So is there a large difference in accuracy with a 60 grain load vs. 50? Is there such a difference with a 70 grain load vs. 50 grains... in your rifle? I'm curious why you are asking about that particular load, is all.
In my state, the minimum charge is 60 grains, .40 caliber rifle up to .72 caliber rifle...., makes no difference. So..., I used 70 grains in my .50, and currently use 70 grains of 3Fg in my .54. The .530 round ball will pass through a deer broadside, striking ribs on both sides before exiting, at 110 yards, my farthest shot on a deer ever.
Why 70 for me? Well IF I ran into a DNR officer while hunting, and he questioned my load, I have a fixed amount measure, and I will gladly pour out an amount of powder and he can use his measure and test it. No worries. Now..., I knew when I made my measure that some of the commercially sold adjustable measures can be "off" a few grains, and there can be a problem with the accuracy of a measure if you don't tap the side when done measuring out a powder charge..., so I didn't want to make my measure for 60 grains, and find out that OOOPS what I was using to check my quantity was off by three or four grains.... using a 56 grain load was likely a DNR citation and a court date, But a 70 grain measure that was off, was still likely to be above the 60 grain minimum.....OH and my rifles so far have all shot very well with 70 grains of 3Fg GOEX.
So will 50 grains launching a 177 grains of lead, patched .490 round ball do the trick? It should, all other factors being correct for the shot. At 75 yards, there should still be enough power behind that ball to make a humane, quick kill.
I remember when Mark Baker, author of Sons of a Trackless Forest wrote about using 50 grains of 3Fg in his 50 caliber rifle for deer. Mr. Baker also wrote at that time, a column in Muzzleloader magazine. OH the hue and cry of unethicalness that went up when he wrote such! Dozens of guys busted Baker's chops about it...and none of the negative pundits asked at what range was Baker normally taking his deer. NONE. Mark was taking his deer at 50 yards and often even closer. He hunted a rather dense area of woods, and that was really his max line-of-sight. Folks backed off, but frankly, they really over reacted.
Further on a historic note, there was a time when flintlock hunters actually used rather light loads on deer. They didn't want a pass-through, they wanted to recover the ball, and add it to melted lead, and recast it. Lead was expensive, and sometimes hard to find. In fact I read an account where the guy was way out from settled areas, ended up with only one ball, and to keep from starving to death as he walked back as he was without horse, he used light loads, recovered the ball, and then used his teeth to reform the ball as close as he could into the shape of a sphere, so he could reuse it.
So anyway, if it hits where you point it and it knocks down the deer, you're fine.
OH my software says that at 75 yards your round ball should be going at just about 900 fps with that load, and my farthest shot on a deer mentioned above, my 70 grain load was doing just over that mark, too.
LD