Hi velocity due to chamber "hold" time?

[Pitt300]

I was shooting my Whisper this weekend w/subsonics thru chrono & all was well for 1st seven shots. Then the wind kicked up so I waited a couple of minutes to shoot. All previous shots were around 1050 fps. When I fired the 8th after being in the chamber for a while it came out @ about 1250 fps & was definitely supersonic. This was my last shot.
Have any of you experienced this?
Do you think cartridge got hot in chamber (9.5g VV N110 w/220g SMK) & velocity was temperature related?

[Cornholio]

Very interesting. How did the brass case look? Is N110 known to be a temp sensitive powder?

[Pitt300]

Brass fine.
Same as others.
ALL powders are temperature sensative.
VV N-110 is supposed to be 3% Vel change for every 10*C which is supposed to be one of the more stable powders.
I think the "Extreme" (Hodgdon?) powders are supposed to be VERY stable but I do not think there is one that works in the 300 W environment.

[kurtz]

I have experienced the same thing....usually more pronounced in the supersonic loads but also with subbsonic....if shooting an AR let the rounds heat soak in the chamber adding 10 seconds to each firing interval (30 sec..40sec...50sec...)...this can give you some idea of how long you can leave a round in a warm chamber before it comes up in velocity....some of the tests I have done with H110 and Winchester 296 the velocity will come up 100fps in a small amount of time, especially supersonic AR loads...

kurtz

[Cornholio]

Then I guess it must be the residual heat in the chamber. But 200fps sounds like an awful lot!

[amafrank]

You can get a lot of pressure and velocity change with temp increases. I had some .50 cal loads that were just fine in the winter running about 2950fps through my friends bolt action .50. Temps were in the 30's and 40's at that time. During the next summer the same ammo was running over 3100fps and showing signs of high pressure. The temps at that time were in the high 80's. This was with the same ammo loaded at the same time.
I don't think 200 fps for a heat soaked round is unbelieveable and I'm sure it depends on how much temp rise the round actually saw. You may have actually hit the point where the barrel temp was ideal too....many factors in play.

Frank