I killed a fair number of deer with the 270 back in the days of covered wagons and dinosaurs using crappy Sierra 130gr bullets. Those bullets would penetrate about 2" and come apart, making a gigantic mess. You never, ever, shot close to anything you wanted to eat.
Fast forward a lot of years: There are a number of really good 130gr offerings today that will hold together nicely. I tested a Hornady that peeled back nearly .7" wide and almost flat. I believe that the .270 is more than adequate for Elk.
There are way too many guys today who think Elk are armor-plated, so they buy some SCREAMINGMAGNUMDEATHRAY rifle and then find (a) it's expensive and (b) no fun to shoot. So they shoot 1/2 box from a bench and carry the rest for hunting. Then they make a bad shot on a 800lb Rosey, watch it run off, and then post on this forum about how it likely survived.
Frankly I get tired of magnumitis stupidity. The 270, chambered in an 8lb rifle, is an easy and cheap to shoot caliber. And it is much more important to learn to shoot, than what cartridge you're packing.
Now the 45-70 chambered Guide Gun is NOT easy to shoot, nor is it cheap if you don't reload. That little sucker will kick the crap out of you if you don't know how to properly mount a rifle. I reload, so the cost of the 2500 rounds or so I've put down range isn't a big deal. And you better have a range finder with you and have memorized your ballistics if you want to shoot past 200 yds. And NO, you can't flatten the trajectory out by loading it hot, so don't even bother.
What the Guide Gun is - is a sweet packing carbine. It weighs 7lbs. As long as you don't lard it up with a scope, it remains a joy to carry one handed. My loads give me a MPBR of 150yds - which is plenty when you are "hunting" (not just shooting), and a minimum wound diameter of .458" with the cheapest of bullets. I have a Williams peep and firesight combo, with a brass upholstery tack for a kisser button on the stock. Pachmayr decelerator recoil pad. And I wear a strap on shoulder pad during practice.
IF I were someplace where bitey/scratchy critters were the norm and a .375H&H with 270gr SPs was considered acceptable, I would feel comfortable with the GG stoked with the 405gr JSP @1800fps - because the two cartridges have identical momentum and produce identical wound cavities at close range. And of course I can always shoot 500+gr hardcast which give penetration from nose to prostate on Buf.
