Factory loads are readily available for the 45-70. Most are light [but very effective on deer] but some are very potent like those from Garrett or Buffalo Bore etc. I use the Win 300 Partition at 1850 which duplicates a load they had 100 years ago ! The Marlin should get 1.5" groups, my 1885 Browning easliy gets 1" or better. The Nosler Partition gives much better penetration than the other 300 Gr bullets. The Marlin won't do well with 500 gr due to feeding problems. Remember that the effectiveness of the 45-70 is NOT velocity but making a big hole [" What are you using , a cannon ?? " ] Happiness is a warm 45-70 !