In answer to your question: both yes and no. Your general concept shows promise as the idea of a marksman who is heavy on defense is quite interesting. It adds variety to the class while being easily countered (perhaps too easily) by AP, as well as providing a unique mechanic, the group dash.
However there are some more questionable points to your concept; for instance take the 1 armour = 1% critical strike chance.
**Most marksmen have approximately 19-24 armour **making an average of about 21.5, plus the armour from a level one q and that gives us 26.5 armour at level one (discounting runes and masteries) effectively giving him a 1/4 chance of criting for 200%ad in damage for 8 seconds after every 10 autos. This would give him a very burst-focused laning phase centered around getting 10 auto attacks then going ham while pretty much always trading positively against the enemy ADC. However, due to standard base scaling, along with the q, runes and masteries it is quite possible for this rate to reach 100% without building any armour whatsoever.
So essentially we have an ADC which can reach 100% crit chance every 10 autos mid game. This makes him heavily reliant on attack speed in the mid game which works pretty well with your machine gun theme. However, this presents us with a problem. Building crit chance on him is unnecessary, even a tad wasteful as you are unlikely to enter fights without your passive up if you are at least semi-decent, whereas building attack speed is necessary to utilize the 8 second duration of his passive while ad is needed to increase the damage of his crits.
Only three items provide ad and attack speed, those being trinity force, guisoo's and Bork. Think about what this means for his play-style. Hurricane, trinity and guinsoo's with maybe a Bork with flat armour from there on. Dash in, allies cc, ult, auto like crazy. Due to his method of infinite armour stacking through his q, he will take negligible damage from the enemy ADC and the dreaded mobile bruisers while also dealing highly significant damage to them with the whole ult scaling thing. I'm thinking every adc's worst nightmare.
**Then again, their entire kit leaves them very vulnerable against AP **as his ult does reduced damage against AP targets as well as the fact that the armour is of little to no use. This leads to him being very easily countered by the enemy mid/support/top/jungle while only posing a real threat to their ADC. This could in theory be rectified by building a good engage comp (as this would allow HIM to burst the mage instead of the reverse). Think almost rework kogmaw style except less reliant on team for peel, which is a truly horrifying thought.
On another note, his entire kit only has (essentially) two abilities, one of which is a heal group dash. The heal dash could be really fun but having the heal just makes it feel so overloaded, with this current level of healing with no mana cost on a 4 second cooldown he would be a broken support. A dash on its own would be very powerful, the heal just feels excessive. His q, although it has a high mana cost would make him incredibly good at tower diving, along with his aoe and dash, potentially making him a good jungle or support (jungle would solve his early problems with susceptibility to magic damage easily) and would be very powerful in blind pick and lower elos. Aside from the dash and ult, he is essentially right click to win as he relies very little upon mana. As a result, he would be hard to outplay early due to the fact that you cant really outplay auto attacks. Also, hos ult is not a channel. Let that sink in.
**Overall, I like his role as the "reverse galio" **and think this general idea of the anti-adc is great but due to the sheer simplicity of his kit it is basically right click to win with little else. At least with galio there are cooldowns to play around, is his ult up? when will the shield run out? etc whereas this guy (even when passive down he still has w) is just a constant threat with high mobility and group mobility at that. To make him more balanced I would simply cut the mobility, reduce movespeed and give him a low range, a bit higher than jayce maybe? (it makes sense too, does armored heavy weapons sound very mobile?). This way you make him reliant on choosing the right time to go in, like the old kat, as well as increasing the need for good kiting mechanics while also reducing the oppressive lane and ganking pressure. He would feel like a heavy weapons guys, great at defending against tower dives and holding the fort until some magic comes around and obliterates him.
Please note this is just my personal take on your concept and I accept that many of my assumptions and acts of logic are likely flawed. Thank you for reading till the end.
Not a bad shot for a first concept.
