N4Z
You should be getting way better groups (at least 1/2 of your 6+").
Don't know your level of experience so I will try to cover all bases.
Set aside your handloads for the time being,lets use some cheep factory ammo and try to find out what is what.
Go to wallmart or Kmart or whatever cheep discount house you have handy and buy a stack of cheep winchester or remington, This will likely be 170gr and should cost you about $10 a box. (get 3 or four boxes) The point is we want to compare group sizes as we try different stuff. The standard 170gr stuff will usually shoot ok to good in a rifle that isn't somehow messed up.
Get some good copper solvent,something more potent than hoppes. I like both Sweets 7.62 or Butches bore shine. Follow the directions,scrub the bejeesus out of it with a new brush,will likely require some soaking.
Take a magnifiying glass and a strong light and study the end of the barrel (front end) ,is there any visable wear or nicks or gouges right at the end? If the gun has been cleaned improperly or dropped this might be the case. That last 1/16" of the end of the barrel is by far the most importaint. Bad cases might need to have the crown recut or the barrel counterbored (both need a trip to the gunsmith). I had a real old,real abused,win 94 in 32 win sp that shot worse than your 6+" . I took a mouse ball,(the rubber coated ball bearing from a computer mouse) cut the rubber off and coated it with J+B bore paste. I then used it to polish the crown untill I had a bright clean ring about a 16th inch wide that looked clean and uniform with a magnifying glass.
Gun went from 6-8" 100yd groups to about 3" . That is about the best I can do with Iron sights and my eyes.
Eyes-- how are yours? Have you had someone else try to shoot this gun? If some snotnosed 24yo kid (with 24yo eyes)can get good groups out of this gun I would suggest a peep sight. Your Marlin likely has the mount holes drilled and tapped (unless it is 30+ years old) so go to brownells or midway and all you need is the sight and a set of screwdrivers. Some will sugest a scope but I personally think that is the wrong way to go with a lever gun ,The peep sight will work as well or better than the scope and the peep will be no more expensive than the scope mount alone.
Is the trigger buggered up? It doesn't need to be super light but should be fairly crisp. Most Marlin triggers I have fingered are fine to very fine,your gunsmith should be able to clean it up for a reasonable price.
Basic factory ammo.
Heavy duty cleaning.
Decent sights (or good eyesight and factory sights)
Check of barrel crown and trigger.
If going thru this long post gets you down to the 3" range (@100yds) then the next level I would suggest beter (higher priced) ammo or reloads and you might find some ammo the gun likes that will give you 1 1/2" or there abouts.
The 1 1/2" group might remain a wish but most Marlins and Winchesters will settle into the 3" range unless they are messes up .
Good luck!
Now 6" is probly good nuff to down a deer, but my AK in 7.62x39 makes tighter groups with inexpensive Wolf ammo, and that bothers me.
