Matt, if your say 205 pounds at 12%bf right now with 17" arms.. and if you keep training arms intently and with purpose,, when you bulk up to say 230 pounds and maintain that same 12% bf, you'll probably have 19" arms, cause your arms will follow.
if your a rare individual that has awesome genetics for arm growth, then 'maybe' you could get to 19" arms without adding body mass. but thats unlikely to happen, otherwise you'd already have 19" arms and would not be posting threads about arm growth. so being you don't have great arm growth genetics, then you just need to train harder/smarter and put on more overall size. its that simple
You do NOT need Tren, and you do NOT need to gain 'unnecessary' body mass.
and I'll disagree with big ben, yes you 'might' actually need more Tren (or deca) and need to put on more size to get to 19" arms. big ben may not need that cause of his genetics,, but YOU are you and got to decide what YOU need to get to your goals, may not be the same as ben because of genetics.
if you want 19" arms, and more tren and 30 pounds of more muscle on your body will get your there with focused arm training along the way . then fucking go for it if you want it .
(bodybuilding is a vain sport.. heck other guys out there with 19" arms who got there with running gear and putting on body mass, may NOT want you to get there too. they want to be the only guy in the room with 19" arms and won't admit the truth of how they got there let alone give you direct advice to help you get there , they don't even want you to succeed)
and on a side note: I generally don't listen to training advice on muscle groups from guys that have great genetics for that muscle group. example, if someone has great calves (genetically), don't bother asking that guy what he does or how he trains to get great calves. heck he may not even train calves, he just has a genetic propensity towards great calves.
the guy with the biggest arms in the room could have the worst training advice for arms, he just has good arm genetics, and then use those good genetics as an excuse of "see look at me and my big arms, I must know what I'm talking about", thats not necessarily the case.