Ive never had this setup before & I'm not sure what to use so any help is appreciated. Thanks

From a barrel that long, the velocity will be sufficent to stabilize pretty much anything that fits in your magazine and cycles the rifle. In my 20" 1/9 I shoot NATO pressure 75gr reloads with no problems. If I were shooting a shorter barrel with SAAMI pressure stuff, I might notice some issues, but.... Sorry, I started to ramble off topic, long story short, use decent pressure stuff and you should be fine.

My opinion differs slightly.  I think it is mainly the barrel twist rate that determines bullet stability and not barrel length.  You can try some heavier bullet weights, but I would not buy large quantities of a heavier bullet load until you have tested some samples.

Mainly I'd not recommend over the 69 grain match type bullets in a 1 in 9" twist barrel.  There may be some low 70's grain weight bullets that will work, depending on bullet length and shape.

Jeff

I'm no expert, but this is how I understand the situation, please feel free to point out any fallicies you notice in my reasoning.

I agree that it is primarily twist rate that determines stability, but velocity plays a part as well. For example, say you take a bullet and drive it at 500 fps and it takes .0016 second(s) to get to the muzzle, with a rate of twist of 1/10 from a ten inch barrel, after all is said and done the bullet will be rotating at 625 rotations per second. (1 full rotation in .0016 seconds, 1 second divided by .0016 equals 625 rps) But if the bullet is instead driven at 1000 fps it will take .0008 second(s) to reach the muzzle, meaning it made the same full rotation in 1/2 the time, for a final rate of spin of 1250 rotations per second. Obviously these numbers were chosen for ease of number crunching and not accurate calculations of rate of spin. But I didn't feel like breaking out the ballistics books and trying to calculate acceleration, etc.

But the same principles apply regardless. To put it in a nutshell, the faster you drive the bullet, the faster it has to spin to avoid shearing.

Wow, does your brain hurt after that.Great info!

Quoted: Wow, does your brain hurt after that.Great info!

My brain didn't just hurt, I actually had to go jump in the pool after that one, my head started to overheat. I almost spontaneously combusted!!!!

try anything

55grain will work great for you

I felt compelled to come back and clear up something from my earlier post, when I spoke of acceleration, I was perhaps misleading in that acceleration has nothing to do with a bullets final rate of spin, All that matters is how fast the bullet is traveling when it leaves the muzzle, this combined witht the barrels twist rate will allow you to calculate rate of spin accurately.

Just wanted to clear that up.

My 24"  1/9 SSBBL AR likes 62 gr but it loves 69 gr.  With 55 gr I get fliers.  Try an bunch of different weights and see which works best for you.  Theres no real formula.  Each barrel has its own preference.

I'd guess that you're actually overstabilizing the 55's because of the long barrel and excessive(for 55 gr) twist rate, remember that M193 was originally designed to be fired from a 20" 1/12 twist barrel. It's true that each barrel has it's own preferences, but only up to a point, one can pretty accurately calculate a rounds RPM's by kowing muzzle velocity and rate of twist. And from there you can usually determine whether or not it will stabilize properly, if it's right on the border line, or if math makes you break out in hives . Then you can certainly just go shoot it and see what happens.

It should be noted that with match bullets, overstabilization is less of a concern because of their more uniform construction, you can spin them much faster than an unbalanced bullet and still expect good accuracy.

This gun is a V-Match Bushmaster 24" barrel and it loves 75 gr. Blue box Black Hills. Not the best group in the world but pretty good for 100 yards and some wind blowing.