8" Piston SBR Project

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opsoff1
New Member
Posts: 1
Joined: Wed Jul 01, 2020 2:03 pm

8" Piston SBR Project

Post by opsoff1 »

Long time reader - first post...
Sometime back in the the beginning of the whole COVID / Work at Home thing, I started considering building an SBR in 300Blk. I wanted a really short / PDW style carbine. So I started reading and researching. This forum is a treasure trove of info. With that said - a HUGE thank you to all the board members here - absolute wealth of knowledge and this allowed me to accumulate a generous foundation to begin this build and it added immensely to the pure fun of it.

Legal stuff up front - eForm-1's were filed and approved prior to this effort getting underway. (Avg turn-around was 26 days from submission to approval.)

I actually built two complete rifles - one an 8" Piston version and the other a 10" DI version (pistol length gas, .096 port, std carbine buffer spring & H Buffer). The focus was on the piston system that would run subs unsuppressed.


The Piston Build:

BARREL
• 8.2” 1/7 CMMG barrel
• Pistol length gas system
• Original Gas Port: .096”
• Change 1: .101”
• Change 2: .108”

GAS SYSTEM
• Superlative Arms Piston Kit – double set screw retention
• Run with no restriction or bleed off.
• Piston preload spring was removed – unknown consequence.
• Piston/rod assy – custom cut length - shoulder to port center length - 4.0295" (Thank you Superlative Arms!)

MUZZLE DEVICE
• Midwest Industries – Flash Can; Zero back pressure. (REMOVED - useless)
• Surefire Warden - superb flash can - but provides no back pressure
• Maxim Defense – Hate Brake – provides measurable booster ability. Shown by improved cycling of subsonic ammunition. This device actually does provide a noticeable increase in back-pressure and helped significantly in achieving the reliability.

BOLT CARRIER GROUP
• Carrier: Std Superlative Arms Piston Type Carrier (NOT lightened)
• Bolt: M-16 9310 MPI Nickel Boron coated bolt.
• Cam Pin: Roller Cam Pin w/ Nickel Boron Coating
• Bolt Assist Spring Added: (Adams Arms Bolt Spring)

BUFFER ASSY
• Maxim Defense Proprietary Buffer to the CQB Gen 7 Stock – modified with replacement of weights.
• Original weight: 3.2 oz
• New weight: 2.75 oz

BUFFER SPRING ASSY
• 3 springs were tested
• OEM @ 1.9 lb/in
• Medium Spring @ 1.6lb/in
• Light spring @ 1.3 lb/in (Installed)


AMMUNITION

Building/Reforming Brass
Cut .5.56mm LC brass to 1.358” (Chamber Max: 1.368”)

LOAD DATA
I tested hand loads with:
150 gr FMJ Hornandy
180 gr HPBT SMK
190 gr HPBT SMK
220 HPBT SMK

And the following powders
H110
IMR4198
IMR4227
A1680

The 150's ran with everything (both guns) - velocities ranged from 1400 - 1700 and I am sure I could get more easily.

I also ran some of the Maxim Defense (mfrd by Fort Scott Munitions) Solid Copper Spun loads - 115 gr & 190 gr. The 115gr is snappy and ran fine in both rifles - velocity is up in the 2200 fps range.
The 190 would not cycle - too slow - chrono #'s were down in the low 910 range. Interestingly enough - The other upper w/ a 10" bbl and DI system - wouldn't function in that either. The charge weight for this load was aprox 10.4 gr of A1680. The 190 solids have a rebated bearing surface so they clearly don't build pressure like a std jacketed bullet.
By comparison - I had a light charge of A1680 behind the 220's - (11.4gr) and that ran fine in the 10" DI gun with velocity in the 930 fps range.

FINAL LOAD
SUBSONIC - 100% reliable - unsuppressed.
220 gr Sierra HPBT
12.3 gr A1680
Wolf Sm Rifle Primer
2.230” OAL

Avg Vel: 1041
Ext Spread: 15 fps
High: 1047
Low: 1032
10 shot sample
I have since run over 150 rds of this load and had 100% reliability.
Ambient temps ran from 38 deg to almost 80 deg

MAGAZINES
Mags are a big deal !!! They have a lot of impact on how the BCG cycles (speed)
• Current go-to: Lancer Systems 10 & 20rd 300 BLK type. Specific to the cartridge, especially <200gr bullets. These are OUTSTANDING magazines.
• Magpul 10 & 30 rd P-Mags – internal ribs push heavy bullets towards center and also have excessive spring pressure that slows cycling. NOT recommended.
• GI 20 rd aluminum mags appear to function acceptably
• Light bullet loads/supersonic ammo (i.e. 115,125 & 150 gr) are short enough to function in the P-Mags.

FURNITURE, OPTICS & ACCY's
Rail is a Daniel Defense MFR-XL and as a neat bonus, the hexagonal ID matches perfectly (with clearance all around) with the hexagonal OD of the Hate Brake.
Grip - Magpul MOE-K+
RDO is a Sig Romeo 5 - great glass, small, light easy to use.
Trigger is a CMC single stage - so far so good.

So that is really it in a nutshell. Fun build(s) - Great foundation of info gained from this forum (THANK YOU again). I wanted a reliable, short PDW style carbine running a piston that was 100% reliable with subs. DONE. Finished length is 22.75" Cycles great, reliable, clean, VERY concealable (it fits under my pillow...) Accurate - while not match grade - it was surprising. At 50yds shooting offhand/standing and running 2 chest/1 head drills - groups were easily within tactical accuracy requirements. (see target pics)

In the end I have to throw a giant shout out to two people in particular. Tony at Superlative Arms and CJ at Maxim Defense - enormously helpful and generous with their time and knowledge. The kit that their companies produce is absolute top notch. You get what you pay for - the fit, finish, design and engineering was amazing. (and no I don't work for them or get paid by them) Same goes for Daniel Defense, CMMG, Magpul and Sig.

Thanks again to the board members!

Opsoff

https://photos.google.com/photo/AF1QipP ... azDVuOM1YE
https://photos.google.com/photo/AF1QipP ... sYpMmwO2MM

ETA - can't get the pics to display...
mebgardner
New Member
Posts: 6
Joined: Wed Dec 29, 2021 11:02 am

Re: 8" Piston SBR Project

Post by mebgardner »

"• Bolt Assist Spring Added: (Adams Arms Bolt Spring)"

This was an interesting design choice. I noticed you opted for the SA piston kit, and yet added this part from the AA piston kit.

What does this addition bring to the build?
FartinSpartan
Member
Posts: 36
Joined: Thu Feb 23, 2023 3:52 pm

Re: 8" Piston SBR Project

Post by FartinSpartan »

Thank you for sharing your build info and some things you ran into along the way.

Perhaps not many people build piston-driven AR's or even have any esteem for them but some of us do. The longer length of time before it will be necessary to clean the weapon to keep it running due to crud buildup etc. is worth it to me.
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