The only problem i've ever had with my factory barrel and it's the heavy one , is it would heat up fast and my group would open up a little. I usually would take 25 minutes between shots so it would cool down i've seen 300 mags and 7mm do the same thing. I just put a broughton barrel on it, I have 34 rnds total through it so it's just getting broke in .The other day I fired 2 sighter shots at a target and then started my 5 shot group at another when I was finished I had one big hole
My total time shooting was less than 9 minutes now I love it, any big magnum will heat up. My advice is if you go with the 300rum get a used action and put a hart ,broughton , or other match grade barrel on it and you will be happy. I bought my sendaro new and I still have the old barrel it never changed from 10 shots to 50 shots to 100 the only reason I changed the barrel was because the 155 amax was running 3400 fps and it heated the barrel up and moved point of impact because of heat! Now the barrel that's on it gets hot shooting it fast from shot to shot but in the end you have one big hole .Again any big caliber will heat up.