I am shooting a Rem model 600 6.5 rem mag this week. Been using H4831 and R-22 with 120 gr bullets. Problem is it seems most the powder burns outside the 18" barrel. Would a faster burning powder be more efficient in that short of a barrel? I would like to keep muzzle flash down by burning more powder in the barrel instead of in the air but at the same time keep pressures safe. Any suggestions?

No, a faster burning powder will not improve performance. Despite how much powder may burn outside of the muzzle of your short barrel, the best pressure/velocity curve requires pressures to be kept as high as possible for as long as possible as the bullet travels down the bore. Heavy charges of slow-burning powder accomplish this better than faster powders, which give pressures that drop off faster, giving less sustained push to the bullet throughout the length of the bore. About the fastest powder I would feel comfortable with in a cartridge with the expansion ratio of the 6.5 Rem. Mag. would be IMR 4350, and I think the powders you are using now are better. The only "cure" for the problem you are experiencing is a longer barrel. (Unless you are willing to accept substantially lower performance than your cartridge is capable of!!)

I pretty much agree with El Deguello (and even know the sad and intimidating anthem), but if muzzle blast is your concern, you would give up very little in velocity by going to a faster powder and would, at least theoretically, cut your muzzle blast a bit. The drop in muzzle pressure might also result in a more accurate load.

With the 120 grain bullet, 4350 should be fine, but you may not improve muzzle blast unless you back off to something more like 4320 or 4895. With your short barrel and loads of roughly equivilent chamber pressures, you will probably be within 50 fps of top velocity with a faster powder anyway. If you can afford to give up this much velocity, then try the faster powder, but still don't expect your gun to sound like a .22 Rimfire.

Post Script: The slower powder is not actually burning outside the barrel (it is consumed in the first few inches), but the results are the same -- increased muzzle flash and blast.

The concern isn't really the blast itself or the noise. I'm more thinking about the amount of powder being burned outside the barrel. I chronographed some Remington factory loads today. They were purchased some time in the 1970's. There was a top of 2825 and a low of 2725. Avg 2750. That's pretty much where I keep it with the H4831 and R-22.

I have to agree with your choice of H-4831 powder.

H-4831 From 54.0 grains to 59.0 grains
Remington 9-1/2 Primer