kyhuntclub,
Varget is the first of, what I consider to be very successful, 'Extreme' powders put out by Hodgdon. The basic premise of these powders is they have shorter, thinner kernels like a faster burning powder except that they have a new chemically formulated and heavier application of burn retardant sprayed on them. Read most any reloading manual and they will tell you powder speeds are determined by size of the kernel. A thicker, longer kernel is a slower powder than a thinner, shorter kernel. With a few exceptions IMR is still the old formula. Look at them compared to any of the newer Hodgdon Extreme powders and you see quite a difference in size. Yet, the burn rates are very similar.
This allows more powder in a case and less chance of an over pressure (within reason) searching for more velocity. How it works is when temps/pressures get up to a certain point, the retardant chemically comes apart and makes new molecules that cool the burn. Again, within reason. There is only so much retardant that can be sprayed onto each kernel. And in the chemical process of high heat and temp, once all the free electrons have been joined to new molecules, forming the cooling formula, there is no more left and the powder continues to burn as it did before. Only now you are above temp/pressure, and the overpressure signs you normally see gradually, come on very quickly.
Varget was formulated to work best with moderately heavy match bullets in the .308. But, a very good rule of thumb is if it works well in the .308 it will probably work well with .223/5.56mm. The ratio of bullet diameter to case proportions/capacities is pretty close with both rounds. Varget works well with .308 and 175 gr. bullets, but change the diameter to .243 and you find it likes medium to lighter bullets. For heavy bullets in .243, I prefer H4831X....2 speeds slower than Varget. For .260 Rem with heavies, I like H4350. That is also my 'go to' in case I don't have any RE-19 that I normally use in my 7mm-08.
It's good to run a few ladders with the different speeds and see what your rifle likes. What I explained here is 'rule of thumb'. I've never seen "thumb" in any precise measurements. You have to get there yourself. But this information is a start on which way to go if you can't get what you've tried, working.
