[Pretzel]

Heya folks, after a few years being Out Of The Game (Covid + work schedule just meant it was overall easier to just not play since ~Feb 2020), I'm hitting a milsim at an abandoned hospital in PA with a friend April 1st and 2nd, so it's time to build something that's hopefully going to be a bit nuts.

The plan is for a highly durable, lightweight/maneuverable, and accurate rifle for both indoor and outdoor play with mixed environment being pretty common around here. I don't tend to use a pistol, so a shortish AR platform that can sling BBs out a fairly long distance is golden.

The HPA going in it now is temporary; I have a Gate Titan, EON Gearbox, Warhead Ronin, modify J cage ceramics bearings, and Siegetek SSGs that will be going in these externals eventually, but with my work schedule and in need of a gun that works sooner rather than later... spare HPA engine to the rescue! Plus, 3d printing alignment jigs is fun.

The intent accuracy wise is for consistent torso sized groupings at around 170-200' (55-65yds ish), shooting between 1.5 and 1.86J (depends on the field...) with 0.36 bbs. Bare minimum I'll be satisfied with is 150'/50 yards accurate on a pie plate (not just "can reach out to and ding 2/10", that one's easy), longer is better as we all know. I've done 120' pie plate 7/10 before with random stuff from the parts bin, so 150+ shouldn't be too difficult (famous last words eh?)

A quick rundown of the parts going into this, items in italics are already completed/waiting for final assembly, and those in bold are "waiting on shipping/in progress".

Externals:

• Mancraft CNC receiver set (Cerakoted in H-267 Magpul FDE )
• PolarStar UGS or Line to tank in backpack
  • If line: 8 pos RS buffer tube, FDE Crane Stock or PDW stock hmm...
  • if UGS: Magpull MOE
• Strike Industries Gridlok 11" Rail system (Cerakoted same)
• PTS EPGC grip (dark earth)
• RA-Tech Steel "Magic" Receiver Pin (3 part to really lock the upper in place)
• Ambi Charging Handle
• B.A.D.A.S.S. Ambi fire selectors (APS I think?) See Update.... Made it work because I have a machine shop and am fueled by spite when things don't work.
• Retro Arms Steel Receiver Pin (screws in place and actually black, unlike the picture which made me happy)
• Gate EON trigger SPEED HPA Trigger (had a MAXX initially and it had issues)
• Random outer barrel from the parts bin, 8" OAL (holds the hopup nicely without wiggle)
• Replica Evike Special Gemtech can with RC cloak inserts, although the inner barrel extends about halfway into it reducing its effectiveness.
  • May or may not add an extension, but 2" is 2" and shorter is better for indoor/cqb use.
• Vortex Spitfire 3x Gen II
• G&P DBAL-A2 in tan Nixing this for weight and lack of need for now
• Flashlight
• Sector Optics T20x thermal optic on offset mount (hehehehe)

Internals:

• PolarStar Kythera with 0.015 metering screw
• Custom 3D printed centering/alignment jig for Kythera+hopup
• MAXX M4P hopup (have both the hard and soft nubs in 4mm, will try both and see which gives the best performance)
• Airtech Studios Advanced Hopup Chamber Inner Barrel Ring
• Stainless EDGi 275x6.05 (the Paragon Armory one, which is why it's a bit larger ID)
• Begadi SHP or TNT TR bucking (need to test them to pick)
• Milled "half gearbox" to hold trigger, grip, fire selectors

Still to do (aside from overall assembly of course):

• Turn the OB down slightly so it fits in the receiver (Macraft made the bore about 0.3mm too small for most barrels, very frustrating!)
• Mill new ambi control insets on the gearbox because the Mancraft receiver is ever so slightly out of normal spec... you can see a trend here. Turns out the receiver is so out of spec it can't take ambi selector transfer gears... ffs. But I have a lathe and mill, so I made a transfer bar for it. See post on page #2 for details.
• Dial in hop/pressure for the correct Joules with the 0.36 0.32 bio BLS BBs I use, and then run some volume calculations through Wolfram Alpha my overkill calculator to get the correct volume reduction I'll need for a spacer to put in the Kythera to reduce my "dwell" so I'm not wasting nearly as much air, and making the rifle far quieter. Using a digital pressure gauge plugged parallel in the line for really accurate PSI information. Overkill, but fun. <-- Turns out this step was a giant waste of time, and was totally unnecessary; see post #16.
• Pick what bucking/nub combo to use
• Take pictures of everything I have so far hopefully this weekend once I have time...
• Order flashlight and pressure switch that works with my DBAL as well (and install the high pwr blocking screw on the DBAL so I don't blind anyone!)

I hope to have most of the parts in bold here before too long so that I can finish assembly and get to tuning, as I'd love to not be frantically trying to finish it the day before the game for once! Ideally I'll have an update Sunday after I do a bit of the machine work needed. (<-- Lol this didn't happen)

 

[Trash]

PA with a friend April 1st and 2nd

Kythera is going to be deadly there. Hope your going to be shoulder and shoulder with me on the URG.

I too am looking to work on my semi auto build. Although I am going to be a bit more obnoxious and have a master key on my gun.....for corners !!!?

 

[Pretzel]

Kythera is going to be deadly there. Hope your going to be shoulder and shoulder with me on the URG.

I too am looking to work on my semi auto build. Although I am going to be a bit more obnoxious and have a master key on my gun.....for corners !!!?

Yup! URG indeed, I'm always green team.

As a minor update, the 0.015" metering screw came in, replacing my already slowed down 0.019, but I want as slow a nozzle as possible to load the BBs gently (and therefor more consistently). Dropping me from 0.183mm^2 to 0.114mm^2, so the nozzle should move about 38% slower (ish) at the same pressures. This will be partially counteracted by the fact that I assume I'm going to need to run around 120psi+ to get any sort of energy approaching 1.8j out of a 247mm barrel. Weirdly it's a 1.5mm hex in the screw, despite everything Polarstar being imperial otherwise?

Flashlight is on order (ETA 20-30 days oof). It's a Surefire M600DF clone from china, but the reviews are actually really good and it apparently holds up on real rifles as well, so I'll just cut a lexan protector for it and print a little housing out of whatever filament I can find that's closest to FDE and call it "more than good enough for airsoft".

 

[Trash]

Yup! URG indeed, I'm always green team.

Have you had a chance to read though their TACSOP?

1.86J with your goal of .36 bbs is kind of slow approx. going to be around 330FPS. I was looking at the AO and measuring things out the longest indoor distance we will see is about 300ft. I've run some past indoor ops with them and I think I am going to be switching to .25s for this. obviously accuracy will be down but over all you wont have too far that you need to reach out and touch someone.

I'm not sure if your goal is to make something for this OP specifically or something for overall.

have you seen any photos of the AO yet? one of our guys was able to find the address and send us some. Let me know if you wanna see those, I am not sure how much information the organizers want out there.

 

[Pretzel]

Heavier is slower out of the gate, but quickly out-velocities lighter. The ATP (which is reasonably accurate from what I've seen) puts a 0.25 at only marginally faster Time of Flight (if an extra 0.03 seconds lets them dodge you're shooting at the flash anyhow) at 1.88j (close enough) up to 110', then it drops to being around a full third of a second slower TOT by 200' than a 0.36: Section VII-B-09: Mass Comparison of Projectiles Fired at 1.88 Joules

The 0.36 is actually moving faster by 60' and flat out has way more energy for the entire flight as it retains it better, so brush, winds, etc are... lesser concerns... ish. It's still a 6mm BB regardless...

But most importantly for me (outside of the nice boon to accuracy!), they accelerate down the barrel slower so I can take advantage of the strong ability to "joule creep" HPA rifles to bring my gun up to 1.8J with 0.36s from whatever it would be shooting with 0.25s, which will let me use less air overall, so more efficient and quieter.

Downside of course is $$ for the ammo...

I've seen some AO stuff, one of my guys linked me some.

 

[Pretzel]

Minor update: Milled new selector holes in the gearbox... turns out the receiver is too narrow for any ambi gears... RIP. Not buying another Mancraft receiver that's for sure!

Somehow lost my lathe chuck key and QCTP mounting stud (???) during the move a few months ago, so replacement ones on order ETA Monday. I can hopefully finish up the bit of turning I need to do then to mount the barrel as well as tweak the rear receiver pin so the PDW stock can close over it in case I go that way (it looks really cool with the PDW stock...)

Begadi shipment might actually be getting on a plane to the US soon based on tracking...

 

[Pretzel]

Just a few days later and, well, I realized why the begadi package was so much to ship... Wonder why they didn't use a mailer tube, but I was unaware the vario piston head adjuster didn't break down... 😅 long box!

Lathe parts are in and mounted, so I can finish off the barrel and receiver pin. Had to Dremel down the chuck key and re-tap the lathe compound slide to get an extra ~3mm of thread in it for the bolt, but that's working now.

Begadi stuff are here and ready as well, really just need to test between the SHP5 and T.N.T. APS-X buckings. And make the hard nub for the MAXX hop solid and not squishy. Probably a tiny 3D printed spacer or similar? Easily done at least.

 

[SiliconeSword]

Curious to hear about what bucking you like better, should be interesting.

For the nub, Marco and I recently replaced the little rubber piece in a Maxx nub with epoxy and molded it inside the chamber with hop arm that moves it. Similar to my SS-nub in a way, or bedding a rifle action. Idk if it actually did anything, but it certainly feels more solid.

 

[Pretzel]

Got the barrel and rear pin turned down and mounted, so here's a quick shot of the externals put together, although I still need to tan the cheekplate of the stock to really tie everything together. Also still waiting for the flashlight, but it's shipped at least!

Internals hopefully happen this weekend.

 

[Pretzel]

Made progress on the uppers internals while waiting for work stuff to happen today, spat out 4-5 of the rear alignment jig as I modified it to function better. Might still put out another one to try and get fitment just a hair better, but it may be good enough... We'll see, they only take half an hour to print in ABS at least.

The spring presses against the lower receiver, and transfers the (rather significant at 104.5lb/in rate!) force through the rear alignment piece, which both centers the Kythera in the upper, but also prevents rotation by having an inner lug that fits in an indent in the Kythera and also extends up slightly into the charging handle groove.

The front of the Kythera is again aligned, with the hopup unit press fitting into the piece to lock it centered.

The Maxx M4P fits essentially perfectly in the outer barrel I'm using, so the wings don't allow it to rotate.

Can is cream filled, but the IB goes about half way through it, so I doubt it will be that effective, at least until I install the volume reduction on the Kythera.

So far I have the SHP5 installed with I think the 6mm MAXX mnub.

Sounds like hard nubs are the way to go nowadays? So may have to switch that, but I'll test first.

Some bonus pics of the parts actually "installed":

 

[SiliconeSword]

Do you think that holding setup would work on my F2 and regular MAXX chamber?

If so, I kinda want some to see if they're better than my current REVO kit

 

[Pretzel]

Do you think that holding setup would work on my F2 and regular MAXX chamber?

If so, I kinda want some to see if they're better than my current REVO kit

Well, it's custom to the upper as well as the engine. Takes a lot of measuring and fitment, so unless you also had a mancraft receiver the taper would be way off, and unfortunately I don't have a F2 to do measurements off of anymore (although I'll be buying one at some point to use for a test bench build).

It's the unfortunate part of "making really cool custom stuff" where it's so custom it doesn't work for anyone else 😅

 

[SiliconeSword]

Guess I'll just have to prompt the guy I'm trading a barrel for my next gun with, an F2 is a literal press fit into an AR-15 upper and I'm just waiting on an outer barrel so I can make a hop chamber and finish the thing. Been planning for 3 years and waiting on a barrel for nearly a year. Supposedly the guy has it though, just a matter of time for him to get the rest of the trade together that's unrelated.
Hyped for the SS-15 lol, long time coming

 

[Pretzel]

Progress on the build, using BLS 0.32 bios because I have Too Bloody Many of them.

So far using the SHP 5 at 120psi, Maxx hop is set to like... 4 or 5? Haven't done hopup/accuracy testing yet, this is just to get an approximate FPS dialed in for my pressure/volume calculator... which is 120 psi apparently for ~1.72j. Consistency is divine, and I can shoot about 5x a second before the metering screw cant keep up (should be fine, even at the slowest it's only dropping by about half a joule). At worst I'm shooting +/-2fps, but if I go a lil bit slower I'm hitting +/- 0.35fps, which I think is the best consistency I've ever had

Range/accuracy testing to come.

The calculator I'm building says with the spacer I'm planning to install I'm running overvolumed by a fair amount, but it's still a lot less than the stock 6:1 ratio the kythera has on a 247mm barrel at 120psi!

Hopefully consistency will maintain with that reduction, or get better (and I might even be able to shoot faster as there will be less space to fill with air!)

 

[Pretzel]

The best part of 3d printing is that you can be proven wrong within 15 minutes, and already have a new spacer going on the printer...

1,738mm3 reductions was too much, inconsistent shots and dropped overall power down by ~0.2-0.3j

Trying a new one at 1,348mm3 (4.469:1), which I'm hoping is the right amount... easy to keep reducing the spacers size if not, and will give me a volume/pressure data point.

 

[Pretzel]

And of course, you can only trust math as far as you can throw it - even a 668mm3 spacer was dropping my fps by 10, so down to 330.

At that point it's only a 12.8% reduction in volume, and with it still being under volumed for the BBs themselves... I could only realistically go to 544mm3 (0.8mm walls), and then 274mm3 (0.4mm walls... pretty fragile at that point) without starting to make weird shapes and through holes etc, and while 274mm3 reduction is probably still enough volume for the BBs, I dunno if saving 5.3% air per shot is really worth it, that's only about 1,050 rounds vs 1,000. And hell, if I used 0.36 I'd probably need the volume anyhow.

During my failed attempts to calculate, I now know that at 120psi the effective Cv of the Kythera poppet is 0.43... so that's... useless I guess, but kinda fun?

Also thank you super slow motion (960fps) cameras inexplicably included in cellphones nowadays for the following details:

Kythera @ 120psi with 12% volume reduction and 0.015" metering screw timings:
5.2ms air release
6.76ms nozzle retraction
12.48ms loading pause
36.4ms nozzle moves forward <-- this is visibly slow to the eye, it's wild.

 

[Pretzel]

Assembly is tentatively complete, accuracy testing still needs to happen but externally...

Well, I was sent the wrong flashlight and the pressure switch for it was broken out of the box, but aside from "different switch" (probably) and a slightly different mounting method for the flashlight (mlok on side of rail instead of picatinny on top, which is where I'd mount the dbal), it's done*:

*Yes the right side selector is missing, still deciding if I hate myself enough to put a through bar in, or if I'll just bolt something in place. Cosmetic only so... Meh.

 

[SiliconeSword]

Looks legit, recently put a shorter upper setup on my M4 and I think I like the look with a massive suppressor more than my old integral suppressor setup. Handguard and receiver set lends itself to a Honey Badger type build, so maybe a PDW stock down the road.......

As for your selector, I'd just print or buy a plug like with what came on my CYMA M4, as it mimics a standard selector switch on an AR-15 pretty much.

Maybe add a suppressor wrap of some sort, that's usually a thing you see cringey guys on YT and IG rocking, and they do look pretty badass.

 

[Pretzel]

Handguard and receiver set lends itself to a Honey Badger type build, so maybe a PDW stock down the road.......

On yours or mine? I have a PDW I had considered throwing on, but it's not Tan on the rear (although I could print a little clip on cover for it in FDE to make it look better), and it's about an inch shorter LOP than I prefer. Also the QD point on it is in a weird position and it kinda sucks haha. It does kinda slap though in terms of looks (from before internal assembly):

Maybe add a suppressor wrap of some sort, that's usually a thing you see cringey guys on YT and IG rocking, and they do look pretty badass.

Honestly the can is pretty thick, a wrap would just be a hindrance more than anything (and having the text aligned perfectly centered and upright is so rare and nice, would be a shame to cover it)

 

[SiliconeSword]

That looks legit, I'd definitely rock that stock