Correct some powders dont do well reduced.I couldn't comment on Varget specifically, but some powders get weird in a bad way with light loads. Secondary explosive effect or something like that- not really a detonation but macht nichts- end result is the same shorts-changer, if you are lucky. The mid load for 2520 was given to me by a guy that for the last few years usually wins events at Perry. Also, I'll use a light load of 4198 based on recommendation that I verified via other sources. Those are the only two light loads I use that are not specifically listed by the powder manufacturer.
The above target shot with 2520 is very consistent with other targets I have shot with this load via this rifle as well as one I had before. In each case, there is evidence of gas coming back over the cartridge. Hotter loads don't do this- I assume the brass is expanding and sealing the pressure from blowing back. Total supposition on my part, but the physical evidence is there. I bet that if I were to fire form some brass then neck size only, the reduced 2520 groups would tighten up a lot. So long as the loaded cartridges cycle OK. I bet the guy killin' it at Perry and elsewhere has his brass very well prepped for his particular rifle.
As you seen you dont get full case obturation with a light load , this is why you get gas blow back. Fire forming and neck size is one solution along with some use of faster powders.
Theres known published data out there that works so no need to do any testing.
H4895 is the only powder Hodgdon claims for thier powders can be safely reduced. For bother cast and jacketed bullets.
The use of faster powders help also as it will have that initial peak pressure that expands the case.
H4895 works well for me in so many applications.
Varget also. So i go with what i can get. Right now varget takes up a lot of shelf space so i use that for most "normal" loads from 223-8mm mauser.