You could use an Armorer contact to build it for you. Use the Mechanic (SR4A, p. 291) as a basis for the Armorer type, giving him a skill rating of Armorer 4. Now, with Logic + Armor of 8, he can buy two hits per test, meaning that he could "phone it in" and get you a respectable gun-cane in 8 weeks. It might take more/less time if he rolls for it, and you can get him to rush the job (SR4A, p 65) to get it with an interval of 3.5 days.
Inv Castle TestUsing the rolls here, your contact could get you the gun cane in 7 weeks. Or 5 weeks if rushed (since 1-2 glitches, there was a critical glitch on the third interval and he had to restart) AND there was a glitch on the 4th test of the rush job, meaning that it might still be delayed, or their might be something
wrong with the function of the cane that could adversely affect you in the future (it only effectively fires 1 out of 6 times, or the pop-out blade gets jammed easily and needs to be repaired to pop-out again).