Hi,

I've started to put some effort into the .32-40 barrel on my CPA #52.   It's a 14 twist and I'm shooting the 200gr tapered Saeco bullet breech seated.   I'm using 41.7gr of Goex Fg and a thin card wad.

At 200 yards the bullets are definitely tipping.   My latest test was on a new target stand and backer that was definitely perpendicular the the ground and square to the shooting bench.

The bullet is 1.04" long.  I've got the bullets seated deep enough to engrave about 1/2 of the base band, but the bullet is not a perfect fit for the throat and only engraves five of the seven driving bands.  

The tipping can be seen in the attached picture.  Is it possible that it's a twist/velocity issue or should it be stable?  I wonder if it's a balance problem due to the not quite correct bullet fit?

I haven't shot this load in any significant wind.  Wind was calm with maybe a 3 mph intermittent breeze. 

Chris.
 

Thank you Frank.  I'll give those suggestions a try.  I haven't been putting a wad under the bullet base as I don't have a .32 cal wad cutter yet but will order one.  I have been doing that in my .38's though.

Chris.

Stability is a frequent topic on this forum.  I think that's a good thing because some aspects of stability are not intuitive and bear repeating.

There are two major categories of stability - static (gyroscopic) stability and dynamic stability.  Most everyone understands static stability, but dynamic stability is a bit more elusive.

With your bullet and twist rate, the bullet is unquestionably statically stable.  However, the combination of short bullet and fast twist can easily lead to dynamic instability and bullet tipping.

There is no such thing as a perfect muzzle exit.  All bullets, to some extent, "wobble" upon exit.  A bullet is dynamically stable if that initial wobble is self-corrected prior to the bullet striking the target.   

Here's the problem.  The faster the bullet is spinning when it exits the muzzle, the greater the initial "wobble."  When you use a twist rate that is faster than necessary to provide static stability, you increase the initial wobble and increase the probability that the bullet will not be dynamically stable.

The rule of thumb is to select a bullet that is long enough to be just statically stable when it leaves the muzzle.  This assures that the bullet will remain statically stable from muzzle to target and gives you the best chance that the bullet will be dynamically stable as well.

JackHughs

Can you explain this a little better?

Quote:


If the base band is engraving, does that mean that the forward bands are to small for the bore?

One other thing is the velocity at the target, if it's in the trans sonic zone at the target, that could cause tipping. Can you chronogragh the load?

Frank

Jack, my bullets don't show the same tipping at 100m, so wouldn't that indicate a stability issue from either bullets bumped up off center, or a velocity/twist problem?

Thanks,

Chris.

Frank I believe the front two bands are a little small for the bore.   

This Thursday I'll give it another try.  I'll try Fg but seat the bullet a bit deeper with a wad under the base.  I'll also try FFg Express and FFFg Express to see if more velocity will help.   

I probably won't be able to chronograph them as the forecast shows rain.  I believe that the Fg loads are supersonic at the muzzle, but will definitely not be so at 200 meters/yards, but then it's unlikely that any BP loading would be fast enough to achieve that.

I'll post my results then, and will add chrony information once I'm able to get it.

Chris.

I doubt if the cause of the tipping is the bands being undersize. I shot a Pope bullet in my 33 Max. It had 6 bands and the first 3 where .002 smaller than my bore size. It shot a 3" ten shot group but only 1 or 2 showed just slight tipping.

I checked my chronograph book for BP loads. I've only shot straight BP once, that was enough. No blow tube (I don't think anyone did it in '86) and no cleaning fouled my barrel so that I only shot duplex after that.

My velocity with 3.5 gr 4759 and FG was 1470 with a 183 gr bullet, BSed in a 28" barrel. Your BC on the SAECO is .201 from Lymans cast manual. I think that would put you at around 1000/1050 fps at 200, near the bottom of the trans sonic range.

By my ballistic program, it's still over stabilized, even at sub sonic velocitys. 1.5 is a well stabilized bullet.

Back when I was developing straight BP loads for 1000 yard shooting, when I ran into problems, I shot a little duplex to see if the gun would even work; then went back to the issues at hand if it did.  You can usually get an idea if the issue is velocity, fouling, ect doing that.