Alright, so I currently have the Force Gun, the Plasma Cutter, the Line Gun and the Pulse Rifle, and I previously owned the Flamethrower, the Ripper and the Contact Beam, but I never got the "Full Arsenal" achievement, for having every weapon.
What am I forgettin' here?

You need to have all the weapons at once....

Ooohh! I didn't know you could have more than four guns at once!!! AAallllright, I gotcha.
Also, do I need to have the gun that you can buy (I think it's like 80 Microsoft points) after you beat the game?

No. Just all the in game weapons, none of the DLC ones (which basically were just new skins for the same weapons). You can only hold 4 at a time, the rest must still be in your inventory for the achievement to unlock.

quote hell hawk

Ooohh! I didn't know you could have more than four guns at once!!! AAallllright, I gotcha.
Also, do I need to have the gun that you can buy (I think it's like 80 Microsoft points) after you beat the game?

Well, to get all of them at one time, you have to deposit 2 or 3 in the safe.
I have to cover this shit in my walkthrough when it's done.....