You can handle Grineer Shield Lancers by shooting at the eye-slit (I *think* the hitbox is a tiny bit above the actual slit, but it might just be that I'm not seeing it right) of the shield or around the shield's edges. I tend to leave Punch-Through mods only to bows because the Shred mod (Nightmare mode reward, fairly common) gives both Fire Rate and Punch-Through, which I find a good combination for a single shot travel-time projectile weapon when Fire Rate improves draw charge up too (since you don't want to waste arrows to non-maxed draws either), or with Opticor (lazor cannon). Most of the time weapons either have inherent punch-through or AoE qualities and thus don't need the mod or punch-through can be replaced with just shooting more accurately and more damage or other mods can be slapped on the slot instead.
But seriously, you don't want to put the potato and Forma on a Latron. Latron is too weak for that. Latron is a semi-auto rifle that has lower damage than many proper fully automatic weapons in the game, even before weapons with superior crit potential are factored in. IMO the only purpose for Latron is as building component for Tiberon, which is a burst rifle that fires three shots that each deal more damage than Latron. However, Tiberon will require Mastery Rank 4, but you kind of want to acquire more MR anyway to unlock access to more weapons and you get it naturally as a side effect of first-time-clears of mission nodes and gear levels.
I do recommend looking into the Tiberon myself though, it's a very solid weapon and if we discount the anniversary-gift-only Dex Sybaris it's IMO the best Burst Rifle to get (also has a good look and really sweet reload animation). If it's a rifle you want in more generic terms and not specifically a semi-auto rifle, my own personal go-to assault rifle is Soma Prime, which is more like a long range and highly accurate light machine gun than an assault rifle though, but I prefer a gun that combines high DPS, accurate marksmanship, and huge magazine size into one stylist package (the fact that with Madurai passive activated it goes full Paul Verhoeven on anything made of meat is just a great bonus).
EDIT: Zhuge is a fine weapon too, and getting two Argon Crystals isn't that hard (besides, you have to get two in one go anyway, since Argon Crystals have half-life decay every server day rotation). Just tag along to farm some Void and you will inevitably have more than two of them at the end of an hour or two.
Anyway, but if you wanted specifically semi-auto rifles which are not sniper rifles, the selection is actually fairly scant. I suppose one could count something like Opticor to be one, but actual semi-automatic rifles are limited to just Latron and Grinlok IIRC.