I'd bet they'd still have the M4. But the SCAR is a good choice. I know our SF uses them. Our SF uses just about anything they want, any infantry battalion can use its funds to purchase specialty stuff, and individual soldiers can get CO permission to carry a different personal weapon into combat. My buddy, the head SDM instructor at the Stryker Brigade Advanced Infantry Skill Center, took his M14 (actually a M21?) to Iraq. I got out just before deployment orders, but I was taking a G17 and a Stoner 7.62 (which I never got because I got out...). We did recommend the Grendel as the SDM cartridge, but we got a new Brigade CO that cared more about pretty uniforms than weaponry.
For pistols, I don't see the US going outside of NATO framework save the specialty stuff. SF commonly use the .45 in 1911 pistols. Pretty plain looking pistols, but they have had a lot of work done on the inside. The Marines custom make their own 1911's, AR's, M14's, and bolt guns from scratch. The match weapons and sniper weapons anyway, and their version of SF get 'em. Standard issue is likely to be 9x19. The army just doesn't invest much interest in sidearms. They are almost like paperweights.
I'd say in ten years the biggest change would be that if we had new weaponry, that it would be dispersed like this. First it would go through development, then SF would get a say on it, then it would be issued to Ranger units, then the fast reaction units would get 'em, Stryker and Airborned units. Stryker units have lots of funding, they get some cool gear. After them, the rest of the infantry gets the new stuff. The non-combat units will likely get the M4's until enough contracts are filled to make a complete change. This is how it is going now. I've seen some pictures of soldiers in Afghanistan that have some weaponry with fused distance grenade launchers built into the rifle. I have no idea what it was, I've never seen it before. A Stryker unit had 'em.
The cartridge will be 5.56, but there is call for a different round. I recommended the 6.5Grendel, but I was looking at it from a sniper point of view. Others like the 6.8SPC.
Interestingly, we also fielded a test mule, the .50Beowulf. I recommended that one for checkpoints, for vehicle stops. Also for door breaching. My view was that it was easier to carry a couple of uppers rather than a Mossberg pistol grip which could only be used for breaching. Sufficient to knock out most light engines, our unit didn't go for it, opting instead to create new checkpoint doctrine using the Stryker's .50BMG, which worked very well. But the Coast Guard does use the Beowulf and the Barrett .50BMG to knock out boats from helicopters.
I'd say give your SF guys whatever you want. Be more conservative with regular units, and more so with the non-combat units. Some of those guys still have old M16A2 rifles by the way, and the Marines used them for half of the last decade as well.
Also know that each time a war happens here, we upgrade to an obnoxious degree. We have the debt problem because of this. The same thing that happened to the Soviet Union is happening to us right now. They overspent on war junk and it bankrupted them. We are doing it now, even though we don't need to. But the political system here is in bed with the military industrial complex, and short of walking in and shooting them both for infidelity, it isn't changing.
Oh yeah, speaking of ten years of development, which would be spurred more so by a Chinese invasion of Europe, you will see a LOT of robots on the field. We have ones now that carry gear, check for bombs, little backpack spy gliders, drones, and the Air Force plans on going full robotic in the near future. No more pilots. The drones will be able to do maneuvers that would kill a man. We also have an exoskeleton that turns infantrymen in to herculean killing machines capapble of lifting volkswagens single handedly. I can only imagine how that exoskeleton will be plated. Raytheon makes this, it won't be fielded soon, but if an invasion like you have planned for your book happens, you never know.
We fielded the M16 fresh off the lines in Vietnam. Using the wrong powder, wrong twist for the bullets, wrong barrel materials, no foward assist, and no cleaning kits for such an intricate weapon system. This FUBAR still gives the M4 a bad rep today among the unknowing.
Good luck with your book!
Not just due to logistics and cost but because when fighting modern troops the 9mm is the superior round in my opinion. Whilst the .45 packs one hell of a punch against Jihadists it it very weak against even soft armour.
