It is 100% fact that it's not mil spec. The thing is, they use lower quality parts, don't test properly, their specs are often out of whack, many, and I mean a huge percentage, of their chambers when marked 5.56 are actually 223 and need reamed for 5.56. Also, their gas ports are often much larger than spec of .063 for 14.5" or 16" and this leads to many serious issues that get worse with time.
I could go on forever here, but another Google search will detail it much better than I can. I can't tell you how many I've seen fail in a class where others did not. They will suffice for basic range shooting though and can be tweaked for better reliability, though still won't be a true "fighting rifle" in most professionals' minds.
I can guarantee, with 100% certainty, that DPMS is not mil spec (actually only Colt is and Remington military since they hold the TDP for the M4, but Colt is as close as you'll get with BCM, Daniel Defense, LMT, some Noveske's, etc.). There are rifles out there that are very high quality that exceed mil spec, but DPMS and Bushmaster are not on that list. FN, Colt and now Remington are the only ones that legally have access to the TDP for the M16 and M4. Colt's civilian 6921 is as close as you can possibly get without class 3, then the 6920 would be next. The only difference there is the extra 1.5" of barrel and the semi only fire control group.
Many people only understand part of this, so it gets confusing. There's no reason to buy anything but mil spec or better when it can be had for much less than before. You can find 6920's all day for about a grand online. You can assemble a BCM for a little under a grand. Now, I'm comparing this to a $1,000+ Bushmaster. It would be hard to argue with someone that has a budget of $700 and only will shoot it at the indoor range once every couple months. But, for professional or serious use, only mil spec or better should even be considered.