After 16 years in a row, shooting at Williamsport, I can't remember a 30 Hart ever winning the year end aggregates in the 6 and 10 match.
I don't recall a Record Group or score being shot by one either.
The 30 Goodling and 308 Baer on the other hand have won these areas of fire.
The main thing is, a good barrel to start with, excellent gunsmith tecniques, a competitor who knows how to shoot and load well and works at it to improve himself, and the luck of good relays, is what determines how good the groups and scores will be throughout the season.
A good 300 Win mag will do every bit as well as any 30 cal shooting at 1000 yards. So will a good 300 Weatherby which has also won aggs.
Good brass is another area to consider when choosing a 30 cal to own and shoot 1000 yards.
If you put all the 30 cal inproved configurations in a pile and pulled one out, they would all be somewhat the same. Most of the time, they will use the same or nearly the same load too.
Now the trick is to find a good load, a good barrel, a good gunsmith and do your part and you "might" just get it to shoot.
Just my 2 cents worth.
As an add on here---It also has to do with the numbers of rifles in a certain configuaration. If 50 people are shooting 300 Weathetbys and only 5 are shooting something else, the odds of the 50 shooters getting good results with the Weatherbys are much better.
THANK YOU THIS IS WHAT I WANTED TO HERE. FOR THE MOST PART THE 30 IMPROVEDS ARE CLOSE IN CONFIGURATION THE THE STRAIGHT 30 MAGS WITH DO THE JOB WITH THE RIGHT PARTS AND WORK. WHAT IS YOU FEELING ON THE 30-378.