Ballistics Modelling with the McDrag Program

[ChrisK]

I really like all the work that you have put into this post, very informative. I think that a shorter bullet will track better in the transonic range because of yaw and pitch. The picture because gives a good idea what is happening.
Chris

By Brian Litz

[rsterne]

Longer bullets are less stable, as you say, hence why they require a faster twist barrel to spin them up to a higher RPM.... The gyroscopic stability is thereby increased.... The least stability occurs right around Mach 1, a well known fact, and if your bullet is passing through that speed, you need the fastest twist (highest RPM) for that part of the flight profile.... If it is stable at Mach 1, it will be stable at all velocities....

bob

[ChrisK]

You might find the following interesting, I also subscribe the twist rate rule to stabilize. I shoot a custom 300WM at around 2800 fps using 208 and 220 grain bullets. I also use Brian Litz's Applied Ballistics calculator on my Android phone to calculate the hit at distance with a 100 yard zero.

What I suggest is that your barrel has the fastest twist rate available. You will not be spinning the copper jacket off any of your bullets anyway.

Bryan’s book "Modern Advancements in Long Range Shooting" 

By Brian Litz
To determine how bullets perform in the “transonic zone”, Bryan did a lot of testing with multiple barrels and various twist rates, comparing how bullets act at supersonic AND transonic velocities. Bryan looked at the effect of twist rates on the bullets’ Ballistic Coefficient (BC). His tests revealed how BC degrades in the transonic zone due to pitching and yawing. Bryan also studied how precision (group size) and muzzle velocity were affected by twist rates. You may be surprised by the results (which showed that precision did not suffer much with faster barrel twist rates).

[ChrisK]

By the way Bob, I think your designs look swell and will work as intended.

What I guess that I wanted to point out to people is to make sure the twist rate of the barrel is at least fast enough to stabilize the weight of the bullet for an unknown barrel to you.

Chris

[rsterne]

Decreasing BC with insufficient spin is a fact, and the modern trend in benchrest is back to a Stability Factor at Mach 1 (worst case) of 1.5 instead of the 1.2-1.3 that shooters were playing with a few years back....

Bob