That is the issue with TAC you can not get enough in teh case so the vel are down a bit. But if youa re not looking for that then its works. I am cheap and like powders that take the least amount fo powder to give the msot vel. That way I get more laods from each pound of powder.
For the person asking about fast versus slow powders:
What it effects is the pressure curve. The fast powder burns faster reaches its peak quicker.
Why you choose one over other: Its based on bullet weight but actually how easily the bullet will go down the bore. You want to match the powder to the bullet. A lgihter bullet will be farther down the barrel at the same point time wise as a heaver bullet. This means you need a powder that will burn faster to fill that larger empty bore area faster. If you try to use that faster powder with a heavier bullet it will peak and go down to soon and you will see high pressure signs before you reach the top vel the round is capable of. With a heavier bullet the slower powder will build pressure slower in line with the bullet and match up with the volume of bore that is behind the bullet.
A note and warning: It is OK to go with a faster powder with a heavier bullet. You will not get the best velocities but its safe. The other way around is not. It is never good to match up a slower powder with a lgihter bullet. The reason is you can get a secondray ignition which is more like a secondary explosion. What happens is the bullet actually gets out ahead of the pressure curve and then as the pressure is not pushing it it begins to slow until the powders pressure curve catches up with it and slams into it. This for whatever reason ( Actually I could explain but I am fairly certian no one wants to be put thru that :a43
casues a secondary pressure head to form and becasue of this unique situtiaon the push actually reaches peaks far above the normally max chamber peak pressure. These secondarys can have peask in the 120,000 PSi range. Worse yet is this is happen not in the thick chamber area of the barrel but the thinner area farther down the bore. This has been docuemnted with pressure testing equipment and has also been further confirmed by dissecting barrels that have used these types of loads and found large bulges 4-6+ inches down barrels from the chamber. THe aount of pressre it takes to bulge a .800-1.00 barrel is in the 100K + range.
For the most part because of the limited case capcity of the 6.8 all powders are fast enough. But some trying to use whatever powder they may have around or can get becasue of the shortage regardless of low vel may run into real issues they were not counting on. I really think people forget or do not realize that while very safe IF you follow the rules hand load has the potential to be very dangerous. It is for the most part setting off a controlled pipe bomb 3" from your face. Usually when a person first starts reloading they are very super careful and have their eyes squeezed shut the first time they set off their first reload. Then as things go well over the months they start to push things with nothing blowing up. Then they may push or do something wrong and again no bad things happen. Its usually after 2-3 years of loading when you figure you ahve everything figured out and people feel nothing bad coudl really happen and they are not follwiong standard precautions and think some do not matter along with giving out similar advice to others. Then it happens. It may not even be from something crazy. Maybe a case used one to many times. Using load info for one brand case in another. Whatever. They get a full out case rupture. The mag blows out the bottom of the gun, you are in cased in a cloud of smoke. Once you open your eyes as you are jumping up trying to figure out what just shook your world you start looking to see you have your limbs and edigits. Next you are pawing your face and looking for a mirror as you are sure you must be disfigured. Once you AS$H$L3 unclinches you start to look for what happened. You finally see the cause whihc may have left a few brass shavings in your hand or face. Ruptured case. After thinking back thru it and possibly looking thru notes if you are still keeping them you begin to see you missed something. It is after that you tend to come full circle and now realize that those checks and rules and warnings were there for a reason and while many times you can get away without going by them you just may not. Then how you reload and what you recommend to others changes. Making sure to error on the side of being cautious and to always check everything and go up slowly and never take anything as a given. After 23 years of reloading Been there down that long ago and now have seen countless others walk that path. Just something to chew on as there are things you woudl never even consider that can and will bite you in a$$ when you least expect it.
Anyways twhat wil normally happen is you will see lower velocities. When using faster than ideal powders you will reach pressure signs on your cases fromhihg chamber pressure before you reach the better velocities. With slower powders you wil likely not be able to fit enough powder in the case to get the velcoities up but if you do (with larger cases) you will also have far more unburnt powder exiting the muzzle causing huge muzzle flashes as well as unusually large amounts of carbon build up in the action etc.. There is a general range of powder burn rates that work for any given bullet in a given cartridge so there are usually plenty to choose from and of differnet rates that are safe.