Aliens' M41A1 Pulse Rifle not a rifle
 

Watching the Aliens movie again, I noticed since the Marines' rifles were built from a Tommy gun and Hicks even stated that it fires 10mm caliber, and judging that they didn't mod the Tommy magazines much, it seems more like a submachine gun than a rifle to me.

Yes but the audience is not supposed to know that it is really a submachine gun.
Also the 10mm is not the real life 10x25mm, but some fictional 10mm caseless round.

Good point, I forgot about the caseless ammo part. Then if we are going along with the fictional logic of the gun, it could be like the HK G11, it's just the projectile itself. Without the casing, it would make the entire bullet just the bullet itself and that in turn would be small. So i guess it could be a rifle caliber that's 10mm.

Thinking on that, I wouldn't the advantages and disadvantages was of the caseless ammo that the G11 offered. Why was it canceled? It clearly was a breakthrough. Was it because the ammo is too new and that no army would adapt it because of production cost?

If I recall correctly the H&K G11 was canceled after the reunification of West and East Germany as part of some budget cuts.

But you're basing that assumption on the fact that you KNOW that the Thompson SMG was used as a base weapon for the fictional Pulse Rifle. The audience would not know this.

The technical specs of 'future weapons' may be different. We only identify SMGs when we know that they fire Pistol calibers. But the weapon can be anything, and the 'fictional' 10mm "caseless" (which isn't really caseless because we can see the .45 acp brass flying in most sequences) could be an official rifle round, just like the 5.56x45mm is officially an 'intermediate' cartridge for an "assault rifle".

I already corrected my assumption on that, MPM.

I used to be an "Aliens" fan, and my recollection from reading the (semi-official) literature is that in the "Aliens" universe, rifle (M4A1) and pistol caseless ammo is distinguished by the length of the cartridge only. There's even a quote about it in one of the books, though I can't remember which one.

But that quote in the book could be inaccurate since a lot of writers might not know anything about guns in general.

I'm pretty this one was familiar with guns. I remember the book - I just figured out it was the one called "Earth Hive" - also made a reference to a character using an "antique" S&W 1076 as his sidearm.

The fact that he would even try to explain differences in ammo types means he's quite obviously a gun geek, as well as a sci-fi geek.

So according to the book, they measure ammo by the entire length of a cartridge? So 10mm overall length for the rifle would be small.