I agree about getting a real HK91 and not a clone. The HK91 is a great rifle with a few easily corrected but serious weaknesses.
The trigger is heavy and poor, but an inexpensive and excellent fix is available from www.williamstriggers.com.
The sights are mediocre, but the “1200 meter sight”, which can be found on the gun boards such as www.hkpro.com or www.sturmgewehr.com is much better, and can be drilled or fitted with an insert to provide the ideal aperture.
The narrow, hard plastic butt stock can be punishing, but the butt end piece can be replaced with a wider rubber part from HK (from the HK21 or 23 belt fed machine guns) that is much more comfortable, and which avoids slippage. From an HK dealer or the above gun boards.
The magazine release button is suboptimal, but a “Tac-Latch” provides much more ergonomic operation in a low-priced drop-in part, from www.taclatch.com.
For optics mounting, the Brugger & Thonet mount is best, because it does not damage the receiver, and provides a see-through view with the iron sights if the optics are fogged (or worse.) It provides a lower mount than the original STANAG clip on mount. Avoid the B-Square mount, because they can cause receiver damage from over tightening.
The HK91 may not quite be capable of the same accuracy as an M14/M1A, but it never malfunctions, and nothing ever breaks. – Mr. Bravo
JWR Adds: As I mentioned in my novel Patriots, the only other major design flaw of the HK91 is that unlike an AR-series, M1A, or FAL, the HK91 does not have its action lock in the open position after the last round in the magazine is fired. In the stress of combat, this could leave someone thinking that they still have ammo when they are actually holding a rifle with an empty magazine. (Read: potentially deadly.) This can be avoided by loading the last two or three rounds in each magazine with tracer ammunition. When you see a tracer flash, you swap magazines.
OBTW, tracers are banned in California, but then again, so are HK91 rifles. So if you live in California, there are a couple of more reasons to vote with your feet!