Umbrella Prometheus 6.03 EG inner barrel w/ R-Hop and Prowin M4 Hopup

Type: Assembly
Lapping Service?
~3 Week Build Time

A truly timeless, tried and true setup.

Be aware, shots will be erratic/inaccurate for the first 5,000 rounds put through, it takes a little time for the patch to break in and conform to the shape of a BB. Once you play a couple of games you should be shooting extremely accurate! 

Prometheus 6.03 EG inner barrel with installed R-Hop modification. Available with a pre-assembled drop in kit for compatible TM spec M4 type AEG/HPA Airsoft guns.

The perfect upgrade for any gun, to increase accuracy, range and consistency of each shot fired. Out range anyone else on the field with incredible range and accuracy despite FPS.

Our handmade R-Hop has been an integral modification we perform for all our builds as users can experience 30% increased effective range over non R-Hopped barrels.

Purchase in a complete drop in assembly configuration for a true plug and play upgrade process if your build uses a Tokyo Marui spec M4 style hop up unit.

If you just want the Prometheus Barrel R-Hopped click here

Please note that there is a little slit in the patch near the end of the barrel, that slit is intended to help vent air pressure on high RPS builds to keep everything more consistent. This does not effect performance negatively in any way on any build no matter the RPS or sector gear, strictly meant to improve performance on high RPS. 


  • Inner diameter: 6.03 millimeter 
  • Polished finish
  • Precision, hand fitted R-Hop patch
  • R-Hopped glued into hop window 
  • Umbrella Armory specific R-Hop 2.0 patch design and material
  • Requires flat bucking and M-Nubbed / R-Hopped style hop up arm for use (included/assembled with full set)
  • Use .28 gram BBs or heavier to see optimal long range advantage over non R-Hopped comparisons


  • SUS 304 stainless steel inner barrel
  • Rubber R-Hop patch
  • Rubber soft type Madbull blue bucking
  • Aluminum ProWin Hop-up unit with plastic rotary style adjustment wheel
  • Resin 3D printed UA Hop-up flat arm and reinforced C-clip
  • Rubber barrel stabilization, air seal, spacing O-rings*


  • * = Barrel + Flat-Hopped Bucking + M-Nubbed Hop-up Unit variant only
  • Lifetime warranty
  • Made to order for some variant options, up to 4 week lead time
  • Manual/FAQ
  • Return policy


What is it? Its a grinding method to microscopically remove material from the surface of the bore. Using extremely fine diamond compounds, we can remove material from the inside of the barrel that is uniform and consistent throughout the bore. Leaving a mirror-like finish that's very time consuming to achieve on a steel barrel.

As a result, the following happens.

  • ~2% increase in FPS
  • Increased shot to shot FPS consistency
  • Increased accuracy downrange
  • Easier barrel cleaning/less stubborn residue buildup

Below is an example of a Prometheus Barrel 6.03mm, pits, and bore marks can be seen within the barrel. With lab instruments we've measured this low/high spread as 5 microns. These microscopic holes tend to catch BB residues, and increase the internal friction of the barrel slowing down the BB.


Below is that same barrel with our lapping process done to it. We're able to reduce the internal bore surface roughness to below 0.5 micron. Resulting in a mesmerizing mirrored effect on the inside.

Customer Reviews

Based on 6 reviews
joseph lennox
6/10 on my order parts are 10/10

but i got the 1-3 day shipping it took 4-5 days to get here so they gave me the usps shipping when i did not pay for that which was a little annoying but i enjoy the parts

Vincent Castello

I order the complete barrel and hop up kit. Order took about a month to get but that’s fine as stated they are made to order with a given lead time.
Everything was packaged nice and delivered, came with extra prints to space the hop unit. I installed it my ICS MK18. For ICS it was not a “drop in” unit, I knew that fully and was prepared to modify as needed. Once modified took it outside to set the hop. I was amazed at the accuracy and consistency of every shot. Had no “zingers” or strays in a grouping. I’m able to tag a 6 inch plate pass 150ft.
Well worth the money and air if want pure accuracy.

Mason Brammer
Shoots beams

I got a it with a lapping surface and it shoots beams, I can hit targets over 250ft accurately with .28s. Yes it’s expensive but worth it

Gavin Herr
Not worth

Came broken, no hop was being applied due to dogshit construction. Just go to a shop and have them custom make an R-hop it’ll be way cheaper


Great quality and works great.