Not tested this but an idea for packing all guns (while you have 3). Worked in WaW, not sure about Black Ops.
1. Get your 3 guns, buy claymores.
2. Teleport to PaP Machine.
3. Put in your first weapon to be upgraded. Stay right in front of the machine.
4. Just as you've put it in rapidly press the arrow button til you get out claymores.
5. As soon as your claymores are out hold the square button and do NOT let go.
6. Continue to hold square and you should automatically take the PaP'd weapon.
7. Let go of square.
8. You should still have all 3 of your weapons. (hopefully)
9. Do steps 3 to 7 for each weapon to be upgraded.
Doesn't work. As soon as you pull it out it won't let you get the 3rd gun. Treyarch probably anticipated a 3 gun glitch would be possible and patched it. And also you can't get more than 3 guns.