Ey man you should try doing what I usually do to get a correct size and handposition. First import the original model you're going to replace, this way you can position and scale it down to match the original as best as possible. Focus on the first handposition (the one by the trigger) make the triggers overlap since this handposition is the hardest to do right. If it's positioned correctly you can drag all the options (muzzle_flash, gun_grip etc) to your custom model (in the items menu) and delete the original weapon.
All maxed out.
P:AMD Phenom II X4 965
G:AMD HD Radeon 6970 2GB
M:16 GB Kingston Hyper X 1600 DDR3
M:Asus M5A99X-EVO