To make any of the antimatter ammunition for K2 you need charged antimatter fuel cells (from K2). Those however have been disabled when the integration with K2 is on and as such, there's no way to actually make that ammo. Is this intended or is it an oversight?

What mods other than K2 and RFP do you have? I just checked and it requires antihydrogen gas, not K2 antimatter fuel cells.

I'm not sure which mod might cause a problem so i'll give you a list of all the ones we have. (without RFP and K2)

Todo List
AAI Signal Transmision
Advanced modules
Afraid of the dark
Aircraft
Akimbo vanilla weapons
Auto deconstruct
Automatic train painter
Big Brother
Bob's adjustable inserters
Bottleneck
Combat mechanics overhaul
Death Markers
Disco science
Double speed belts
Even distribution
Evo gui
Extended vanilla: Fusion reactors (This only adds more tiers of portable fusion reactors)
Extended Vanilla: Mining drills
Factorio Library
Factorio standard library
Factory planner
Flow control
FNEI
Helmod
Honk
K2 - Additional locomotives
Lightorio
Long and Far reach
Lovely Santa's library of knowledge
LTN
Miniloaders
Mining and crafting speed research
Module inserter
Nanobots-Early bots
NiceFill
Orbital Ion Cannon
QoL research
Realistic Reactors
Reinforced Walls
Repair turret
SLP - Dyson Sphere
Splatter Guard
Squeak through
Tree collision
Ultimate belts
Ultra fast inserters
Vehicle snap
VoidAll
Void Chest +
Walls Block spitters
Vortik's Deep Core mining
Warehousing
Walls block spitters
Waterfill

That should be all.

No idea what could be the problem. Can you try it with just RFP/W and K2 to make sure it is another mod messing with it?

(posted twice, oops)

I did what you asked.
I have only K2 and RFP loaded and i still have the base recipe from K2 for the antimatter weapons.

You still have to have RFW loaded if you want the weapons.

Wait, i think I know why. I made K2 compatible with RFP and RFW at the same time, but not RFP alone... Here's a link to it, use that with RFP until I fix it.

Ah i see. Yes that seems indeed to have been the problem. Thank you for clarifying.

Something i just found that also should be mentioned, the He3-He3 cells don't work with the Fusion Locomotive from the K2 - Additional locomotives mod.
I would guess the same applies to the antimatter locomotive.

Yes it does. You should make your fuel type "antimatter-fuel" in the presence of K2.

Fixed in 1.6.0.