Can very small changes in flash hole dia effect accuracy?

[Larry Pletcher]

Daryl,
Obviously there is a difference in vent condition between my mild load and your load. Using 75 grains in a .40 is fairly stiff and I'm sure supersonic .Do you suppose that there is a pressure level or a velocity that above which vents remain clean?  I can't measure pressure, but many of us can deal with velocity.

Suppose we suggest that when a velocity reaches 1000 fps, the barrel pressure is sufficient to keep the vent clear.  The velocity may be the wrong level, but testing would allow us to zero in on where barrel pressures keep the vent clean.  i'd guess your 75 gr load may be nearing 2000 fps.  Any ideas where this change might take place?
Regards,
Pletch

[Daryl]

Closer to 2,300fps, Pletch.  I'm sure the pressure would keep the vent clear at even less than 1,000fps.  I suppose partical plugging could happen with a large bore pistol, with light loads.

[William Worth]

I would suggest that perhaps a way to measure vent restriction would be to contrive a means to measure bleed down of applied air pressure, much like doing a compression check on an engine cylinder.

[Dphariss]

Have you changed powder brands, granulations or powder lot numbers within the same brand?
How about patches? Are these from the same bag or same piece of cloth? Has the lube dried or in some way changed over time?
There are are a number of things that could effect this.
While not likely with a GM barrel the barrel may have had a wire edge somewhere in the bore that has been removed by the shooting over the past year.
The point being that there are other things besides the vent diameter to consider.

Dan