Is not velocity velocity?

[bellavite1]

So I found a substitute for Varget for my 308 load .
Well, kind of...
I normally load a 175 gr SMK over 42 gr of Varget to get 2550 out of my 20" 700 barrel, with 5 shots groups as small as 1/3" at 100 yds.
I have now been loading the same bullet over 41 gr of H335.
Although lacking a chronograph the groups are zeroed just as the ones with Varget, so I assume: same bullet, same zero=same velocity...
BUT my groups are 2 to 3 times bigger than the worse groups I shot with Varget.
I thought that velocity is velocity, but apparently there are other factors at play.
Could somebody explain me how the same bullet pushed at the same velocity (and therefore, I assume, same pressure) is affected by what powder I use to push it with?
Confused...

[MarkCO]

Mathematically, in order to attain a certain MV with the same bullet, the same amount of energy has to be transferred to the bullet. The energy under the "pressure" curve can be loosely connected to the energy, but it is delivered differently based on the ignition speed, flame temperature and burn rate. A powder that has stopped burning and is right on the edge of no longer expanding when the bullet leaves the barrel, theoretically, will produce the best accuracy. A powder that is still burning when it leaves the pressure vessel (barrel) will have a higher variability and thus not produce groupings or velocities that are as consistent as the theoretical prime. Bullet obturation in the bore, asymmetrical engraving due to higher ignition speeds, ignition turbulence, gas cutting of the base and or heel, are all interior ballistics issues that have an effect on the bullet as it goes through the near transonic, which occurs somewhere between the muzzle to several feet in front of the muzzle, and supersonic flight.

[XC700116]

Just roughly looking at the Hodgdon data, I'd bet that H335 load is smoking out of the barrel significantly faster than the Varget load was.

Also a 100 yd zero won't (or shouldn't) significantly change from load to load considering same bullet out of the same rifle, so that's not a very good indicator of velocity, only a chronograph is really going to show you what speed it's running, or you can take it out to distance (like 800ish) and back calculate from the drop along with the conditions.

I've also never had much luck with H335 for accuracy with heavyish for caliber bullets. Velocity consistency just hasn't been there for the heavier stuff.

You might have significantly better luck with RL15, or IMR 4064.

[Circuits]

If you haven't measured MV with the new load you cannot assume same MV.

Wider groups indicate more variable MV, if nothing else, so your new powder is less consistent than your old load.

Same overall zero with wider dispersion in group MV will yield larger groups.

[TheBelly]

if no chronograph, check the bullet drop at 200, 300, etc. The farther out the better. That will give you a good indicator of several things, mv is one of them.

It's all about eliminating variables. There aren't too many variables that are going to show themselves at 100 yards with a rifle caliber.