the 300gr remington hallowpoint seems popular for deer. Can someone explain why as I thought it was a big enough bullet to not need any more mushrooming. Just asking as I have just got a 45-70 marlin and am going to reload for it and cabelas has remington 300 gr hollowpoint for so-so price

You are correct. But, just try to convience many hunters that they don't need the hollow point. You will likly find sales quite a bit different, with and without!

It is simple. I have been shooting a lot of deer with a hard cast WFN out of my 45-70 revolver and internal damage sucks. Deer go three to four times farther and I lost a few with lung shots. The boolit is too fast and the pressure wave in front of the boolit pushes tissue out of the way. When opening the deer, the lungs are intact.
The 300 gr Hornady does a perfect job as will a softer, expanding cast boolit. You need to slow the bullet inside the animal so it does work.
Slowing the same type hard cast boolits with a .44, .45 or .475 results in very quick kills, massive internal damage and little meat damage. Shot too fast and killing effect goes down.
I have been learning the hard way.