8" Piston SBR Project

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8" Piston SBR Project

Post by opsoff1 » Wed Dec 02, 2020 2:17 pm

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:

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

• 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!)

• 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.

• 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)

• 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

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


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

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

And the following powders

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.

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

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.

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!


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

ETA - can't get the pics to display...

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Re: 8" Piston SBR Project

Post by mebgardner » Tue Jan 11, 2022 3:51 pm

"• 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?

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