Barrel Length
 

I'm looking at building an SPR, which would normally use an 18" (457mm) barrel. I've found a King Arms outer barrel that is 20" (508mm) which is fine with me. But I intend on attaching a 250mm suppressor to it so that I could accommodate a longer inner barrel. My question is, since I will be using an M4/16 platform to build off of, am I going to run into issues if I use say a 650mm tight bore?

Yes this is a LONG gun...by my calculations, total barrel length including suppressor would be almost 30"...

If you want to use a 650mm barrel you'll need a non-ported bore-up cylinder set.

There's nothing wrong with having a long rifle, but my question is, why would you want a longer inner barrel? It really doesn't make any difference in range or accuracy...

I thought the general consensus was that a longer inner barrel would increase accuracy...obviously there is some point at which this becomes detrimental, however, the barrel lengths I am looking at are about the same as those found in a PSG1...

Well if barrel length had anything to do with accuracy, my HFC beretta with it's 100mm stock barrel wouldn't be dead accurate at 30 meters now would it? :p
That was the hard evidence that convinced me. The biggest factor in range and accuracy is compression. As long as the air behind your BB remains compressed until the BB is out of the barrel, your going to have great range and accuracy.
That being said, tightbores affect accuracy, not necessarily because they're tighter, but because they're more consistently the same bore down the barrel.
It's all about keeping that BB in the very center of that barrel until it leaves.
With longer barrels, there's a higher chance of dirt affecting the shot, compression issues will be more apparent, and your really just keeping the BB in the barrel longer than you have to