I may SBR an AR of mine, and of course, I have questions.

A barrel of less that 7.5" will NOT spin the bullet properly to stabilize it in flight
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This I didn't know - the classes in ballistics I've taken say otherwise... such as, handguns still stabilizing bullets, short barreled rifles still stabilizing bullets, etc. I'll have to read the threads over on ST, I'm PTK there same as here. :)


EDIT: I read those threads (and participated in one a while back) and neither says anything about the twist rate vs the length vs the velocity effecting bullet stabilization. I don't see how a shorter barrel won't stabilize a bullet (other than VLD bullets, that's easy to understand) for things like 55gr... I mean, my PLR-16 will stabilize 69gr bullets quite happily, no baffle strikes. But then again, it's a 9" barrel.
 
Most AR-15 barrels have a 1:9" twistrate. It needs 9" of barrel for a full rotation. For a bullet to stabilize it needs to rotate/spin 1-1/2 complete rotations (minimal).
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It's not barrel length so much as muzzle velocity. For example, a Glock 17 has a twist rate of 1 per 9.84 inches. That does not mean the Glock needs a barrel 14.76" long for the bullet to stabilize.

For a bullet to stabilize, it must be spinning at a given RPM when it exits the barrel. RPM is determined by multiplying muzzle velocity by (720/twist rate). The minimum RPM for stabilization is based on bullet diameter and bullet mass (weight).

So a 55-grain .223 using a barrel with a 1:9 twist rate will have some minimum muzzle velocity to stabilize and it is that muzzle velocity which depends on barrel length.
 
It's not barrel length so much as muzzle velocity. For example, a Glock 17 has a twist rate of 1 per 9.84 inches. That does not mean the Glock needs a barrel 14.76" long for the bullet to stabilize.
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For the sake of argument, this discussion is on the AR15... not the glock or any other handgun firing standard handgun rounds. You are correct that barrel length is NOT the only factor. A glock shoots a heavier bullet than the 55gr found in the common .223rem. Since the bullet fired out of the glock has more mass, surface area and moves slower, it can stabilize with ease out of a shorter barrel.

Still, this post is regarding a 7.5" barrel for an AR15. Whether it's a 1:9 twist shooting a 55gr or a 1:7twist shooting a 62-65gr bullet, the fact is a 7.5" barrel will be hard pressed to stabilize.

I mean, my PLR-16 will stabilize 69gr bullets quite happily, no baffle strikes. But then again, it's a 9" barrel.
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PTK - This falls under the same principle. The 69gr bullet is probably traveling at a slower speed. The barrel is probably a 1:9twist, so the heavier bullet is compensating for the short length. I would bet that it would not do as well with a 55gr bullet. Try it. Do a 50yard test... see what happens.
 
The deciding factor is muzzle velocity. If just happens that muzzle velocity is proportional to barrel length for a given cartridge.

A 7.5" barrel may just happen to be too short to achieve minimum muzzle velocity for stabilization for a commercial .223 cartridge with a 55-grain bullet using a 1:9. Jack up the powder load, get more muzzle velocity and a 7.5" barrel may work.

If you could get a 55-grain bullet going fast enough, a 5" 1:9 barrel would work. Speed is key.
 
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