9mm accuracy using VV N320 Big Difference

[stephensackro]

Wanted to find a really clean burning powder for 9mm and bought a pound of VV N320. Everyone says that it is a very clean burning and in my testing I found this to be true. Loaded up 100 rounds, some at 3.5 gr and some at 3.8 gr using 124 gr FMJ RN bullets. Min and max for these is 3.4 gr starting to 4.0 gr max. Went to the range (50 yards) yesterday with my KelTec 2000 to test these rounds. My previous load for these 9mm rounds were Win 231 @ 4.2 gr which was very accurate but dirty. Started shooting the 3.4 gr of VV N320 and found they were all of the place and shooting about 5 or 6 inches low. Next I tried the 3.8 gr VV N320 and they shot about where my old Win 231 shot and were accurate. Seemed like a big difference for just .3 gr more powder. Was thinking that maybe ? since the N320 is a fast burning powder and my barrel length on this carbine is 16 inches, that the fast burning powder at 3.4 gr didn't have enough "push" for the 16 inch barrel. When I switched to the 3.8 gr, more pressure and velocity really helped accuracy? Don't know if my thinking is right or could it be something else. I will be loading at 3.8 gr and might even try a slower VV powder like N330 for this rifle. Your thoughts.

 

[BillM]

You're theory is correct. Ran into the same thing using VV N320 with a coated
147 gr bullet. It chrono'd faster out of a 5" pistol than a 16" carbine. Had to
up the powder charge .2 gr to get back above the power factor floor (USPSA PCC).

 

[Unclenick]

I think what happens is low pressure just isn't sealing the case against the chamber walls very well, allowing enough gas to escape to affect the interior ballistics. But up the charge just enough to get past the threshold needed to get a good seal and it starts to behave.

If the load works OK in your pistol and not in the carbine, the first things I would look for are differences in the chamber and throat and bore diameter dimensions.