Hello...  I am new to this caliber, having just bought my first rifle of this type.  It is a Rock River LAR-15 with a 16" 1:9 barrel.

I have read about the twist history and although I know a lot of people are getting 1:7 barrels for even heavier bullets, I am wondering what my options are.

First, it was my understanding that the standard 5.56 load has more pressure than your standard .223 load?  So with the same 55gr bullet, the 5.56 load would have higher velocity?

Somewhere I saw a chart that showed the optimum grain for my twist to be around 70?  Someone also mentioned a common surplus round that had a higher grain weight?

I am familiar with the 62gr M855 and the 64gr M856.  I know there are commercial loads in the 75 and 77 grain size, but don't know of any surplus in grains other than 55, 62, and 64?

I am happy to use PMC .223 or XM193 for practicing and plinking, but would like to know which cartridges would be best for accuracy and what cheaper option do I have other than the three I named?  

Thanks!

+1 to what Zukhov said, because the Ammo FAQ will cover everything in much more detail, but:

1:9 twist means 69 grain or lighter bullets.

5.56 is loaded to a particular velocity specification for the projectile weight, regardless of pressure.

.223 is loaded to a particular pressure maximum, regardless of velocity or projectile weight.

You're unlikely to find actual 5.56 surplus with a heavier bullet than the 62 grain SS109 or the 64 grain M856 tracers.

Best accuracy will probably come from handloads (assuming you handload) or good factory match ammo with a 69 grain or lighter bullet (think Federal Gold Medal Match).  "Acceptable" accuracy will probably not be hard to come by with most ammo as long as you are not trying to shoot outside the limits of a given loading.