The Peasant Railgun, a weapon of mass game-breaking destruction.
Relies on a few basic rules in the DnD system (and total ignorance of the rules on the DM's part): readying actions and the length of a combat round (6 seconds).
 Creating a Peasant Railgun
1. Hire a ton of peasants; let's just say that it is two thousand two hundred and eighty. Line them up in single file; this will form a chain of peasants two miles long. It'd have been four miles back in MY day (witness me hiking up my 2nd Edition suspenders).
2. Buy a ladder. Just buy a standard, ten-foot ladder. Disassemble the ladder into a bunch of rungs and a pair of mighty ten-foot wooden poles. Hand a pole to the peasant at the back of line.
3. First round of combat. Peasant at the front of line readies an action to throw the pole at the enemy. Every peasant behind him readies an action to hand the pole to the peasant in front of him.
4. Next round: peasants fire off their readied actions, passing the pole two miles down the line and hurling it in six seconds or less. Pole accelerates to the speed of 1188 miles per hour, or Mach 1.546875 in dry air, at 20°C/68°F, at sea level on our planet.
5. Peasant Railgun can be reloaded and fired in less than 12 seconds.
6. Variations - Really, your choice. Weapon is scalable, you could use your peasant railgun to fire a number of things at a really long range. Add more peasants to make the weapons even faster; paint them red to make them fasta. Use gobbos to make a DnD grot cannon. Hurl pointy bombs for HEAT weapons. Severed heads make an impressive psychological warfare tool. It's even more wild with a bag of holding - place a team of fighters in it for DYNAMIC ENTRY over castle walls and shit, hurl some fucking bear cavalry directly into enemy lines, who knows. You can also throw a halfling monk to take full advantage of Flurry of Blows at 1200 mph. Combine this with the 15,000,000 gold-a-day trick and you're ready to absolutely ruin your DM's day.
8. Motherfucking PROFIT
 Practical Applications
A campaign I was in recently employed a peasant railgun in a large-scale battle. Our mage was a dumbass and decided to launch an alchemical flask from said railgun and into the heart of the enemy forces. It blew up and killed a bunch of the enemy. Our DM got back at us by making it tear a big-ass hole in the time-space continuum. The same mage decides to approach the hole, and when he touches it, it blasts him across the fucking map and vaporizes some more enemy troops.
Our mass was 3.6287896kg (8 pounds, the weight of a first edition 10 foot pole), our speed was 536.448m/s (1200 miles an hour). The final kinetic energy was 520,005 Joules. This is similar to 1200g of TNT, or a quarter of a stick of dynamite. It's not exactly a weapon of mind blowing destruction, the maths breaks it before the GM has to. It gets worse (for the weapons sake) when one considers drag, that would tear the projectile apart into a flaming swarm of splinters before it reached the end of your peasant chain. The only way this thing works is if:
- a) Your GM allows for real world physics to translate into the game
- b) Your GM doesn't actually know real world physics
An alternative use that requires a looser DM but allows real world physics (partially) to still come into play: Instead of a regulat 10-foot pole, use a heavy lance or, if your GM will allow it, a ballista bolt or whatever made of Riverine. As Riverine is stated to be immune to anything except for things like disintigrate, it can be argued that it would be immune to the compression forces and such entailed in a Peasant Railgun. Because of this, your railgun is now armed with a truely unstoppable shell. Assuming you simply extend the railgun farther, possibly by forming an open loop part way through, you can accellerate up to near light speed, allowing you to do such things as: Break the earth into a radioactive cloud! Explode the moon! Simultaneously eliminate all of existance with your near infinite mass bolt! Hold all of creation hostage! Terrify the iving fuck out of your DM! And More!
If your DM is paying attention and bothered reading the Dungeon Master Guide (right where it says Simultaneous Activity on pg. 24 and Adjudicating the Ready Action on pg. 25-26) they will know that 6 seconds is no where near enough to reach the end of the line and if you're not in combat, ready action can't be used. They can also tell you the rules on readying actions dictate none of the peasants are ready to catch the pole, meaning they're going to drop the pole right at the start. Even if they do manage to pass it forward there's one other problem: an improvised thrown weapon has -4 to-hit, -4 to-hit for using a 2h weapon with 1h, and -2 per range increment, and impossible to throw beyond 50ft. Even if you had the implied momentum, the final peasant would have trouble hitting a stationary barrel.
If your DM has ever read a physics textbook, or has the physics knowledge of an 8 year old, the projectile would shatter under the strain and the recoil would vaporize the peasants. 20th-century railguns need to be rebuilt after every third shot, and they're made of stronger stuff than 0-level NPCs.
As always with any (obvious) bullshit tactic you might try to pull on your DM, Rule Zero can trump you.