What's happening with my gun?

There are many factors that determine a gun's power.  The ones that give you trouble are the ammo type and wind resistance.  The ammo you are using seems to be one that can be affected by the wind rather easily. 

Indoors you don't get any wind resistance, while outdoors has plenty of wind resistance.  If I shoot normal rods from a TR without fins, the wind resistance will cause the ammo to stall around the 80 foot mark, causing the ammo to fall down sharply, while if I add fins this does not happen and it shoots a good 140 flat.

The wind resistance is your problem, you need to probably add fins that will curl around the plane when pushed, but when they are let go, fins pop out, collapsible fins are like this.A fin construction for small missiles that includes two members that are otally secured together at one end with the other end of one of the members being pivotally secured to a structure and the other end of the other member being slidably mounted on a rod with tension springs biasing the two members together to form a generally triangular shape of a fin. The two members are maintained in a flattened out relation inside an opening of the missile housing by securing means around a plurality of the fins located around the periphery of the missile which securing means maintains the fins in a retracted position until the missile is launched from a launch tube at which time the securing means is removed to allow the tension springs to bias the fin members into a triangular fin shape for stabilizing the rocket.A real example of this type of missile/ammo is the Type 79 Jyu-MAT / 79 Tan-SSM.The Type 79 Jyu-MAT (79þGþ&ʘ> nana-kyuu-shiki-tai-syuutei-tai-sennsya-yuudou-dan?) is a Japanese SACLOS guided anti-tank missile that entered service with the JGSDF in 1984. It was initially issued to coastal defence units, intended to destroy troop and vehicle landing ships as they approached the shoreline.The missile is stored in a cylindrical transport container. On launch the missile is ejected from the tube by a solid rocket motor. After traveling a safe distance from the operator, the Daicel Chemical Industries flight motor ignites and takes the missile to its cruising speed of approximately 200 meters per second.
The missile is a thin cylinder with two sets of four pop-up fins positioned along the body of the missile. The warhead is either a shaped charge for use against tanks, or a semi-armour piercing fragmentation type with a delay fuse for use against landing vessels.The missile is steered in flight by command signals from the launcher transmitted over a guidance wire spooled out behind the missile. A xenon lamp on the rear of the missile allows the NEC manufactured launcher sight unit to compute an offset between the missile position and line of sight to the target, and calculate steering corrections based on this offset.
The Type 79 missile can be fired remotely, at a distance of up to fifty meters from the tripod mounted guidance system. It can mounted on a Mitsubishi Type 73 jeep, similar to the Type 64 MAT and the Type 87 Chu-MAT. The Type 79 is also in use with the Mitsubishi Type 89 IFV.

It's just perspective.  Inside everything is close and it looks like the gun shoots far.

Outside everything is farther off and it makes the gun look like it doesn't shoot as far.