Piston conversion, 300 Blackout

[Ape_Factory]

After my foray into subs for 300 blackout, I've decided to convert my SBR upper to piston for long term reliability and overall cleanliness. It'll be suppressed 100% of the time. Currently I use a Superlative Arms gas block in recirculation mode with a Rainier Arms titanium BCG. It'll cycle everything, zero issues. Super easy to set up and it pretty much worked the moment I slapped everything together. I will only shoot two loads, 110gr supers and 190gr subs. (plus a plinker 200gr. sub round for burning money at the range).

I also share the lower, an ADM ambidextrous with a 5.56 upper, Rubber City Armory Titanium bolt carrier which is virtually the same weight as the Rainier bolt carrier. I'd rather not have to adjust the buffer and keep the lower functioning on both rifles. With that said, I have an extra plane jane non-ambi lower and I've ordered another matching trigger for it just in case I can't make this work.

I ordered the Superlative Arms piston conversion kit and mistakenly ordered the wrong gas block diameter. But overall, it's a quality kit from the looks of it. My main issue is the weight of the bolt carrier. It's 9.4oz and my Rainer is 5.70oz. and the RCA is 5.6oz.

I then noticed Adams Arms had a 300 blackout-specific lightweight bolt carrier and I'm wondering if I should go that route vs. the Superlative. My only issue with the Adams Arms is the rod/spring possibly interfering with my barrel nut, see photos below.

Beyond that, I've ordered an RCA bolt and firing pin coated in their proprietary black nitride process and will also use the POF roller cam. It's been my experience that this coating is slicker, easier to clean and needs less lube than anything I've come across. Looking at the Rainier BCG, which is also black nitride, vs. the Rubber City Armory, there's no comparison, especially on the bolt itself. The RCA is much, much easier to clean.

Photo of the Adams Arms kit in 300 Blackout, you can see all the cutouts to reduce the weight:


Here's a few shots of my current upper, barrel nut and distance between the upper receiver, barrel nut and the gas block. The Barrel nut is a Smoke, for their carbon hand guard.




One thing I did notice is looking at the barrel nut and shape of my barrel, I could probably machine out about half an inch, a slot, for the piston rod clearance as it doesn't actually touch the barrel and does nothing to secure anything. Worst case scenario, I use the SA kit with the Adams bolt carrier.

If anyone has direct experience with the above, or suggestions, let me know. Adams doesn't have much in the way of actual measurements on their site and I've reached out.

Ultimately, I just think I'll have issues with a standard weight bolt carrier in the SA kit with what I want to do and cycle subs reliably.

 

[Neurotic]

Since you're spending a bunch of money, what about a handguard with a shorter barrel nut? I'm not certain how much shorter they can be, but I think the MI one's are only like 1.25" long.

Edit: regarding the reciprocating mass, do you restrict the gas port much? If so, I'd think you'd be able to make the heavier BCG work with less restriction but the same buffer as the 5.56 setup.

 

[Ape_Factory]

Since you're spending a bunch of money, what about a handguard with a shorter barrel nut? I'm not certain how much shorter they can be, but I think the MI one's are only like 1.25" long.

Mostly because I've already spent a bunch of money on the hand guard I have now and I actually quite like it. If I'd had the epiphany of a piston upper before this later build, I probably would have done things a bit differently. If I can't get it to work, I've actually thought about going with an LMT monolithic but I have a barrel that shoots well and I don't want to give that up either, especially in this day and age. I think I can get it to work but I won't know til the parts are in-hand. That's the sucky part. If I can get a hold of measurements from Adams, that'll at least help me make a more educated decision. And the Adams is back-ordered for months too which doesn't help.

 

[supercorndogs]

Adams Arms has a list of handgaurds that work with their kit. To my knowledge they all utilize a stock barrel nut. It is needed to for the return spring on the piston.

If it were me, I would leave it alone if its running. Its probably going to be louder, and just as dirty with the piston, also taking a gun that is running on gas, and switching it piston, does not mean it will still run.

 

[Ape_Factory]

You bring up good points. I may try switching to bleed-off mode on the SA gas block first and see if that helps a bit. I knew subs were going to be dirtier, just didn't realize it'd be that much worse than supers.

 

[hlee]

Most of the dirtiness, when shooting suppressed, comes back down the barrel and out of the chamber (because of suppressor back pressure), not through the gas block/tube.

 

[Ape_Factory]

Most of the dirtiness, when shooting suppressed, comes back down the barrel and out of the chamber (because of suppressor back pressure), not through the gas block/tube.

I sorta knew that...wasn't sure how much of a difference moving to piston would make. I do use a flow through suppressor (OSS) so no additional back pressure. I'll try bleed mode with the SA gas block first and see if that makes any difference whatsoever.

 

[supercorndogs]

I was running a YHM can with my Adams Arms kit. It makes quite a bit of back pressure, so your dirtiness results my be different than mine.

 

[thehun]

Superlative is a much better design...

 

[the monk]

Subs are very dirty, its just the way it is. I dont think your going to get around that. As someone who has been in blackout since 2012 and shot thousands of rounds (primarily subs) in all sorts of configurations. I think you would better off going with a Sionics or Forward Control Designs NP3 Coated BCG. Their ability to cleanup with just a wipe of a rag is very real and allows a gun if properly built and spec'd to run on the dryer side.

 

[Ape_Factory]

Superlative is a much better design...

The SA looks like it'd work with more hand guards, etc...but the BCG is HEAVY. Not sure it'd work with 300 subs and cycle reliably. That's my only issue with it. Can't figure out why no one has made an alternative carrier out of titanium so it'd cycle supers and subs reliably. Anderson's 300 blackout kit will cycle 300 subs but I'm looking at either modifying or replacing the Smoke Composites hand guard and barrel nut to get it to work.

 

[thehun]

I mean if you are stuck on a piston 300BO just get a MCX...or BRN180

 

[the monk]

I agree.

 

[Ape_Factory]

I can't stay I'm "stuck" on it but if I could get it to cycle correctly for a longer period of time sans cleaning, that's a bonus. Right now it's a moot point with the unavailability of everything. I'll likely get the Anderson and just see if I can get it to work with my current hardware. The barrel shoots, I've already form 1'd that barrel and the rifle is light without accessories. Moving to an entirely new upper is taking a risk and I've never been a Sig fan.

 

[the monk]

Just to clarify, when it gets dirty does the rifle start to have malfunctions?

 

[Ape_Factory]

As of now, no, trying to see if I can improve on what I have for long term functionality and cleaning ease. I'm ammo challenged like most folks so I'm not going through 1K rounds at the range these days or trying to run it til it stops. Just surprised how dirty the BCG gets shooting subs and how quickly that happens (first sub gun). I may need to change my minimal BCG lube regiment as it tends to dry up from what I've noticed vs shooting supers. This is shooting 50-100 rounds of subs with some supers as well while testing loads.

I did receive RCA bolt/firing pin along with a JP Enhanced bolt last week which appears to have a very similar coating. I'm going to give one of those a whirl as well.

Thinking out loud, it literally just dawned on me the existing bolt's gas rings may not be functioning perfectly leading to more contamination further up the bolt? But...the gun has zero issues cycling anything I put in there. I bought a range of factory ammo to try and no issues with any of it so far. Sub or super.

On another topic, anyone ever try a Riflespeed gas block? I'd loose the SA's bleed mode but man it'd be a whole lot easier to change between gas settings for supers and subs using something like this! https://www.riflespeed.com/RIFLESPEED-Gas-Controls_p_12.html

 

[the monk]

Sound like you built a good gun and now you are on a mission to keep tinkering with it. I would encourage you to spend some time over at 300blktalk.com.

300 blackout was specifically designed to not have to change gas settings.

Gas ports on 300 blackout guns can be tricky based on the length of the barrel. That's why most people run an adjustable gas block. It allows you to have a larger gas port, and then dial it down for perfect functioning. Or because you want to turn the gas down so when you shoot subs it's super quiet. Your can is not quiet so there's no reason to do that.

Dial it to where it works with the loads you want it to work with and shoot it.

 

[hlee]

If it ain’t broke, fix it until it is…

 

[Ape_Factory]

If it ain’t broke, fix it until it is…

It's a learning process and I'm just curious. I'm ok with experimenting and finding what works and what doesn't. Nothing I'd like to try is going to break anything but I might find a mix of parts and settings that further increase the reliability and durability of the gun over the long run. Push comes to shove, I go back to what I have right now as I know it works. It's just dirty.

On the not changing gas settings, that's the way I have it set now. If I reduce the gas and set it properly for subs, it won't cycle supers. I'm assuming I'm adding wear/carbon into the BCG by not adjusting down for subs or no? I've had numerous people comment on how quiet the rifle is when shooting subs with the OSS. Even with a "quiet" can, the cycling makes a hell of a racket so it's kind of a moot point on an AR. I do go to the 300blk forum on occasion.

Moving on...I did manage to make it out to the range today with the intention of running the SA gas block in bleed mode. Went to adjust it and it was stuck, very close to being frozen, something I didn't think was possible with the SA. I was able to free it up and got it to rotate freely and into bleed mode. I'm guessing I have, at most, 500 rounds through this rifle, half sub, half super.

I was under the assumption there would be a point where the gun would not lock open on an empty mag using bleed mode, just like restrictive. I guess it doesn't bleed off enough gas to do that. I tested both ends of the bleed off spectrum, minimum to max and it cycled both subs and supers, suppressed. I couldn't tell the difference in felt recoil honestly but it seems to shoot slightly softer with the gas block in restrictive mode.

I left the magnetospeed at home in order to concentrate on function so no results on velocity changes. I don't believe the SA gas block bleeds off enough gas to affect velocity greatly between minimum and maximum bleed setting but others have documented a slight loss in velocity between restrictive and bleed mode. There was a POI shift vs. running the gas block in restriction mode.

The new RCA bolt and firing pin were used today but I haven't had a chance to tear the gun down to see if it looked better/worse.

Met a few fellow experimenters at the range today...one had been working for a year developing his own cartridge which he called the 6mm max if I remember correctly. It'll basically work with nothing more than a barrel swap on an AR15 but will hurl 88 grain bullets well over 3,000fps. He was using a Lab Radar and had nothing but good things to say about it. Hopefully he's on the hide and can share more as I don't remember all the details.

 

[the monk]

On the not changing gas settings, that's the way I have it set now. If I reduce the gas and set it properly for subs, it won't cycle supers. I'm assuming I'm adding wear/carbon into the BCG by not adjusting down for subs or no? I've had numerous people comment on how quiet the rifle is when shooting subs with the OSS. Even with a "quiet" can, the cycling makes a hell of a racket so it's kind of a moot point on an AR. I do go to the 300blk forum on occasion.

As stated before by @hlee, the majority of fooling induced by a suppressor happens through the barrel and not the gas block. By turning down the gas you are slowing down the system. As I said previously, you can turn down the gas to just cycle subs, which with a can designed for suppression and a gun optimized can be 9mm quiet. No offence, but if you have only used an OSS can you have no idea what quiet is.

 

[Ape_Factory]

No offense taken. I havent personally owned any other suppressors but Ive been around many others and I couldn’t tell any difference between them as a spectator standing to the side. Especially with hearing protection in place. There’s usually at least one other suppressed rifle every time I go the range.

I have shot subs with the OSS and no hearing protection and while I would not do that regularly, it wasn’t objectionable by any means. Given we have actual measured data from sources like Pew, its within a few db of restrictive baffle cans. I’ll take that given all the other benefits of the OSS including less gas restriction and thus less fouling.

Am I under the illusion its as quiet as a bolt gun shooting subs with a Harvester 30 on the end? Hell no. I bought it specifically for the AR’s and it having no effect on cycling and to take the edge off the 308 with a muzzle brake.

So your opinion is a piston upper wont reduce the amount of fouling or run longer between cleanings when shooting subsonic 300 blackout with a flow through suppressor?

 

[Clayman]

So your opinion is a piston upper wont reduce the amount of fouling or run longer between cleanings when shooting subsonic 300 blackout with a flow through suppressor?

There is almost no restriction with oss and it's not great at suppressing subs because of that. So the blow back would be relatively nonexistent with a properly tuned di anyway. Tuned di > piston nonsense.

Will it probably be a bit cleaner, sure. But as stated by someone earlier itt most of your blow back comes down the barrel when shooting suppressed.

How clean do you need it and how few intervals? I have a 14.5 noveske that has seen over 1000 rounds with an Omega 300 or Sandman K attached without cleaning. It's dirty but it still runs fine. That doesn't have an adjustable gas block either, just a vltor A5H3.

People have gone many thousands of rounds without cleaning their di guns. I believe Tim's (military arms channel) bcm has over 7k rounds without lubricating or cleaning.

As far as reliability I don't consider piston AR's to be a more reliable system. Especially piston kits.

 

[Ape_Factory]

Makes sense, I've not run any of my DI's til they don't although I'm working towards that particular experiment. I guess I need to reach that point before realizing a piston conversion is going to be more trouble than it's worth. I appreciate the feedback.

On the OSS and subs, my experience and those who have both shot my rifle as well as stood next to it, commented on how quiet it was. It's literally a laugh out loud moment when I fire a round for both myself and others watching at the range. Dunno, maybe it's all relative but I'm super happy with it.

I've read Pew's review on the Q bolt gun with an 8" barrel and I didn't notice any first round pop and it's definitely quieter than supers on the same rifle. The guy next to me audibly knew when I switched to subs. Maybe there's something different going on with an AR vs. bolt and a slightly longer barrel, 10.5" with the OSS. I also have the lighter Ti version. Sort of a hands on vs. reading it on the internet situation. My experience could not be more different but I absolutely get why most will take Pew's data vs. my experience. And sure, I might consider something different if it were mated to one gun that would shoot nothing but subs. This is a one can on three different calibers, all DI AR's and it works really well. I don't notice an increase in fouling with supers suppressed vs unsuppressed.

 

[Clayman]

Makes sense, I've not run any of my DI's til they don't although I'm working towards that particular experiment. I guess I need to reach that point before realizing a piston conversion is going to be more trouble than it's worth. I appreciate the feedback.

On the OSS and subs, my experience and those who have both shot my rifle as well as stood next to it, commented on how quiet it was. It's literally a laugh out loud moment when I fire a round for both myself and others watching at the range. Dunno, maybe it's all relative but I'm super happy with it.

I've read Pew's review on the Q bolt gun with an 8" barrel and I didn't notice any first round pop and it's definitely quieter than supers on the same rifle. The guy next to me audibly knew when I switched to subs. Maybe there's something different going on with an AR vs. bolt and a slightly longer barrel, 10.5" with the OSS. I also have the lighter Ti version. Sort of a hands on vs. reading it on the internet situation. My experience could not be more different but I absolutely get why most will take Pew's data vs. my experience. And sure, I might consider something different if it were mated to one gun that would shoot nothing but subs. This is a one can on three different calibers, all DI AR's and it works really well. I don't notice an increase in fouling with supers suppressed vs unsuppressed.

Something to keep in mind on the pewscience oss review is short barrel 300 blk subs vs a 20" 308 super. So on the same host supers and subs there would definitely be a difference especially with a short barrel.

If you really want piston I would look into the sig sauer 300 blk options. PWS also makes a 300 blk upper. I know a couple people that had issues with different kits.

I could be wrong but I think a few manufacturers that use pistons don't actually offer the 300 blk rifles/uppers with a piston. LMT, LWRC and Barrett come to mind.

The thing with some piston guns is regardless of their adjustment they are oveegassed suppressed. SCAR, HK, etc. You can watch the most recent garand thumb hk video and see how gassy that thing is. They are also less accurate, more moving parts, more impulse, etc. I've never shot a piston gun in 5.56 that compared to a well tuned, short dwell length di gun.

 

[Ape_Factory]

It's not a matter of really wanting piston, I was just hoping it'd help the rifle run cleaner and cooler. I've read about the inaccuracy, the impulse and obviously more moving parts. Just took the BCG out of the 300 black and it's not as dirty this time around but my round count wasn't the same this time out as I wasn't testing loads.

I'll check out the 300 blackout forum and see if I can gain some knowledge on the SA in bleed mode.

 

[oldfatguy]

I mean if you are stuck on a piston 300BO just get a MCX...or BRN180

Maybe Brownells has solved the problems with the BRN 180 300BLK running subs. Had one that never ran, 4 rounds at most before FTF or FTE. Tried 11 different mags from 4 manufacturers. Returned it for a refund.

OFG

 

[hlee]

My 300 BO (DI) runs equally well with supers or subs, with or without can with either ammunition. It will not lock back reliably with factory subs, but that has been rectified by hand loading subs for it. Factory subs are expensive too. This is with a gas block full open all the time- even though it has an adjustable gas block. This is with a Q thunder chicken suppressor. Subs with the can require no hearing protection- even for my sensitive ears. Does it run clean? No, it blows fouling back into the action and magazine. But, it has never failed to function. I'm a relatively low round count shooter- Ill shoot 50-100, maybe. And, I don't have any issue pulling the BCG and wiping it down with a rag and adding a bit of oil to it before putting it away. Piston kits are a "solution in search of a problem."

 

[dwtaylorpdx]

FWIW - (lurker here...) I have a sig in 300BLK and one of the benefits I notice at least compared to my Ruger 556 (Got to cheap to ignore... LOL) is cycle time, it just feels quicker, I need to set up my go-pro and see if I can time it... I noticed it when shooting running jack rabbits, It may just be the gun but I can get the second shot off quicker...

I can use the 300 with supers or subs and just to lower the BCG recoil I turn the piston back 1, or if I'm strictly shooting it out farther (Coyotes or sage rats) I shoot the lighter end supersonic and turn the gas down 0 so it doesn't cycle at all, I have a slightly easier time holding the shot together with no bolt jumping around...

My Ruger is also a piston and it likes to be throttled back a click with the can on.. which seems to reduce the gas out of the ejection port..

Same Can both guns.
Ruger has a TImney.
Sig has a Trigger Tech.

Relatively new to the AR platform.. Compared to a bolt..