Re: 223 v 308 on steel
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: osuarchitect</div><div class="ubbcode-body">In short... yes, a 223 shooting 55gr FMJ at 3200-3400FPS will do more to AR500 than a 308 FMJ @ distances closer than 100 yards. Alot of it has to do with bullet construction and velocity more so than energy. I've pondered this for a while and not been able to come up with a good reason why. Some 223 55gr FMJ's won't touch it, while others might leave a a dimple, like the dimple on a golf ball. It also depends on the angle of the target...alot of factors come into play.
My hypothesis is that because the bullet is light and fast, it doesn't have enough energy to really move the plate and let the plates motion absorb part of the energy. It dumps its energy quickly and in a small frontal area. The other reason I think has to do with the bullet construction. Typically the cheap-o 55gr FMJ rounds have thick heavy copper jacket. In comparision to the 308 there is a lot more jacket vs. lead in the 223, in essence making it a harder projectile. This is just my theory.
The heavy match 223 rounds, 68+gr, amax's and v-max's won't hardly do more than take off the paint. </div></div>
This thread has gotten me thinking about this subject a good bit. It has convinced me that it's much more about strain rate than it is energy. 55gr to 77gr 223s will have the same energy (1). What's different is the strain rate. Take silly putty for example. If you take it in two hands and pull it slowly apart, you can stretch it almost to infinity and a very very very tiny hair like string. However if you pull very quickly, it'll "snap" with a clean break. You're not applying more force in one case than the other, you're not giving it a chance to "flow". Same thing with these steel targets. If your displacing the steel so quickly it doesn't have a chance to bend and relieve the stress, only to break. These rounds happen to be on the border line of that velocity.
(1) Powder burns and releases pressure that's dumped into the bullet. Energy = Work = Force * Distance. The Force is pressure* crossectional area of the bullet. If the work is constant then the energy is constant between rounds. You'll note time in barrel isn't a factor and neither is the bullet weight. Muzzel Velocity can be figured by (energy/mass of bullet)^0.5 * units constant. This does assume you choose the correct powder burn rate to keep your pressure the same between bullet weights, but that's the manufactor's job.