I read in the book "The Black Rifle" that the reason the AR-15 causes a devastating wound, is because of the high velocity at which the round penetrates forcing exspansion of tissue and fragmentation of the bullet. Unlike a low velocity .45 that causes the wound due to its size and mushrooming projectile.

My question is, is the M4 stlyle short barrel versions as effective as the longer 20" versions?

I'm sure the answer is "NO", but I wonder how much of a difference there is?

Part of that is answered in this post www.ar15.com/forums/topic.html?id=104957

Go to the link in the second post, it discusses ammo and velocity.

In order for it to fragment it has reach a certain velocity and it may not do so with the shorter barrel. E.g.- 2700+ FPS causes fragmentation. a 20" barrel is around 3000+ FPS and a 16" barrel may be on the border, possibly 2500 to 2700 FPS.

The preban ArmaLite AR-180 uses a 1:12 twist barrel and the optimum bullet weight is 55 grns.
With this bullet and the AR-180's 18" barrel about 3160 fps of MV is achieved.

Now with carbines your accuracy will be fine out to about 300 yds.  (Meant to be a rough guide only.)

Users of the short-barreled M4's in the Afghan are reporting that with the issue rifle/bullet they're having trouble knocking down the enemy over 100 yards.  Multiple shots are required.

The only thing to worry about is shot placement.

Quoted: Users of the short-barreled M4's in the Afghan are reporting that with the issue rifle/bullet they're having trouble knocking down the enemy over 100 yards.  Multiple shots are required.

That is to be expected for several reasons:

1) Fragmentation of M855 maxes out around 100 yards.

2) If the round doesn'g fragment it acts more like a .22LR (not much better - similar to a 5.45x39)

3) Hits on the body in combat are random.  With you and your target moving you can't aim at the heart and expect to hit it (you are not sniping - you're in a firefight); that is why you aim COM (Center-of-Mass) to maximize your probability of a hit against the target.

4) If your random hit doesn't hit a critical spot (i.e. heart, aorta, liver etc...) the small hole isn't likely to bleed much - not enough to cause an immediate cessation of hostilities at any rate.

5) Even if you do hit in a critical area w/o fragmentation the bleed out could take 15-30 seconds through the small hole (time enough for both of you to squeeze off several rounds at each other).

If you get into firefights over 100yards with an M4 you are at a disadvantage - the M4 is much better suited for 'typical' combat distance - Afghanistan combat is proving to be the exception to the rule (either that or the our guys are not using artillery & CAS enough - which I doubt).