Re: Sell an H&K to buy .308 AR or SCAR17?
You may be the one and only person that I have EVER heard refer to the HK91/SR9 series rifles as "collectors guns"/"safe queens" and NOT "working rifles!!" That is some funny stuff right there, my friend!
Trust me when I say that the HKs are built for utter reliability, hard duty and reasonable accuracy. Yes, some people "collect" them, but personally, I shoot the hell out of all of mine (and they are still worth considerably more than I paid for them years ago).
As to your questions re: the SR9...the biggest drawcacks of anything in the HK91/SR9 series rifles are: 1) the difficulty in properly mounting optics; 2) the terrible factory trigger packs; 3) lack of a free-floating barrel which can cause accuracy problems depending on how you are setup and how you are shooting the rifle; and 4) a complete lack of aftermarket support to get the rifle fitted to you the shooter and your specific needs. (With ALL of that said, I wouldn't part with my SR9TC or my MSG-90 clone if you beat me to death with them).
As for optics, you are stuck with a claw lock mount which puts the optics closer to the moon than the barrel (clear the rear sight) which necessitates a cheek riser or adj. stock, OR you have to go with a picatinny rail adapter, rings, etc. Either way, there are mounting difficulties that can be overcome, but they are still not as secure or as ideal as a forged picatinny rail built in to the receiver (such as with an AR/308 type rifle). The other issue is that you are limited to the length and bell size on any scope you chose for the SR9 because of possible interference with the cocking lever. If you go too long (over 13" or so) or you go with a large bell scope, you can have problems.
As for "destroying" brass...this is one of the most overstated and easily remedied issues with the HK91/SR9 rifles with the addition of a $30 port buffer. I shoot good reloads from my rifles all the time and with a port buffer installed, the brass does not take anywhere near the beating it would in a rifle not equipped with the buffer. I reuse 99% of the brass I recover from shooting my HK rifles/clones for multiple reloads.
Now, as for accuracy...I will respectfully disagree with some of the other posters who claim that MOA or better accuracy with the basic SR9 rifles is the norm. From my experiences with the platform and across the board with the SR9s, the basic models will achieve 1.5MOA or so with quality ammo (not surplus) and with a shooter that knows what they are doing. The SR9T and SR9TC variants are MOA or slightly better capable because of the vastly improved PSG1 trigger packs, as well as the more ergonomic stocks/grips that give a much better fit/hold to the shooter that further enhances accuracy potential.
Personally, I would keep the SR9 and do the following. First and foremost, get a Bill Springfield trigger job done on it! Bill's triggers are just night and day compared to the gritty, heavy, generally rough battle-rifle trigger found in the stock SR9. An improved trigger can greatly improve the performance of your rifle and give you a fighting chance at better accuracy.
http://www.triggerwork.net/hk9xrifles.html
Second, you are going to have to mount a scope to wring the most accuracy out of the rifle (especially at extended ranges). I recommend getting one of the MFI picatinny rail adapters, along with a set of rings that will give you the proper height with whichever scope you select, and a low-power variable (2-7x, 3-9x, 2.5-10x or 3.5-10x) or fixed power (6x or 10x) scope. There are some great values out there right now, including the Bushnell Elite 10x which will work on the SR9 in terms of length/bell size. Check out SWFA or some of the other optics dealer/site sponsors here for more scope options...just remember to keep in smaller/shorter (40mm obj or so and no longer than around 13" at most). All the Bushnell Elite 4200 (and some other models) are on clearance right now at most all retailers and they have some great optics choices for your SR9. Here is a link to the MFI mount I mentioned:
http://www.hkparts.net/shop/pc/HK-Low-Profile-Scope-Mount-For-All-HK-Rifles-Pistols-118p762.htm
Here is an example of one of the great deals on 10x Bushnell Elite 3200 scopes:
http://www.midwayusa.com/viewproduct/?productnumber=129037
Finally, get some decent ammo for testing purposes with your rifle. Federal Gold Medal Match is really the universal standard for across-the-board accuracy testing. Grab a couple boxes of 168gr loads and shoot some 5 round groups for accuracy (prone or from the bench).
All told, with a trigger job, mount/rings/optic and some good ammo...you'll be out <$500 or so and you'll have a very good, roughly MOA-capable rifle on your hands. Get that SR9 out and put it to work for you!!