Leade / Throat Angle... 3/4degree - 4degree? Which is better?

[jb1000br]

Recent PS articles have me wondering...

Seems Jim Boatright prefers a 4 deg angle for VLD's,8 deg total) while we conventionally run 1.5deg, 3 deg total) for normal tangent ogive bullets,SMK) and David Tubb,IIRC) prefers 3/4 deg. per side,1.5 deg. total) for his 6XC chambers.


I did not see any proof,other than theoretical) in Jim's writing, or elsewhere to show one angle is better than another...

Am I missing something?

Does anyone have some evidence to show one is preferred?

JB

 

[KHGS]

Good theory, and as he alludes, would require setting the barrel back a thread every so often,fairly often depending on chambering) to maintain the leade angle. The angle begins to change with each shot fired. Shallower angles tend to last longer & maintain their actual angle better. I still like 1 1/2 degree angles as the best compromise.
Keith H.

 

[DTubb]

From my experience when I changed,in a 7'08) from a 3 degree lead to a 1 1/2 degree lead I got a little more than 100 fps increase in velocity with the same pressure signs.
This was the same barrel and same components and the bullet was seated into the rifleing.

The more easily the projectiles transition into the rifleing the less distortion would seem plausible to me.

I cant really say that I get any more velocity with a 3/4 degree lead over a 1 1/2,when I did those tests it was less than 20 fps difference)but there is no question that the bullet certainly is eased into the rifleing to such a degree that it is sometimes hard to measure,as compared to a 1 1/2 lead which acts like running into a wall with the bullet - there is no question whether you are into or off of) where exactly the bullet is in relation to the rifleing.

I feel that jumping the bullet to the rifleing is prefered with the 3/4 lead,since you dont worry so much aboutg seating depths).

I would also say that my long range elevation is more consistent when jumping to the rifleing,10-20 thou off).

There are some very notable long Range shooters that are using 1/2 degree leads.
DTubb

 

[EricStecker]

JB,

I relayed some thoughts on this subject to your post on benchrest central 1,000 yard forum. Since others may not see both forums I'll share those thoughts here.

Remember the point of Jim's article was to address alignment. His goal is improved alignment so that greater accuracy can be achieved. To that end his suggestions are worth considering as a 4 degree angle allows a secant ogive,VLD) bullet to be held,or first touched on a jumped bullet) by the portion of the angle that is near the tops of the lands. A 3/4 or 1.5 degree angle will touch a tangent ogive bullet at the portion of the angle that is near the top of the lands but it does not do this on a secant ogive bullet. If it is better for a tangent ogive to be touching the portion of the angle near the top of the lands then it is reasonable to suggest this would be better for secant ogives as well.

This situation has nothing to do with velocity. If you think more velocity is important then use more powder. More velocity does not always equal accuracy. Consistency and alignment does.

For Jim's suggestion to be an improvement for some it is important to know how the 4 degree angle affects accuracy if the secant ogive bullet is jumped. Even though we know that putting bullets that have the same ogive shape into the rifling usually produces better accuracy I understand that shooters have reasons for jumping the bullets,to avoiding sticking a bullet and dumping powder in the action).

Another item that must be addressed is the throat wear. If it is true that a 4 degree angle turns into a 1 degree angle after a few hundred rounds when then this is a waste of time. I believe that if throats erode at this rate then it would make sense that a 1 degree angle would become a freebore in the same number of rounds. Why would this erosion stop at 1 degree? I understand that the angle is different but is it that different. I wonder.

Certain cartridges and powders erode throats faster than others. It may be proven that with the right cartridge and powder combination the improved accuracy would be well worth the effort of finding the combination that does not erode the lead angle at a rate that is unacceptable.

If a 4 degree angle does make a difference it will go a long way in explaining why secant ogive VLD bullets are difficult to "dial in". It will also have an effect on the realized BC of the VLD bullet as better in bore alignment produces a bullet that will stabilize sooner reducing the excessive drag that results from this initial in flight instability. The fact that most,if not all) leade angle are 1/2 to 1.5 degree could be the very reason why VLD bullets are considered sensitive to seating depth.

The best way to know if the 4 degree angle is better for secant ogive bullets is for someone credible to do some controlled testing. As I suggested on the other forum the reamer manufacturers have the most to gain by this situation. If secant ogive bullets are consistently more accurate with a 4 degree angle and tangent bullets are consistently more accurate with a 1/2 to 1.5 degree angle reamer sales would double overnight.

Regards,
Eric Stecker
Berger Bullets

 

[jb1000br]

David and Eric, thanks for the comments...I'm about to be at my limit of testing things soon, so I may have to stick with convention at this point. Hoping to get some results from those that have actually tested this as well...

JB