45/70nut,
With some frequency, there will be a thread on one of the forums in which the Original Poster bring up the typic of "over kill."
With a 300gr bullet in your 45/70 at a velocity of 2000fps, you are rapidly approaching the point of Over Kill, if you have not already exceeded it!
Almost always, over kill is NOT a factor of the rifle being too big for the job at hand, but rather the factor of poor bullet choice.
During my early days of loading hunting ammo for my 45/70, I mistakenly used the same methods of load development that I had successfully used for years with my typical high velocity centerfire rifle.
Wrong! Poor choice! The 45/70 is a different breed of cat and therefore optimum loads have little similarity to those in the high velocity centerfire rifles.
My first year hunting load was with a 355gr Wide Flat Nose cast bullet and a velocity of 2300fps. I tested that bullet to a touch over 2500fps.
Yes, that first critter was dead, but quickly brought up the question, just what in the world had I turned loose on the game population.
HUGE wound channel. Not the minced bone and tissue I'd expect from the high velocity rifles, the bone and tissue was simply gone leaving a huge wound channel and this from what is basically a non or limited expanding bullet.
Thankfully, the bullet never gave me the accuracy or consistency I desired so I again went looking and thankfully some good council pointed out that cast bullets of 400 or more grains in weight provide better results then those of lighter weight.
So, I ended up with a 465gr WFN that I put out the tube at 1650fps. Sooooooooo much better results and NO, IT IS NOT over kill, the results a world better then with the lighter and faster bullet. The deer taken all dead where they stood with only one exception are making a growing pile along with three elk.
Recoil, MUCH LOWER then with the lighter and faster load.
So, will the 300gr jacketed bullet you speak of kill deer? Of course it will, no question! But if jacketed bullets are your choice, you'd be far and away better to go to a heavier bullet rather then the lighter bullet. No matter how fast you drive a bullet from your 45/70, you will never make it into what it isn't and never will be, a long range flat shooting hunting tool.
The 45/70 is an awesome hunting rifle, but the 300gr at nearly 2000fps will NOT kill deer any faster then my 465gr Wide Flat Nose at 1650fps. Dead where they stood is dead where they stood and you will get the critter on the ground with a lot less destruction and just as fast with a large meplat jacketed bullet of 400gr at mid 1000 - 2000fps then you will with the light bullet at 2000fps.
Crusty Deary Ol'Coot