I have a few questions
How familiar are you with shooting the M14 style rifle?
What is your shooting background?
How did you learn your shooting technique?
What are the specifics about your rifle?
What ammo did you use?
What was your shooting position?
What kind of surface were you sitting, kneeling, standing, laying on?
What was the weather like?
I ask these questions to help troubleshoot your target so please don't infer anything negative about my questions, I certainly don't mean to offend.
One of the best group troubleshooting guides I have ever read can be found at this link;
http://www.rifleman.org.uk/Fuller_group_diagnosis.htm
In general there are four things that you would generally review when you look at your groups;
- You
- The Rifle
- The ammunition
- The Weather
The marks on your arm are pretty common when you use a good sling position, I always got them when I fired for score in competition, it's just a broken capillaries in your arm from the sling compressing your skin. That is nothing to be worried about and if you intend to do a lot of target shooting you will either find a good shooting jacket that will help eliminate the problem or you will just get used to it.
All that those marks tell me is that you had a good tight sling and with what little I can glean from the target my guess is that either your position was changing or the rifle has some issues or both combined. A six o'clock hold is good enough for standard combat shooting but if you want better accuracy then you need to hold the top of the front site on the spot where you want the bullet to impact. In my personal experience, I only used the six o'clock hold while in combat in the military but I always used the direct aim point when I was trying to compete for score and/or accuracy. My personal opinion is that a six o'clock hold will usually result in a group of a couple MOA at any range, there is an old saying "aim small, miss small".
The left to right pattern could be caused by your elbows creeping apart if you were in the prone or the barrel could be coming in contact with something. Until you post a little more info I can't really make an informed comment so please post answers to the questions above if you can or you might be able to find your problem by just reading the info at that link I provided. If you have never had any formal marksmanship training see if you can attend an Appleseed shoot. Their training is very similar to the marksmanship training I received in the Marine Corps and it is very helpful, not to mention, they have some really good historical info too.
Good luck and keep working at it, all it takes is experience and technique and you will only get that by doing a lot of shooting and taking range notes and getting some help along the way from others.
