Depends on the M14, there are two different designs.

Thanks for all the feedback, fellas. :) I'm happy to hear that the M14's hop-up is particularly effective; this will be useful when the time comes to upgrade.

Regarding the M14 itself; I own both a 'standard' M14 with long barrel and the add-on rail attachment (ie. afterthought style), and the M14 SOCOM version with the full rail system. Both are excellent guns, though I find them to be heavy and unwieldy. It is this that has led me to want to upgrade the long-barrel M14 to be a proper DMR. I started off with an illuminated 3X scope and bipod attached to the barrel, but I find the accuracy at long range to be somewhat random. I also find it to not shoot as far as I'd like it to.

Based on your suggestions, I should get: Upgraded hop-up rubbers, a flat hop, a tightbore 6.03mm barrel, and ammunition that weighs around .3-.33, if I'm not mistaken. Sound right to you? :)

I only have one question left: I've never modified a hop-up before; do you have any suggestions as to any tutorial videos that I can reference, that you think it's the best at describing and showing a hop-up upgrade?

Thanks everyone. :) Hopefully through these experiences, I can become a gun doc for others, as I know we all start from somewhere, right? :D

EDIT: Here's the two M14's I own, for reference.
Attachment 45183

Before dropping serious cash on upgrade parts, I strongly recommend you read this excellent guide to the DIY aspect of building a marksman's rifle. You'll be able to get the best performance out of the stock parts you're keeping, learn a bit more on the theory, and in the process you'll get VERY comfortable taking apart your gearbox and diagnosing problems (there will probably be tons of them). Bonus: most of this will not cost you very much if you've got the tools.

Before embarking on this project it's important to get a chronograph, it will be impossible to judge your FPS consistency without one.

Lastly, once you've gotten your stock/nearly stock gun to the point you don't think you can improve it any more without upgrade parts, make sure the brands you're buying are any good with this handy guide.

Good luck, hope you don't run out of money/patience before you're finished. This is a big project and it'll be pretty tiresome at times.

what's there to upgrade to put in a flat hop?

if the hop up unit isn't disfigured or improperly cast, there's little need to change it.

How do you install a flat hop?

Like any other rubber. you put it on, install the barrel nub or you put in the flat hop nub, reassemble.

A flat hop rubber is exactly like a normal hop up rubber except it doesn't have the mound on it. hence FLAT hop.