Some rogues are expert assassins or thieves or faff about with arcane trickery. The magic marksman, the sniper, is the archetype that prefers to do their dirty work at a distance. Snipers build custom bolt rifles that fire magical projectiles at their intended target. They use this tool to pick off targets at a distance and cripple the enemy line with shots that would make anyone think twice before leaving the safety of cover. Some are just hobbyists, some are monster slayers but all experts at obliterating whatever gets caught in their cross-hairs.
Magic MarksmanWhen you choose this archetype at 3rd level, you have combined your interests of magic and marksmanship to craft arcane rifles. You gain proficiency with the Arcana skill. You gain proficiency with the Light & Heavy Crossbow and Bolt Rifles. You learn the Prestidigitation and Mending cantrips.
Fighting StyleYou gain the Archery Fighting Style: You gain a +2 bonus to attack rolls you make with ranged weapons.
Bolt Rifle AficionadoAt 3rd level, you also gain proficiency with the Carpenter's Kit.
By taking 1 hour, expending 25 gold, and using a Carpenter's Kit, you can customize a Light or Heavy Crossbow into a Bolt Rifle. The Bolt Rifle is a magic weapon that uses a powerful arcane apparatus to hurl magical bolts through the air from the end of a long barrel. Your Bolt Rifle deals 2d6 magical force or fire (your choice) damage per bolt and has a range of 200/800. It requires two hands when you attack with it. It weighs 20 lbs. It does not use ammunition.
Only you can use your custom Bolt Rifle.
HawkeyesAnd at 3rd level, you can use a bonus action to make a Wisdom (Perception) check to spot a hidden creature or object.
You also gain proficiency in the Perception skill if you don't already have it. You proficiency bonus is doubled for any ability check that you make using that skill.
Homicidal GeometryBy 9th level, you have improved the rifling of your Bolt Rifle's barrel and increased the caliber of each bolt so that it penetrates your targets. Whenever you fire your Bolt Rifle at a target, you may expend you may instead choose to fire in a beam, which is 1 foot wide and 800 feet long, and you make an attack roll against each creature or object in that line until you miss. You may only use this feature once per round.
Use Magic DeviceBy 13th level, you have learned enough about the workings of magic that you can improvise the use of items even when they are not intended for you. You ignore all class, race, and level requirements on the use of Magic Items.
OverchargeWhen you reach 17th level, you can take a bonus action to overcharge your weapon. When you do so, you aim at a point within your Bolt Rifle's range. Each creature in a 20-foot radius sphere centered on that point must make a Dexterity saving throw (8 + your proficiency bonus + your intelligence modifier). Each creature takes 20d6 force damage on a failed save, or half as much damage on a successful one.
A nonmagical object that isn't being worn or carried also takes the damage if it's in the spell's area.
After you use this feature, you can use it again until you take a short or long rest.