Unfortunately, there is (as of yet) no fix for this issue. The only way to repair it is by vendors. The best vendor for repair can put it to 75%, so it's not SO bad...=/

If your barter skill and repair skill is at 100 your vendors should be able to repair it to 100%
at least thats what mine is doing.

If you have the Mothership Zeta DLC there is an item called Alien Epoxy that will repair any weapon you have equipped. Not sure if your repair skill factors in, and I don't think it works on armour.

either you do A. talk to uncle roe at canterberry commons and talk to him about investing in trade vans and invest in crazy wolfgang (when you invest in him his repair skill will be at 75 but also note it costs ALOT of caps to do this!) or B. get the DLC mothership zeta and find some alien proxy to repair your gun. other than that, you can always go to megaton and go into the craterside supply and talk to moria brown and have her repair stuff for you (since i think she has the highist repair skill if you don't invest in a trade van).