Superlative Arms Piston Conversion Kit - 300BLK Build Helpful Info Thread

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FartinSpartan
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Superlative Arms Piston Conversion Kit - 300BLK Build Helpful Info Thread

Post by FartinSpartan »

I got to thinking it would probably be helpful to create a topic for whatever few of us there are who are interested in building their own piston-driven 300 Blackout using a Superlative Arms Piston Conversion Kit. They make a great kit and there ARE people who have this working well -- even if Superlative Arms won't help them and doesn't want to get involved.

As stated by Superlative Arms, they will NOT honor their warranty if you're using their piston conversion kit on a 300 Blackout. They won't talk about customizing a piston rod length for your 300 Blackout either. All conversations have to be about supported calibers like 5.56. Keep that in mind if you plan on needing their help.

I am just getting started with my build after researching barrels, handguards, etc. -- so it isn't a completed success story yet, but I do believe it will be. I may open a separate topic detailing my build later, but I'm really hoping this topic can just be about listing known-good barrel mfgrs/models, buffers/springs, etc. that people have found to work so others build theirs successfully without going crazy trying 5 different buffers and springs, etc.

If the forum admins wouldn't mind, please let this topic stay for those of us who find it helpful. Even if there are only a handful of us it helps or would even be interested, there is still no better place on the net than this forum dedicated to 300 Blackout for such a topic.

I do politely ask that we keep this topic focused on sharing info that helps others trying to build a piston-driven 300 Blackout AR using Superlative Arms (SA) piston conversion kits -- like brands and models of compatible barrels, successful buffer and spring combos, etc. If you're just looking to debate DI vs. Piston, trade insults or dismissive quips like "this is a solution in search of a problem", etc. please do that in a different topic. We're already past debating it.

WHY AN SA PISTON KIT?
There are reasons some prefer the Superlative Arms piston conversion kit to the other option(s). Such as:
  • Better handguard compatibility - SA's design does not require a GI-type barrel nut that has to be timed for the piston rod to be able to travel through it, opening up many other options for handguards
  • Not being forced to go "low mass carrier" -- another deviation from original 300 Blackout cartridge design parameters -- and then having to figure out all those resulting problems behind and under the BCG because you did... etc.
  • The highly adjustable gas blocks these kits come with

IMPORTANT SUCCESS FACTORS
Aside from what all builders of 300 Blackout AR's already face, there are others things that are pretty important to get right if you want to be successful building one of these.

CRITICAL: BARREL SELECTION
Since SA probably won't help 300 Blackout users with custom length piston rods, it's CRITICAL that you choose a barrel manufacturer and model which locates the gas port at the correct distance from the barrel extension and offset from the gas block journal shoulder to match the length of SA's pistol- or carbine-length piston rod.

SA warns everyone that there is no industry standard for gas port length on pistol-length barrels.

There are probably many 300 BLK barrel manufacturers who locate their pistol-length gas ports in a place that will work for the SA piston kit, but without a barrel in hand to check it with SA's fold-out quick check ruler, you don't know unless people share information.

Did you find a brand and/or model of barrel that the gas port location matches the SA pistol or carbine-length piston rod? Please share it here to help others. This is the main reason I thought a topic/thread would be helpful, so people would know which 300 BLK barrels are not a gamble to buy for an SA piston kit build.

Ideally if others will participate and share the barrels that are dimensioned to fit right, we can have a nice list of known-good 300 Blackout barrels for use with an SA piston conversion kit.

Here is Superlative Arms' diagram showing the 2 critical dimensions:
Image

Some manufacturers of 300 Blackout barrels just aren't going to share these dimensions, or the gas port diameters of their barrels -- I talked to several of them. More than one basically told me, I can buy one and measure it for myself when I get it, but they don't give that information out over the phone (or email either) before you buy one. If you're feeling lucky and want to roll the dice with an unknown make/model, I recommend you familiarize yourself with the vendor's return/exchange/refund policy first.

So I'll start this off with a barrel manufacturer I spoke with. Unlike a number of other manufacturers who would not discuss this over the phone or respond to my emails, this company was willing to research and even called me back promptly with the good news:

BALLISTIC ADVANTAGE - GOOD TO GO
Ballistic Advantage told me they make ALL of their Modern Series pistol gas length 300 BLK barrels the same - locating the center of the gas port at exactly 3.970” from the barrel extension and the right offset from the shoulder. I've verified this with the BA 300 BLK barrel I bought with SA's little fold-out quick-check "ruler" that comes in the piston kit, it is a perfect match for pistol-length.

Here's a pic of my BA 300BLK barrel lined up with their quick-ref ruler:
Image

POTENTIAL "GOTCHA" WITH BALLISTIC ADVANTAGE 300BLK BARRELS - THREAD RELIEF
One potential gotcha with Ballistic Advantage barrels if you plan to attach a suppressor -- depending on the part of your suppressor's mounting solution that actually screws onto the barrel's threads -- is the lack of thread relief behind where the threads end, about .060" ahead of the shoulder. BA may or may not even still be threading their barrels in this manner, but I do have two 30 caliber BA barrels with threads cut this way. This may complicate mounting a suppressor, depending on your suppressor or mount. There are topics covering that issue on the forum here you can search if you are affected by it, but you may not be.

UPDATE: NON-ISSUE FOR YHM -- *MY* can, and as a result my QD muzzle attachment interface brand of choice, is YHM -- and as documented in their 5/8x24 Thread Specification Reference on the next to last page of the manual for my suppressor, their 5/8x24 Phantom QD brake mount muzzle device (YHM-4302-MB-24A) brake is NOT affected by the ~.060' of unrelieved area behind the threads. Why? Because there is more than .060" distance inside the QD before the 5/8x24 threads begin -- so the YHM QD mount butts squarely against the shoulder on the barrel before it runs out of barrel threads.

So whether the lack of thread relief on your BA barrel will be a problem for you or not -- as BA told someone -- WILL depend on the design of your suppressor/muzzle attachment device.

I've also seen a picture of a newer BA Modern Series 300 Blackout barrel same length as mine that appeared to be cut differently (relieved), so it is entirely possible BA doesn't cut their 30 cal barrels this way anymore. But 2 30 cal BA barrels I have of theirs are.

If you find yourself bitten by this thread relief problem and your brand/flavor of muzzle attachment isn't getting along, Strike Industries makes 7.62mm timing shim sets that include one 2mm thick (.078") shim. It is slightly rounded/chamfered on the inside diameter and at least on my BA barrel with the thread relief problem, sat perfectly squarely on the shoulder of the barrel, and it's thickness was more than enough to get past the "shank" to the threads. It was also a nice match of OD to the BA barrel, instead of being much much larger in diameter like the YHM shims that came with the YHM-4302-MB-24A can mount.



HANDGUARD SELECTION
Depending upon whether you are going to be using the the clamp-on or solid/set screw type piston conversion kit, you will need to pick a handguard that can accommodate the SA piston gas block . You can review SA's dimensions for their blocks at their website, or for ease of reference:

Image

Image

Rather than duplicate a lot of effort and information and also to give credit to those folks who did the research and reporting, here is a topic over at AR15.com covers this quite well:

https://www.ar15.com/forums/ar-15/THE-P ... 26-732204/

Me, I'll be using a BCM MCMR since it can handle an SA clamp-on gas block. SLR Ion and Helix handguards will accommodate a set-screw block, but not a clamp-on.

BUFFER AND SPRING SELECTION
This is an area I have not nailed down yet -- but since 300 Blackout cartridge design parameters don't really call for special buffers and springs, I'm expecting with the right amount of gas coming through the block it should be possible to get the job done with relatively standard gear.

I'd love to be able to keep the same spring and buffer setup in the lower if I want to swap uppers to a 5.56 etc.
Last edited by FartinSpartan on Mon May 08, 2023 4:16 pm, edited 45 times in total.
FartinSpartan
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Posts: 36
Joined: Thu Feb 23, 2023 3:52 pm

Re: Superlative Arms Piston Conversion Kit - 300BLK Build Helpful Info Thread

Post by FartinSpartan »

Another barrel manufacturer dellet recommended, which makes 300BLK barrels specifically made to be used with Superlative Arms gas piston conversion kits, already pre-dimpled for the solid/set-screw type piston kits, and even with gas ports sized for your usage -- is X-Caliber. Here is a link:

https://www.x-caliber.net/configure-you ... piston-kit
FartinSpartan
Member
Posts: 36
Joined: Thu Feb 23, 2023 3:52 pm

Re: Superlative Arms Piston Conversion Kit - 300BLK Build Helpful Info Thread

Post by FartinSpartan »

Edited OP section about my Ballistic Advantage barrel after finding that I was not affected by the lack of thread relief behind the muzzle threads. My YHM QD mounts have more than .060" of.... "lead" inside them before the 5/8x24 threads begin. So the brake mount butts firmly and squarely against the barrel's shoulder before it runs out of threads on the barrel. A .060" or thicker shim as someone mentioned using is not even necessary for me.
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