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by shu246
Thu May 15, 2008 9:26 am
Forum: Reloading Forum
Topic: Burn rate, Fast or Slow
Replies: 11
Views: 2494

Re: Burn rate, Fast or Slow

My understanding is:
..Smokeless powder burns more efficiently as pressure increases. Higher pressure means a cleaner burn, no unburned flakes thrown out the front. Also means enough pressure to expand the brass case against the chamber wall and seal against soot blowing back; cleaner ejected cases.
..So look to achieve and keep a high pressure during most of the burn - without, of course, exceeding the SAAMI max pressure at peak.
..Heavier bullets, other things being equal, cause higher pressure because they accelerate slower and delay opening up the combustion space in the barrel.
..Surface hardness of bullet (swaged lead < cast lead < copper jacket), other things being equal, causes higher pressure because friction in barrel also delays opening up the combustion chamber volume.
..Bullet-to-barrel contact area (round nose < full wad cutter), other things being equal, likewise increases friction and thus causes higher pressure.

So, pick the bullet and muzzle velocity you want, then look for a powder which will give you high pressure without exceeding SAAMI max pressure:
..For targets, a light swaged lead bullet at 900 fps will hold down the cost of bullets and keep recoil low; a fast burning powder will reach peak pressure early, completing most of the burn before the bullet exits.
..If you want expansion of bullet cross section in terminal contact, you will need a higher velocity; higher velocity, while avoiding barrel leading, in turn means cast lead with gas check or a jacketed bullet; a slow burning powder will have a pressure curve that rises slower in time, keeping pace with more slowly opening combustion chamber volume without exceeding max pressure; much of the burn may continue after bullet leaves the barrel.

I have heard said, and have no reason to disbelieve, that regardless of barrel length slower burning powder yields increased velocity. With a snubby, you still get more bullet velocity with slower powders - along with more muzzle flash.

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