Varget/Sabot/Unburned Powder/Temperature

[student]

Howdy, folks, I'm new here, drifted in with the NEF/H&R refugees, and I am really glad I did.  Heres' a situation I ran into, and some questions, that maybe you can help me with.

I'm trying to work up new 50 gr .223 nylon sabot loads for my .308 Remington VS.  A few summers ago I had used IMR 4985 with these sabots, and got so-so accuracy and good (3800 fps) velocities with no pressure signs.  This was in 90 degree plus weather.

In trying to switch to Varget, I looked at .243 58 gr. Varget data and reduced by 10%.  (my sabot plus 50 gr vmax bullet is  56 gr.)  I have tried this load this winter in cold temperatures, like 38-40 degrees, and have gotton velocities much lower than I expected, and today (38 degrees) noticed lots of unburned Varget in the barrel and on the snow in front of my rest.  

The question is, can I attribute all of this to cold weather?  I thought Varget was supposed to be resistant to temperature changes?  Or is the 50 degree difference between winter and summer too much for this powder?

Is the solution to this unburned powder issue one of going to a faster powder, or to a magnum primer?  If so, would that be a winter only load that would have to be replaced with something tamer (like my original Varget approach) for summer?

Appreciate your comments and suggestions.

Student

[Iowegan]

I really doubt the temperature had much to do with the unburnt powder. For any powder to work well with sabots, the bullets (sabots) must be taper crimped very tight. What happens with sabots is they are slick and will compress. The result is poor ignition because the bullet leaves the case much sooner than conventional bullets. If you have any factory Remington 308 sabot ammo, measure the diameter of the neck. It will be considerably smaller than 308's loaded with regular bullets. A roll crimp won't help you much because it only squeezes the mouth. You need a taper crimp die that will squeeze down tight. Not to worry, when the sabot is fired, it will puff up to bore size under pressure and your powder will get a good burn.

[student]

I had actually used a Lee Factory crimp, although it was pretty light.  You make a very good point, though, and I will play with heavier crimps.

I wonder if temperature can have an effect on the diameter of the sabot itself?  I might freeze a few and measure them.

Thanks for your ideas.

Student