature: Solid

Element Classification: Metal

Period Number: 2 Group Number: 2 Group Name: Alkaline Earth Metal
 
#26
Re: 375 CHEY TAC twist rate

Bobby,

It has already been done, and a very long time ago for that matter.

Today we call that configuration a ring airfoil, and it works.

- Noel
 
#28
Re: 375 CHEY TAC twist rate

tried to pick up a smoothbore from Dean a couple of years ago for my Windrunner. He had been experimenting with them and accuracy was not good but if IIRC he was getting near 5,000 fps with a huge near 2.0 BC with a tungsten dart. That would have been sweet.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: lrs50bmg</div><div class="ubbcode-body">FSDS......!!</div></div>
 
#29
Re: 375 CHEY TAC twist rate

copper jacket around tungsten cylinder (tube) with a melting of zinc or aluminum center?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Noel Carlson</div><div class="ubbcode-body">ROAH,

The majority of mass need's to be behind the CP for dynamic stability, but we are focusing upon more than the simple relationship between CG/CP. In a built-up projectile, density distribution is very important.

Greater stability can be achieved, at equal rpm and mass, if material with a higher specific gravity is on the outsiide of the projectile body, relative to a less dense core, compared to an equal monolithic mass of the same configuration.

- Noel</div></div>
 
#30
Re: 375 CHEY TAC twist rate

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Noel Carlson</div><div class="ubbcode-body">Bobby,

It has already been done, and a very long time ago for that matter.

Today we call that configuration a ring airfoil, and it works.

- Noel </div></div>


Lets do it up then!!! I will build a rifle just for testing them.
 
#31
Re: 375 CHEY TAC twist rate

Mechanic,

That is the type of thinking that will produce an excellent bullet, but tungsten will run you afoul of the BATF... same with the W flechette.

Bobby... ;-)
 
#33
Re: 375 CHEY TAC twist rate

Mechanic,

I have never worked with bismuth, but the specific gravity is right. Given it's place in the periodic chart, I would expect it to have physical properties similar to lead, in which case it will fly apart at high rpm. As a point of personal interest, it would be worth looking into. Cost of manufacture will likely out-weigh feasibility.
 
#35
Re: 375 CHEY TAC twist rate

Mechanic,

I noticed some of it's alloys expand following solidification. This must be the same material chamber casts are made from. I do not do die casting, but there might good design potetial for someone who does.

Combine this with an integral copper engraving band ring, and it would probably work in producing a decent VLD projectile.

Best,
Noel
 
#36
Re: 375 CHEY TAC twist rate

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Augustus</div><div class="ubbcode-body"> If you are going to use the 350 SMK the 11.5 twist is the way to go; however, if you wish to optimize the 375/408 you will want a tighter twist. I have an eight twist that is too slow for the 6.5 and 7 cal bullets Noel is making. It dont really like the 350 SMK, the Rocky Mountian jacketed hollow point came apart. Noel is sending me some 6 cal solids that should work well in the 8 twist, I will post how they work as soon as I know. I have a 13-6.5 twist barrel on the way from Bartlien that should work well with Noels 6.5 and 7 solids. There are some jacketed alum tipped bullets that should be available soon from various sources. To make a long story short, there is much work to be done before this chambering is optimized and perfected. My belief is that the 6.5 and 7 cal bullets fired from 6.5ish gain twist barrels will bubble to the top. I know this probably just muddies the water for you but there are simply more questions than answers right now. Right now the 11.5 twist using the SMK and the Lost River/Hooker solids are the only game in town that has any extensive testing. I dont think the 8 twist has much of a future unless it is with the jacketed alum. tipped bullets or Noels 6 cals. The projectiles with the most potential for high BC are giong to require the 6.5 twist (I THINK) Hope this helps. </div></div>


Hi,

Does anyone have some new info concerning 6.5 & 7 cal bullets, and best twist rates needed to use them ?

Thanks.
 
#37
Re: 375 CHEY TAC twist rate

Armeca,

There are a six 416R LGT barrels, suitable for the ZA 6.5 & 7.0, that should be showing up on rifles in the next four to six weeks.

Augustus will be receiving his 6.0 caliber projectiles in the next five to ten days, and this will tell the story on the 8" constant-twist barrels.

Sorry for the information delay.

Best,
Noel
 
#38
Re: 375 CHEY TAC twist rate

I love Bobby's idea with trying a ring airfoil again with new technology. I seen some pistol bullets made that way. Used for terminal performance is hard material not external ballistic performance, but they still got me thinking.

If I could just come up with the cash I have a smith who can work with me on building a rifle for this projectile also. Lets keep this idea going
wink.gif
 
#39
Re: 375 CHEY TAC twist rate

Montana,

You will see a revival of this concept, and soon.

The resources of a major University (and it's supersonic wind tunnel) are playing a part in the development. I would not expect the cartridge to find wide general use due to price, but the military should be able to discover a cost effective application.

Jeff VanNiel has an interest in this project, and will likely be demonstrating it.

Best,
Noel
 
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