The first concern is time. If it is truly 1500 years AFTER the year 2200, what the hell are weapons going to look like? What will they fire and how hard will they be to reload?
I personally don't like the idea of laser or ray guns. As seen on a documentary of "future wars" they would most likely emit a silent long lasting invisible beam capable of burning a target at an almost infinite range. BORING!
Stepping back in time to gunpowder is tempting but a bit unbelievable. So I move on to an only slightly futuristic concept of rail guns which is more my style. Those are cannons or handheld guns that fire a metal projectile with electro-magnets, as demonstrated here by the US Navy...
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That projectile was fired at Mach 7 and didn't hit the ground for ~120 Nautical Miles because it followed the curvature of the earth! I presume the bigger ships in OT may have that kind of thing but the concept can be applied for a rifle application.
The benefit of this type of weapon is the familiar reloading characteristics where a magazine of projectiles will need to be supplied and that mag could also contain the ______ fuel/energy required to charge the weapon. It could retain a standard-ish design characteristic and it also makes a hell of a sound!
I have always liked my guns short and stockless with the ability to either fold out a stock or screw on an extended barrel for those hard to reach radioactive spiders. Simple designs like this with some slight modifications such as bigger magazines to accommodate the energy pack should suffice.
Going entirely electromagnetic is a good idea because I can see plenty of scrap, able to be turned into ammunition whether it be from parts of the landing ship or wreckage from the civilization's past. The weapons will require this magical energy we are all trying to harvest and it is a concept not too far out for current gamers. In most future shooters the rail gun is the last weapon obtained and is the most powerful so giving it to them strait away would be awesome in anticipation of the gun that shoots bee's to be released later that year.
Any vehicle mounted guns can be larger but instead of a standard projectile they could fire handful's of shrapnel at longer intervals sort of like a massive shotgun.
The variety of weapons will be non-existent at first I am sure. A single type of rifle on the landing craft and none equipped to vehicles. Those sort of upgrades can occur later on.