I have had a bigger problem shooting the heavier bullets in the slower twist barrels than lighter bullets in faster twist barrels. That said, I always recommend that you try a variety of bullets in your rifle to see how it actually does.

In answer to your question though, if I didn't have time to test, I would go with 62 grain or heavier.

Al

 

I have a 1-8" that just loves 77 grain, 55 grain not so much.

 

It's the length x velocity of a bullet that determines the twist needed to adequately stabilize the bullet. Bullet construction will change the rate of twist needed also (e,g, lead core FMJ vs steel core)

IMO, the 1:7 twist will only stabilize the longest bullets for a 223 cartridge. Length of a bullet in most cases is proportionate with weight (e.g. a 22-250 can stabilize the long bullets with a 1:10 or 1:12 twist because of higher velocity)

A good all-round twist for a 223 is around 1:9, this will enable you to shoot a greater range of bullets weights available for the cartridge .

http://kwk.us/twist.html

 

I don't disagree with the above comments, but don't be surprised if 55's shoot good. I had a couple hundred 55 z max loaded and sighting in for my 9 twisted. We were going prairie dog hunting and my brother wanted to take his 7 twist psa ar. We sighted in and he shot quite a few prairie dogs out to 200 yards with the 55's.

 

I have had decent preliminary results with 55gr bullets and my new 1-7 twist barrel. Range was under 100yds but still better than expected.

 

Buy a box of each and test at 25 and 100 Yards. 55, 62, 75.

You test to find out what it likes to eat.

 

Optimal for a 7T is 90gr. But unlike some of the posts above will lead you to believe it will stabilize smaller bullets as well. I have never seen a bullet over stabilized. I have however seen bullets blown apart by high velocity and fast twists. .224 90 gr bergers would not hold up im my .22-6.5x47 once the barrel heated up. I have shot a lot of 40gr and 55gr ammo loaded for my walking varminter. I loaded a whole bunch of ammo for it when it was a 14T. When it came time to rebarrel I went with a 7T and I was not going to pull the bullets and have some sickness that forces me to shoot all the loaded rounds I can find. So even the lighter bullets in a 223 will hold up to its velocity window.
IMO if your not going to be able to load the 90 bergers with the boat tail seated at the shoulder junction because of magazine constraints, run the 75 A-max. If your going to run factory ammo I had really good luck with some cheap privi 75gr rounds.
The heavy bullets are actually only needed for shooting longrange, but it is not going to hurt a thing to have a barrel capable of shooting the long range bullets. There really is not downsize of a fast twist barrel...except that whole bullet blow up thing and the modest 223 velocities are not going to hurt a thing.

 

Testing is only way to know for sure, but IMO put a 1/9 barrel on it.....

 

I like everything about the 6920, but it just happens to have a 1/7 barrel. That's not something I really want to change, I'll just have to alter my choice of ammo I suppose.

 

My 1/7 is odd. Loves 75-77 ammo (match grade) loves 55 fmj. Shoots 62 green tips poorly.

Gotta try and see what works.. your never know until you know.

 

That is interesting, I guess I will have to play around with different loads until I find a sweet spot. I have read that some 75gr bullets are too long to fit in an AR magazine...

 

You should be fine with 75 gr... normally the 85+ is to long from what I understand.

 

It's more about bullet length than weight. For example a 50 grain TSX may behave very similar to a 62 gr lead/copper projectile due to the length. Most people tend to stick with comparing weights because before copper alloy bullets it was easy way to compare, most bullets of the same weight were approximately the same length. That isn't the case anymore, so as everybody is saying test it out.

I can shoot 50 grain V-max's and 77 grain SMK's with similar accuracy in my 1:7 twist BCM for what it's worth.