.355 vs. .356 speed question

[rocket99]

I have been shooting .356 125gr coated lead (black and Missouri) thru my P320 X5 and enjoying that combo.  Yesterday I picked up some .355 125gr Blue Bullets to try.   I loaded a hundredish rounds using my same recipe to compare.  Accuracy was the same with both the .355 and .356 ammo.  

 

All other things being equal, will the chrono notice any difference?  I don't have access to one until next week.

[stick]

You shouldn't really notice a difference from the .355 or the .356.  You might see a slight chrono difference from the one size to the other.  Usually the .355 are jacketed and the .356 are lead.  

[IDescribe]

You are likely to see a decrease in velocity, going down a thousandth in bullet diameter, when the other relevant factors are equal, equivalent to somewhere between .01gr and .02gr of powder.  If you care to know why:

First is simple -- the bigger bullet obturates better (fills the rifling grooves) and thus contains gas better.

The second reason is a little more complicated -- smokeless powder burns faster the more pressure it's under, and the faster it burns, the faster it builds pressure.  It's a positive feedback loop.  So if you do anything to slow the acceleration of the bullet at the beginning of the burn (such as you do relatively speaking by using a fatter bullet that resists entering the rifling more), you end up with higher pressure for the same mass of powder through the burn, and you end up with a higher muzzle velocity.

People often think if do something to make it easier for a bullet to pass down the barrel that it will result in a faster muzzle velocity, but in practice a slower initial acceleration results in higher pressure and a higher muzzle velocity.

All that said, there are a number of variables that affect powder burn, and when you switch bullets, typically all else is NOT equal, so in terms of how much of a difference you'll see, no one can tell you.  Could be MUCH slower, or a little slower, or pretty much equal.  It is also possible but unlikely that the narrower bullet will end up faster.