From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Spinning bullets and asymmetry - (Was: Re: Long range rifle)
Organization: Hewlett-Packard Fort Collins Site

Kwong Wong (blkcat!Kwong.Wong@uunet.UU.NET) wrote:

: I don't think I agree with you about what happens on the first shot out
: of a clean barrel.  Yes, the velocity will be different from about 20
: fps to 70 fps from average depending on the cleaner residue that might
: be in the bore.  In the M1A and probably some other rifles as well, ther
: is a slight compensating effect in that low velocity rounds are launched
: at a higher angle of departure than high velocity rounds.  I run a
: chronograph on practically all my handloads and most factory loads also
: and I have noticed a tendency for the low velocity rounds to go high and
: left with this particular gun.  At 100 yards, the velocity difference
: does not have a chance to show itself as bullet drop so difference in
: angle of departure is all that is seen.

You're absolutely correct.  M1As and M14s do have a compensation situation.
That's caused by the gas port being about in the middle of the barrel and
it's at the bottom.  Escaping gas from a lower velocity shot will typically
cause less upware bowing of the barrel; less downward pointing of the muzzle.
This does cause those lower-velocity shots to print higher.

This is one thing the military match-grade M14 armourers found out in their
machine rest tests of completed rifles.  Groups at 600 yards had a lesser
elevation spread in MOA than those at 200 yards.  Kind of a nice thing as
Lake City Match ammunition typically has a 50 to 60 fps velocity spread.

But this doesn't happen with bolt guns, except for the Lee-Enfield and the
Remington Model 788 with their rear-locking bolt lugs.  It's more pronounced
with the Enfield than the Remington.

BB