8” .300 Blackout Hack-job

**JerDerv** · 07-19-19, 11:53

Having decided on my first suppressor (yay finally!) I started digging through my parts bin. With a few lightweight nick nacks here and there and an SBA3. I decided to build a somewhat lightweight .300 blackout pistol.

I decided on the CMMG 8” barrel. An 8” 300blk is the minimum length for a lot of suppressors. Cmmg’s barrel is profiled lighter than the others I came across and it features a 1:7 twist. A close runner up was the faxon 9” 1:8 twist.

I used a Samson lightweight gas block and fit a YHM QD brake as this pistol will be the home for my YHM Resonator. Im a fan of ALG handguards but they don’t sell an 8” m-lok. I already had an ALG V2 laying around that I picked up cheap used, out came the hacksaw. Sanded flat and square I dabbed on some alumi black and I had my 8” m-lok ALG.

The non forward assist Aero upper got an AIM surplus lightweight bolt carrier outfitted with an RCA adjustable gas key.

I chose titanium and aluminum components in place of steel for a few of the lower parts. The castle nut and end plate are 7075 from damage industries. Take down and pivot pins, buffer retainer, and grip screw are grade 5 titanium.

Wanting to keep it compact and lightweight I decided to try out LWRC’s UCIW receiver extension. The buffer and SBA3 needed to be shorter, out came the hacksaw.

With a short length of pull I prefer a steeper grip angle. I already had an Ergo swift grip laying around, I like the grip angle but they are longer than needed so I cut the bottom off.

Keeping it simple I topped it off with some Daniel Defense irons. Total weight came out to 4lbs 12.2oz

**opngrnd** · 07-20-19, 23:45

How does it shoot?

**SBRSarge** · 07-21-19, 00:34

JerDerv,

Nice clean and compact build.

I used the same barrel on a SBR and have found it to shoot really well.

**JerDerv** · 07-21-19, 12:34

Originally Posted by taekwondopreacher

How does it shoot?

So far i got the sights lined up, and tuned the gas key to lock back on empty with magtech 200grn subsonic. A box of different supers and subs show up this Thursday, plan is to get a feel for accuracy and put it through the paces unsuppressed. Once the suppressor shows up I’ll do it again.

Originally Posted by SBRSarge

JerDerv,

Nice clean and compact build.

I used the same barrel on a SBR and have found it to shoot really well.

Thank you!

Glad to hear, so far so good over here. I have read some older complaints with suppressor misalignment, I’m eager to check mine.

**JerDerv** · 07-26-19, 22:42

Suppressor arrived at the FFL today, suppressor fits great, wish i could bring it home. I put 150 rounds through it today, 115grn 123grn 150grn and 200grn subs. Shooting subs suppressed is an experience i have been looking forward to for a long time. I also put the can on my KS47 and .308 bolt gun, really happy with the performance. The cmmg barrel to suppressor alignment is great. Function was mostly great, shot fine for me the first 125 rounds. I handed the gun to a buddy to try out with subs and it shot two rounds and then failed to feed, he smacked the magazine to make sure it was fully seated and charged the rifle. Fired two rounds and click, failure to feed. Dropped the mag, pulled the BCG and inspected the gas key and adjustment screws. Everything looked tight, put the carrier back in seated the mag and shot the remaining 25 rounds without another issue. When i got home and pulled the gun out the clean, the bolt carrier was extremely sticky in the upper, the bolt was difficult to unlock and the carrier movment in the upper reciever was sluggish and sticky. The lubricant was gone and everything was covered in carbon. The carbon began to gum up the carrier bearing surfaces and the bolt was not at all free moving within the carrier. I disassembled and cleaned everything, im assuming the failures to feed where due to short stroking because of this lack of lubrication and filth. This is my first time owning a suppressor and the first thing i have noted is that my gun may require more lubrication than i typically apply. I have been using lucas gun oil, perhaps there is a better lubricant out there for suppressed ARs?

Here are some quick pictures i snapped with the Resonator attached.

**SBRSarge** · 07-27-19, 03:56

I love shooting suppressed, but it sure does gunk up the action from the back pressure.

I usually keep a can of lube in my range gear and, at the first sign of the BCG slowing, give a shot of lube onto the BCG, bolt body and locking lugs.

A higher viscosity lube might stay a bit better than a thinner one, but I have good luck with synthetic Amsoil motor oil and CLP as well as the much thinner Super QCG. It is more a matter of keeping the thing wet, than of what you use.

**recchavo** · 08-22-19, 18:28

Love my 300 Blackout

Sent from my iPhone using Tapatalk

**JerDerv** · 08-30-19, 11:40

Thanks for the compliments! I have put another couple hundred rounds through it. I have learned a few things. Tuning the lightweight carrier and .8oz buffer to cycle unsuppressed with subs leaves it very over gassed when shooting supers suppressed. Extraction was starting way to early and the extractor was leaving huge marks in the brass and in some cases even stretching the brass. An adjustable gas block instead of an adjustable gas key would be better for this setup however id like to avoid an adjustable gas block on this build so i made a new buffer that can accept 2 standard weights so i can load it up to 2.1oz 2.9oz or 3.6oz. Ill be playing around with that some more.

Last time i shot i took another picture.

Got some new parts in the mail yesterday, all PSA Custom bolt catch, safety, and mag release button. Shaved a few grams but more importantly bigger bolt catch extended mag release and ambi safety are all improvements in my book. Trying a different trigger as well, its from unbranded AR.

**RVTMaverick** · 10-29-19, 09:26

You Tricked me, You haven't built a HACK Job at all, She's SWEET!

And,... How about NO Oil....>>>-------> https://www.youtube.com/watch?v=bWJ8rjsvFas Enjoy!

Originally Posted by JerDerv

Thanks for the compliments! I have put another couple hundred rounds through it. I have learned a few things. Tuning the lightweight carrier and .8oz buffer to cycle unsuppressed with subs leaves it very over gassed when shooting supers suppressed. Extraction was starting way to early and the extractor was leaving huge marks in the brass and in some cases even stretching the brass. An adjustable gas block instead of an adjustable gas key would be better for this setup however id like to avoid an adjustable gas block on this build so i made a new buffer that can accept 2 standard weights so i can load it up to 2.1oz 2.9oz or 3.6oz. Ill be playing around with that some more.

Last time i shot i took another picture.

Got some new parts in the mail yesterday, all PSA Custom bolt catch, safety, and mag release button. Shaved a few grams but more importantly bigger bolt catch extended mag release and ambi safety are all improvements in my book. Trying a different trigger as well, its from unbranded AR.