Originally Posted by Daryl in Az
Yes, really; but you missed my point. The smaller diameter bullet only has a higher BC if compared to similar weight bullets in the larger caliber. If you compare, say, 124 grain bullets in 9mm to 155 gr bullets in .40, or 148 grain 9mm to 180 grain .40 of the same type/manufacture, the .40 generally has a higher BC and SD. IOW, medium-weight-for-caliber bullets compared to medium-wieght-for-caliber bullets, or heavy-for-caliber compared to heavy-for-caliber bullets. Apples to apples, so to speak.
Now, if someone's determined to shoot a 124-135 grain bullet for self-defense, and wants to have the highest BC and SD, then the smaller bore makes sense. IME, that would only be the case when addressing a very specific need.
BTW-once a bullet starts to expand, the SD changes. How much depends on how much expansion there is, and what shape the bullet assumes. For that reason, SD pretty much goes out the window, and penetration becomes more dependent on mass and momentum. Bigger and heavier bullets have more momentum, and leave bigger holes.
Then we agree as that is what I said in my first reply to the OP. SD matters, regardless of diameter. Correct on the SD changing during expansion, it's why bullets must be of the identical construction to have a realistic comparison. Bonded bulelts & monometals have changed the ideas on SD, but it's still valid for evaluating most bullets peformance.
"Given adequate penetration, a larger diameter bullet will have an edge in wounding effectiveness. It will damage a blood vessel the smaller projectile barely misses. The larger permanent cavity may lead to faster blood loss. Although such an edge clearly exists, its significance cannot be quantified".
Last edited by fredj338; 04-12-2011 at 10:21..