Would a 20in .223 barrel be able to stabilize a Hornady 75gr A-Max @ appro. 2850fps? This would be in a gas gun.

Barrel length had much less to do with it than twist rate.

1-7 should be good to go.

1:8 is worth trying but 1:7 generally for heavy 223.

What they said.

Correct. It's not about barrel length, but about rifling twist rate. You will need a 1:8" or faster twist for the 75 A-max. Plus, you will need extra length in the magazine to seat the bullets a little longer.

Never no until you try, i successfully shoot Hornady 75 bthp out of my Remington R5 with a 1-9 twist.

Twist, altitude (air density), air temp and velocity determine if a given bullet weight will stabilize. Rate of twist is by far the most important.

Greenhill's formula for stability:

Twist = \frac{C D^2}{L} \times \sqrt{\frac{SG}{10.9}}
The actual formula was:[3]

S = \frac{s^2 * m^2}{C_M\alpha \div \sin(a) * t * d * v^2}

where:

S = gyroscopic stability
s = spin rate in radians per second squared
m = polar moment of inertia
CMα = pitching moment coefficient
a = angle of attack
t = transverse moment of inertia
d = air density
v = velocity

Simplified by Miller:

{t}^2 = \frac{30m}{sd^3l(1+l^2)}
where:

m = bullet mass in grains
s = gyroscopic stability factor (dimensionless)
d = bullet diameter in inches
l = bullet length in calibers
t = twist in calibers per turn

Simple as can be. Or simply use JBM's calculator: http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Sorry, went brain dead. Yes twist rate is key to question, forgot to give that info, it's a 10 inch twist.

http://www.shilen.com/calibersAndTwists.html

The only reason 1:7 twist barrels were ever used in 5.56 rifles is to stabilize tracers. Tracers are pretty freaking long. 1:7 will overstabilize almost anything that fits in an AR mag. Heaviest you can fit in there are 77 grains, and some lighter ones probably still won't fit due to the length or shape. 1:8 works a lot better.

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and this, Remy 5R, 20" 1-9 twist does okay with the 77gr SMK and tipped SMK's

Try 55 grain bullets in a 1:7, or even 62 grain ball ammo. There is a reason the m4 is a 2-3 moa weapon. The 1:7 was a compromise between stabilizing the tracer and overstabilizing the ball rounds. Sad thing is, tracers are still not remotely accurate. Bad decision. But all milspec barrels are still 1:7.
There was some talk of switching to 1:8 but i haven't seen it yet. 1:9 sounds a whole lot better.
I am not by any means an expert on this sort of thing but I know a little about service rifles. Apparently there are some 1,000 meter 5.56 rifles out there... they use handloads with like 90 grain custom bullets, and they still prefer slower twist than 1:7.

You will be able to shoot the 75 gr BTHP match bullets easily out of an AR with a 1-8 twist. The 75 gr A-Max is a different story. The OAL will be longer than a magazine will allow,so you will have to shoot the rifle as a single shot. While you can seat the bullet shorter than the suggested 2.390",most magazine's will only allow an OAL around 2.260"

The 75 gr BTHP and 77 gr SMK's can be seated at 2.260" safely,and will shoot out of any AR.

You can load 75gr A-Max bullets short enough to work in ARs without the ogive being in the case if you use Steel not polymer mags. It is close but works. Have done that before, don't remember the length though, set it by what would function through the mag.

I have a 1:8 20" RRA bull barrel and it shoots anything I throw at it and very accurate.
But so heavy...