1. For the barrel, M4 or heavy and why?
Mostly personal preference. The M4 barrel will be lighter by about 8oz and being in front of the barrel nut the difference in weight will be immediately noticeable in the handling. Generally it will also be more accurate than most of the people using it, even with a bench, sandbags and a big scope.
The heavy barrel will handle heat a little better and be more rigid; both of which should give you a tiny fraction of better accuracy and longer barrel life. In real world use though, the primary difference is that the heavy barrel is heavier.
2. For someone who cleans his guns well after every trip to the range, chrome-lined barrel or no?
Chrome-lining makes the bore easier to clean and the lubricity can make extraction a little easier in marginal situations. Chrome-lining will not make a poorly produced barrel any better or more reliable though and low-quality chrome-lining that flakes off can actually reduce accuracy as the chrome flakes off in the throat area.
Chrome lining also reduces the theoretical accuracy because it is impossible to deposit the chrome lining equally - meaning it is thicker in some spots and thinner in others. As much as you might think this would affect accuracy negatively, my experience has been that few people can shoot well enough to notice the difference and that the main determinant of accuracy is the quality of the barrel, not whether or not it is chrome lined. If you take a Mike Rock match barrel and chrome line it (like the GTS Operator barrels do), the barrel will still perform like a match barrel.
Chrome lining also is supposed to extend the life of the barrel (provided the chrome remains in the barrel). I've shot at least one chrome-lined barrel to the point it wasn't performing well for me at 500yds+. I didn't see any significant gains in barrel life over unlined barrels. A recent NDIA Powerpoint put the lifespan of an M4 chrome-lined barrel at about 10k rounds. That coincides with my own experience and if correct, suggests that any lifespan advantage offered by chrome lining isn't very large. At the same time, I've read lots of comments indicating that the chrome-lined barrels continue to provide acceptable accuracy at less than 100ds for considerably longer than unlined barrels. In some cases people have claimed about 150% more barrel life. If you are doing most of your shooting in this range, then that 150% improvement in barrel life is probably worth an extra $40-50 intial investment.
Right now I have a stainless steel match barrel that I am using like I used the old chrome-lined barrel. In another 7k rounds I will have a better comparison of how an unlined barrel holds up to the same use. Right now my gut tells me that if you can buy a better grade of barrel for what it would cost to add chrome-lining, you are probably better off with the upgraded barrel than the chrome-lining. If you can't, then the chrome-lining may be worth the investment.
3. I'm leaning toward flip-up sights, like those made by YHM. Any big downsides like lack of accuracy, durability, wiggle, or anything else?
Flip up front, flip up rear, or both? Since I like to shoot longer ranges occasionally, the main thing I dislike about flip up rear sights is that you rarely see one that offers both apertures and is elevation adjustable. Usually, you either get two apertures and no elevation adjustment or elevation adjustment; but only the small aperture. If you never shoot beyond 200M, this is a non-issue though.
On the flip fronts, they tend to use set screws, which can sometimes work loose and stop the rifle by obstructing the gas port. It isn't an issue that you can't fix by proper installation; but it can be an issue if done wrong. Unless I am using a 1x optic of some sort, I generally prefer the normal front sight base.
4. I like the idea of a grip that has storage for small parts. Useful in the real world, or just a gimmick?
Depends a lot on how you use the gun - I use mine on the range where I am usually within a kilometer or two of a toolbox with all the spare parts I have. I can see where being able to store extra batteries or a spare bolt and firing pin with the rifle would be handy; but make sure the concept is executed well. The Magpul MIAD is an example of a great execution of this concept - it holds stuff in the grip and keeps it from rattling around. The old Olympic grips with the sliding trap door on the bottom would be an example of what I consider less than perfect execution of the same concept.
There's no real reason /not/ to go chrome-lined.
