ammunitionThe bullets for .300 Whisper manufactured by Lehigh Defense have relief grooves. The 170 grain has seven, the 178 grain has five and the 200 grain has three.
What are they there for? And why do the different projectiles have different numbers of them?
In theory anyway they allow for higher velocities.
Blockquote . “The several cartridges and calibers we tested using the new, grooved bullets showed pressure reductions of up to 13.4 percent, or 8500 pounds per square inch (psi). This meant we could load more powder to boost pressures back to SAAMI standards and safely produce higher velocities.”
Barnes (advertising, so Caveat Emptor)
I’ve read that relief grooves help reduce barrel wear.
http://www.ruag.com/de/Ammotec/Armee_und_Behoerden/Weitere+Informationen/Downloads/03_Rifle_Ammunition_for_Special_Forces.pdf
Sometime grooves are there so as to hold a lubricant, for lead (non-jacketed) bullets. I haven’t heard of bullet lube being used with jacketed bullets (other than moly etc coatings).
The .300 Whisper is a special case; usually one want’s a (just-barely) subsonic bullet for a .300 Whisper.
When making subsonic ammo, it is tricky to load things “just right”; even with a fair bit of care, there can be big velocity variations from round to round or from rifle to rifle.
One thing that helps to get consistent velocity is to arrange for consistent “shot start pressure”, which means the pressure that is reached before the bullet starts to move. In order for the bullet to move much, it has to engrave itself into the rifling; this takes a lot of force.
It could be that the grooves are there to help the ammo designer get a more consistent shot start pressure. Why the different bullet weights have different numbers of grooves though I have no idea.
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