There's no gap to bridge between the receiver and upper rail when installing a scope or sights. 5.3 APPEARANCE The mod 0 has a much more distinctive look, which I prefer. The SPR MOD 1 is interesting and I do like ArmaLite (and even built one as a basic carbine) but I also have to say Colt. The original AK-47 was built to use this cartridge. Even though I didn't know Colt had a monolithic upper! The SPR Mod 1's one-piece construction also offers a continuous optic platform (COP). The Mod 1 6.8mm is built with the ArmaLite-developed mid-length handguard and gas system to provide the best in form and function. Changing rails and add-ons on the SPR Mod 1 is as easy as turning a torque wrench, according to ArmaLite. The ArmaLite SPR Mod 1 is a collaboration between ArmaLite and Aero Precision, with Aero performing most of the design and tooling. The gun has a nice feel and the solid rail system/upper receiver is a definite plus. The ArmaLite ran without a hiccup, with the only stoppages being the ones that I intentionally induced by inserting a dummy round. Still when you compare the parts, there is a difference. 5.2 RECEIVER FIT The SPR Mod 1™ is not a custom firearm. However, weight in a precision type rifle is not nessesarily a bad thing, unless you plan to be hauling it all over kingdom-come. The ArmaLite SPR Mod 1 is a kit that contains a true one piece upper receiver manufactured from a single 7075 T6 aircraft aluminum forging. Re: Mk12 Mod0 vs, Mod1 To me, the mod 1 looks a lot closer to an M16A4 with a scope. This rifle is known as the ArmaLite Defensive Sporting Rifle 10, or ArmaLite DSR10, or ArmaLite DEF10, and in California as the ArmaLite SPR Mod 2A. ArmaLite® requires a close fit between the receivers, but normal forging and machining variations may result in play or slight side to side receiver mismatch. The SPR Mod-1 carries an MSRP of $1,554, and comes standard with a chrome lined barrel, two stage trigger and hard coat matching finish, as well as the patented hardened Armalite bolt. Armalite has issued a "Notice of Potential Safety Issue and Recall" for Armalite Models AR-10, SPR MOD 1, SPR MOD 2, M15 and Eagle-15. This is not a manufacturing defect and has no adverse effect on accuracy. It is also about a pound heavier than the mod 1, as mentioned above. But none of that is rollmarked anywhere on my rifle; instead AR-10A is stamped on both the lower and upper receiver. So what makes an SPR Mod 1? Posted: 2/1/2015 9:32:07 PM EDT Quote History. ArmaLite’s SPR Mod 1 7.62X39mm provides all the flexibility of the Mod 1’s one piece upper receiver/rail system but now for the world’s most available cartridge. The ArmaLite SPR Mod 1 LE carbine is a solid, well-built carbine that brings the story of the AR-15 full circle. Special Purpose: ArmaLite also offers the ArmaLite SPR (Special Purpose Rifle) Mod 1 mid-length carbine featuring a monolithic-rail upper, which means the forend and receiver are machined from a single piece of aluminum so that flex between the two parts is decreased, and day and night optics can be mounted on the same plane. Armalite "Special Purpose: ArmaLite also offers the ArmaLite SPR (Special Purpose Rifle) Mod 1 mid-length carbine featuring a monolithic-rail upper, which means the forend and receiver are machined from a single piece of aluminum so that flex between the two parts is decreased, and day and night optics can be mounted on the same plane. The Armalite SPR mod 1 has a quad rail monolithic upper. CDNN sells the SPR Mod 1 as an "M15". Posted: 2/1/2015 9:02:29 PM EDT The M15 is like a regular Ar with round hand guards and A2 front sight.