Guards are mediocre as anti infantry. Their PTRS AT rifles have a 5% chance to instantly kill infantry, and when the RNG god doth shine down upon thee it can mean very good news.
The PTRS is the worst anti-infantry weapon in the game and it does not have a 5% insta kill chance.
Guards PTRS rifles have 5% accuracy at close range, down to 2.5% accuracy at max range. A single PTRS round does 40 damage, which is not enough to kill a full health model, except snipers and ostruppen.
The average DPS of a single PTRS at close range is roughly 0.4; meaning that on average, a single PTRS will kill one model per every 3:20 minutes at close range. At max range it's half of that, meaning that a single PTRS will kill about 1 model for every 6:40 minutes of firing at long range.