COL

[alwie]

All, 

Im new to reloading and have a lot to still learn.  If everything but COL remains the same and COL decreases by 0.010” , my understanding is that the peak pressure will increase. Will velocity remain the same (since only peak pressure increased) or will muzzle velocity increase a little bit?

Thx. 
Alex

[Lemonaid]

My understanding is that you may get a small increase in velocity. One test that I did with a large amount of seating depth change only resulted in 20 fps or so increase, and it wasn't a straight line change, it would go up a bit, down a bit but it trended on the increase.
It may differ for each powder/bullet/primer/firearm, so get a large opinion sample.
The extremes are a different can of worms, if the c.o.a.l. gets the bullet into the lands or if bullet seating starts powder compression it can change a lot faster.

[grayfox]

To add on to what LA says, in theory the pressure should go up a bit, although another factor is that a bit more "blow by" around the bullet happens until it fully seats into the barrel due to the somewhat higher pressure. My GRT tool says Pressure/MV increases but there's not a lot of MV change. When I compute my OBT nodes I generally assume coal changes only affect the timing of the bullet exit while MV is assumed constant. It's a simplification but seems to work ok.
The other factor as I know it, is that for a given charge of powder, there is a finite and fairly constant amount of energy to do the work of accelerating the bullet, so again, MV shouldn't vary by much.

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But, as LA said, at the extremes of too short/too long-into lands, the pressure can skyrocket and do weird and dangerous things. Not a good spot to be in. Stay away from extreme Coal's.

[CZ527 Guy]

Perhaps one of the better sources of YT reloading guidance I have encountered thus far. for what it is worth Ill share the link below.

https://www.youtube.com/watch?v=rRu8NYCq9Y0