It all depends on what you want to skill in. I would first max out the TT gun then go from there. Here is the list of (L) guns you should use in order of use. Remember that you need to look at the HA (Hit Ablilty) and CHA (chritical hit ablilty) to know how your doing on a gun. When you have the gun at 10/10 then you have it maxed out.
If you follow the list of guns this is the most eco way of getting your skills up. Again your going to want to look and see what kind of guns you want to use. IE. Handguns - BLP or Laser or Plasma- Rifles- BLP or Laser or Plasma. ( I would stick with BLP or Laser use plasma as a tagin gun IE Marber Bravo)
Also for fun I have the Swords in there
List of (L) guns and the order in what to use them in:
BLP Pistol
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Emik S30
Svempa S40
Emik S50
Svempa S60
GeoTrek H40 Brody
Svempa T5
GeoTrek H41 Mina
Emik T10
BLP Rifle
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Isis CB5
GeoTrek LP120 Niloticus
GeoTrek LP175 Sphyra
Isis CB13
Isis CB19
GeoTrek LP470 Manis
Isis CB21
Laser Pistol
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Omegaton M2100 - Available from the TT
Breer P1a
Breer P2a
Breer P3a
Korss H350
Korss H380
Breer P4a
Korss H400
Breer P5a
GeoTrek H21 Scrof
Isis HL6
Laser Carbine
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Sollomate Opalo - Available from the TT
Breer M1a
Breer M2a
Breer M3a
Riker UL1
Riker UL2
Riker UL3
Breer M4a
GeoTrek AL15 Taurus
Isis LR32
Isis LR41
GeoTrek AL17 Auriga
Longblades
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Castorian Combat EnBlade-A - Available from the TT
Loughlin Cutter One
Loughlin Cutter Two
Loughlin Cutter Three
Embra Laser Sword C1
Kilic Kek
Rutic Kil
Nano Katana
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Good Luck M8t!
Boo Who You