Hands down the best soldier guns in the game for high level people are the Spasmodic and Collaborator Assault Rifles.  Naturally they are also the most painful to get :(  But here is a general guide to help you through the process.

What you need...

You'll need the following to make your assault rifle.

An Inamorata Assault Rifle (omni) or a Sancrosanct Assault Rifle (clan)

Grey Glyph of Ocra-Xum. Any ql.

A Turn Spirit of Fiery Hatred (omni) or a Turn Spirit of Laethosin the Leader (clan). 80% QL of your gun.

Inamorata/Sancrosanct Assault Rifles.

The assault rifles are obtained by killing sided and/or unredeemed/redeemed mobs of the opposite faction. That is, if your are omni (unredeemed) you will want to go kill redeemed mobs and vice versa for clan. The best places for these seem to be in the various temples throughout the SLs playfields or inside static dungeons. The guns are also not [NODROP] so you can simply buy one from someone if you don't want to go about getting your own. To get a gun of good ql will take some time though so don't expect it to be over in 15 min unless you are lucky. The guns come in ql 100-300 only, so there is no point hunting down lower level ones either.

Grey Glyph of Ocra-Xum

These are even easier to obtain as any ql works. They drop from static dungeon bosses so you can just stroll into a lower playing field thats way under your level. Of course the best way to get them is just to buy them. Hardly costs a thing, with 100k usually being more then plenty to entice a seller to sell you one.

Turn Spirits :(

Unfortunately this is where it gets tough. To obtain a turn spirit you have to kill certain dynacamp bosses in Adonis or Inferno (Only 2 places confirmed, at the moment). The drop rate for a turn spirit isn't all that bad, by best guesses its about 5-10%. Unfortunately there are a total of 32 different types of turn spirits that can drop and in a wide ranges of Qls. While it is possible to buy these parts as well, soldier turn spirits seem to be perticularly rare and thusly VERY expensive if you can find one for sale at all.

If you havn't been to the forums yet and downloaded Sphere's excellent SLs map, I suggest you do so as it is extremely useful and I will be refering locations on it for this guide.

Mull and Turk the Adonis Dryads
   If  your going solo or can't kill in inferno, Adonis is the place you'll end up in. Adonis is especially easy because of the low hp and level of the mobs which will probably all be grey to you. Here you can expect to find turn spirits from ql 120-200.
     First thing to do is get there. Open your map and look for the following place. The easiest/safest ways to get there are by taking your respective garden's statue to Lament Lagoon. If you are omni, go straight north over the cliff. If you are clan you'll have a bit more of a run NW. To make things clear, they are IN the abyss ( underwater) dont expect to find much if your not underwater :P  Also note the location of the yellow cursor in the picture to the right.  This is where Mull spawns, not at the red dot on Sphere's map.  Mull will tend to wander if left alone (but not to far).
     For the most part you'll be wanting to kill Turk and Mull. They have a much higher drop rate of the turn spirits then the other 4 dryad bosses down there (Starks, Skylight, Jukes, Black). All 6 can drop spirits but the latter 4 have much more loot in their drop tables and it clutters up the drops making whats already a rare drop even more rare.  The other 4 drop them so rarely that most soldiers don't bother killing them and just stick to Turk/Mull
     For a soldier the bosses are pretty easy, they can however cast 2 different and stackable dots of 290, and 148 as well as occasionally a good Init debuff. Having TMS running is a good idea. The bosses respawn every 25min with a chance of spawning around 80%. All of them will stand still except for Mull and Jukes who walk around, but always stay within tabbing distance of their starting locations.

Paxos, Beast and The Distracted Snake Tamer
     If you can get to inferno and can manage to get a good team together then you'd be better off in Inferno camping as higher and more usable qls drop there. Don't get me wrong, they are still rare but they are better qls, some easily enough to make a ql300 gun. You will need a good team though, preferably with a good damage dealer or two, a healer and someone for crowd control is nice for the adds you will get. The bosses you are after out here are around the E side of the eastern most incarnator in Inferno. They are named Beast and The Distracted Snake Tamer. These bosses are very powerful not to mention they seem to have a much longer range to agro you with. There are a lot of high level mobs around so watch where you step. A 4k+ crit after TMS has fallen is not uncommon from things in this area. No special tactics here, just try and clear a path 1 by 1 to the boss then drag the boss far enough away and hope you live :)   Paxos ( A boss near the 255 incarnator in inferno also drops them)

Yuttos and Flesh Pouches

Alternativly to camping, If you have lots of money to spend and a fixer friend  handy you can see yuttos about a "Turn Spirit Flesh Pouch".  These handy little items spawn a turn spirit when they are used (Takes B&E to use).  They generally cost between 5 and 20 million credits depending on ql and you can get them from any Yuttos Manual.  Elysium tends to be a  popular place to check since there are two sets of yuttos there.

They are "suppose" to spawn a random Turn Spirit in the ql you buy, but for now they spawn  a specific turn spirit depending on the ql.  That is a ql 222 pouch will always spawn the same type of spirit.

Currently they drop as follows.

A Turn Spirit of Fiery Hatred:  ql 155, 202, 229, 243 and Turn Spirit of Laethosin the Leader: ql 150, 151, 160, 272.

"Nothinman" keeps a well updated forums post here listing all the current known Qls/Types

Yutto's will change their stock roughly every half hour if they are not talked to during that time.  Another popular way is to get someone without faction to loose to kill the yutto but this takes time and most people have faction.

Making the Final Guns

Now that you have all the pieces, making the gun should. First you have to go find a GOOD tradeskiller to combine the glyph with your gun. This process takes 6*ql of gun in Weapon Smithing, 5*ql of gun in Electrical Eng AND 4*ql of the gun in QFT. After combining the gun and glyph you will end up with a XXXX gun of Ocra-Xum. This adds a bit of damage as well as the burst and full auto specials to the gun. While the burst is fast at this stage, the full auto is not.
After you get that back all that left is to simply add your turn spirit (which remember has to be 80% of the ql of the gun at least) to your gun. YOU MUST DO THIS YOURSELF!!!. Adding the turn spirit takes Weapon Smithing 1*ql, but makes the gun nodrop. Adding the turn spirit drops the full auto recharge way down, adds more damage and changes the name of the gun to Spasmodic/Collaborator.
With the right skill levels both of these gun can cap out their Burst and full auto recharge times which gives you some serious special damage potential. :) Enjoy