Well I can't help with the editing (I've got some of my own questions as a matter of fact), but I do wish you luck (I'd like to try it out).
But I DO know a bit about M16 series rifles and AR15s so here Goes:
The M4 is a different beast completely from the SPR.
The M4 is a carbine weapon orginally intended for more special operations and replacement of submachine guns and many handguns. Now its an issue rifle for many units in the Army.
The SPR (Special Purpose Rifle) is a fairly rare beast. Its a custom built rifle made by Crane (a Navy Lab in Indiana), orginally made for the SOCOM Seal Units - now used by some of the Army units that make up SOCOM.
Its a 5.56mm rifle. Uses an 18" long Douglas Stainless Steel Match barrel (standard M16A2's have a 20" barrel , M4s have a 14.5" barrel - add 1" if the flash hider is on). The SPR uses an Armalite (brand) flattop upper receiver (A4 style) and a carbon fiber free float tube instead of handguards. The combination of the match barrel and free float tube make for an exceptionall accurate rifle. The Free Float tube also has rails attached (the one along the top runs all the way to the receiver and helps with the structural integrity). These rails are used to attached various optics, lights, and bipods.
The rifle is build on M16A1 lower receivers and as such have the semi/full auto capability (no burst). This was done as the burst mechanism really screws up the trigger pull even when firing in semi. The trigger group are custom match parts and not 'reworked' military parts.
The rifle is intended as a 'medium' range sniper rifle (or a 'light' sniper rifle) and often use with the 75gr Tap rounds or 77gr SMK Match rounds. They are also working with 80gr and 100gr rounds (to help improve terminal performance at longer ranges - and help with bucking the wind at longer ranges).
Both the front and rear sight can be flipped down when not in use (on most weapons in the M16/Ar15 series the front sight is always up). While you can use a variety of scopes - the one Crane has requested purchasing is 2.5x to 8x variable power scope with a mil-dot reticle (think of cross hairs made up of small dots).
There is also a sound suppressor which can be attached to the rifle.
Hope this helps!