[TL;DR] TTK in local play is about the same as it is in INS2; I am near certain that the higher TTK is a result of bad hitreg, and not of OP heavy armor.
First of all, thanks to the devs at NWI for making this game. It still needs some work, but when it runs properly it's an absolute gem.
As of the time of writing I've logged more than 1.4k hours in INS2, and have been eagerly awaiting the release of Sandstorm ever since it was first announced that they're working on a sequel to INS2, because boy does INS2 deserve one.
What I'd like to talk to you, both the devs and other beta players about, is hitreg. In co-op and PVP I've noticed that often when using a bolt-action sniper rifle, even at close ranges when my reticle was right on the chest of the enemy when I fired my shot, it wouldn't register as a hit. The enemy wouldn't shout, there'd be no blood splattering, nothing (except for me usually getting killed soon thereafter). Given what I've read here, I'm probably not the only person who has experienced that. The same problems when using rifles, in semi or in full-auto, it often seemed like it took many more shots to put down an enemy than it did in INS2, e.g. a far higher TTK. Originally I thought this was due to heavy armor, but after the November 2nd hotfix and with the frame rate capped at 30 FPS I've been able to ADS and get into firefights and aim properly when taking shots without lagging (I'll spare you my complaints about lag as the devs are well aware thereof and this is still the beta anyway), I noticed that even when I should have clearly hit an enemy it didn't do much. With the lag "eliminated", I figured this must either be armor or hitreg, opting that it's probably heavy armor which is ridiculously unbalanced.
So today I loaded a match of checkpoint on local play and lo and behold, bots were dropping at a single bullet to the chest from an M24, 2-3 shots to the chest with an M16A4 would be enough to kill any bot I encountered, always.
This low TTK was consistent across pistols, SMGs and sniper/assault rifles. Obviously it takes a few more or less bullets depending on the gun and caliber, but TTK felt no different than it did from INS2, making semi-auto viable if not preferable to full-auto (semi-auto is next to useless in co-op and even more so in PVP because of the hitreg issues).
I encourage everyone to try this out for themselves, hopefully it will give you a better sense for what combat in Sandstorm is supposed to be like than online gamemodes have so far.
To the devs, I hope that you make lag and hitreg issues your top priority. I'm sure there'll be a lot of feedback regarding gameplay balance to come, both from me and from others, but I don't think we can properly judge the combat and gunplay with the current problems of lag, and especially hitreg as the TTK is completely distorted from what it's supposed to be.
Thank you and all the best.