I was a 45B/91F from 2006-2012.
I worked in the Motorpool, Started my own Battalion sized Small Arms Repair Shop, supporting 4 Companies stateside, 6 Companies while deployed. I worked on my own for the first few years, I was authorized 2 other E4 and below in my shop, I didn't get them until 6 months before i was ETS'ing.
It was a great job. Got a lot of experience, met and worked with alot of cool people. Im currently working for a company doing tech support, customer service and, building AR-15 Platform Rifles. I would say that the training and Army experience did help out with that, but as far as serious gunsmithing, i would say no. Working on Machine Guns, M4's and such is a lot different than building a complete 1000+ yard rifle from the ground up, or bedding actions. Its just not taught in the AIT/ On The Job as a 91F. Your taught to repair what the Army uses. Overall it was a pretty cool MOS though.
Career length? if nothing has changed, you make it to E5 as a 91F. Once you get your E6 you are reclassed to a 91K, which is fire control and tank turret repair stuff. Its a mix of 91G and 91F if i remember correctly.
You can be an instructor at the Ordnance School as an E6. Most in maintenance MOS's end up going Warrant after being an E6/E7 after a while. There weren't many Ordnance E8's that I knew of, E7 was pretty much the ending point, then you retire.
If you are an odd MOS and there isnt work for you in the motorpool, most of the time you get cross trained to work on vehicles. We had gen mechanics, welders and myself doing vehicle services during part of the deployments.
Feel free to PM me if you have any more questions or anything.