Here are a few pictures of the barrel on the 18.5” H&R a friend of mine has. I loaded quite a bit for this rifle and compared most loads in my Marlin Guide Gun.
The bullet of choice proved to be the 500 grain Laser Cast bullet which is hard and penetrated well.
After trying a number of powders we settled on a velocity goal of just short of 900 fps based on accuracy, noise and penetration.
Out front things were quiet until the bullet hit dirt or a pig. Behind the rifle the snap of the primer was quite loud.
We settled on both Accurate Arms No. 2 and Alliant Bullseye for powder. 9.0 grains proving to be the best load with both powders and 860 fps for No. 2 and 870 fps for Bullseye. Extreme spreads were typically single digits.
The shooting was close and the bullets would exit on lung shots and sometimes – but not always - remain in the body when crossing the X.
Care must be taken when charging cases and we charged all case one at a time seating the bullet immediately.
If you choose to do this I will enjoy reading of your shooting results. Bullet hardness and sizing diameter has quite a bit of effect on accuracy. Do not overlook sizing to exact groove diameter with hard bullets – 14 to 18 bhn - in the Marlin rifle.
I had tight spots in my Guide Gun barrel under the rear sight, the stamp with all the Marlin stuff and under the front sight. The tight spots affected accuracy with slower powders and higher velocity but had no discernable affect when using fast powders and hard bullets sized to groove diameter.
In the Marlin Guide Gun 400 fps is easy and quite a bit of fun.
Greg Mushial at GMDR has quite a bit of low velocity load data for the 45-70 and heavy bullets on his web site.
H&R barrel