westy:
I'm gonna assume you are refering to an assault rifle that a sniper might carry with him for backup? In case his location becomes compromised or he is overrun? If I am way off here let me know, but under the above mentioned senario, this would be my line of thought.
1. Reliability- I don't care what weapon, scenario, or anything. I want my weapon to be as bulletproof as possible! I dont want it to fail ever! This has to be my number one concern, because Accuracy, Effectiveness of cartridge, Lightweight, Modularity, Ergonomics ,Compact size and Effective Range all go out the window when your weapon fails. I believe modularity kinda goes along with reliability because it seems to me that the simply designed things tend to be the most reliable.
2. Effectiveness fo cartridge- This is a close one with several others, but no matter what, if your gun works, but it fails to stop your target, then i guess nothing else matters now does it? (although I do believe even a cartridge like .223 would be a sufficient caliber.)
3. Weight, Ergonomics and Size- These all tend to be secondary factors for me. I would agree that having a small, light, and comfortable weapon would be great, but these things are not on the top of my list. If achievable then they are obviously very desirable, but again not priorities.
4. Accuracy and Effective Range- It is difficult for me to place these lowest on my list (lord knows how much I HATE to miss). But it seems to me that in this day in age, any decent assault rifle, has acceptable accuracy to engage a target at ranges inside 300yds. Especially for a sniper, I can't see why a sniper would engage a target outside this range or even anything farther that 200yds. with an assault rifle. Most assault rifles have acceptable accuracy for a backup weapon and are most likely more than effective at such close ranges.