Re: 6mm remington
When I get around a rifle that shoots that poorly I like to totally inspect the rifle for problems.
Here are a few that you should look into before you begin trying different powders and bullets.
Is the action properly supported in the stock? Bedding always helps
Is the barrel free floated or contacting the barrel channel?
If it is contacting the barrel channel is it tight or can the stock be easily pulled away from the barrel?
Irregular barrel contact with barrel channel can cause havoc with tight groups.
Are the action screws tight? Are any of them bottoming out in the metal of the action?
Is the barrel clean?
Is the barrel's crown in good shape?
Are the scope base screws tight with none bottoming out before becoming tight?
Is scope parallax free or close at 100 yds?
How about the trigger pull? If is it much above 4 lbs there is always a struggle to shoot accurately
You said some of it is you. What sort of support are you using to bench rest the rifle? Can you improve upon it?
There are probably a few I left out.
If all those possible issues have been addressed then you can move to handloading
You said you tried 45 grs of 4350. Did you try other powder charge weights? What is your method for finding an accurate load.
Check out your rifle. If it is sound, then please share with us your approach for finding an accurate load.
Hope these suggestions are helpful.