I've been shooting my AR-9 with Maxim RDB for a couple months now.

On the whole, I think I like it. I've had a couple of difficult to explain feeding failures that could have been magazine related (I run the stendos) or short-powder or something. I shoot reloads, weird shit happens. My MPXs are hardly perfection, in comparison.

One thing to note is that I shoot high PF (135) ammo, and it would not shock me if lighter ammo was causing issues with this system.

Finally got to shoot and sight in my Palmetto State AR-V with Maxim buffer system. Has a 9" PSA barrel and Silencer Central Banish suppressor. Ran like a top and the rounds used were a mix of super and subsonic in the same mag. Deffinate difference in sound between them and shot great. Glad it ran with no hiccups and ejected great. This is a replacement for my CMMG RDB that I was tired of fighting ejection issues with. The AR-V takes the CZ Evo mags. I only have PSA mags right now but bought some Magpul mags to try. I need to run this thing through some local matches and see how it does. Way to go PSA and Maxim.

My AR-V hates the Magpul scorpion mag I bought to test. I just bought a bunch of PSA mags after testing it, as they work great, look great, and are cheap to boot.

Well crap. Hope they fixed something or I can offload them on a CZ owner.

@erwos
Most 9mm AR feed failures can be fixed using a barrel with an enhanced feed cone (the funnel-shaped cut that leads into the chamber).  The larger feed cone compensates for many issues including mag height and cartridge presentation angle.

See here: 4-step guide to make (almost) any AR9 run 100%

Won't help with shortstroking, though.  I noticed my Maxim was very sensitive to extra bolt mass and wouldn't lock back on LRBHO.

Hope this helps.

I'm using a JP Supermatch 9mm barrel that, in theory, has pretty good feeding. I may be swapping over to a shorter barrel to make this thing a suppressor host, though.

No problems with short-stroking, but my ammo is 135 PF.

@Joedirt199

Did you have to modify your AR-V in any way to get the Maxim to work?  I'm tempted to get a Maxim for my AR-V but haven't read too many experiences with the combo.  Thanks for posting your's!

Just have to pull the weight out of the bolt. Forgot to do that and thought, "Man this thing is impossible to charge." I butted the carbine extension right up against the bolt so there is no free space between the bolt and buffer system. I also pulled the buffer back some before installing and greased up the balls in the buffer. Not sure if it makes it easier to charge but figured it couldn't hurt. May help keep ware and tear down.

Interesting note here, as I bought 2 RDB systems at different times.  The first one was in pristine, shiny-new condition, and the bearings were WELL greased.  The second one had some rust webbing around the laser engraved serial # on the outside and had less grease on the bearings.  The first one was very low serial number.  The second was much higher serial #.

We know Maxim received far more interest in this than they originally expected, and they sold out their first batches quickly (the carbine version first, from what I saw in availability).  I suspect some later batches were hurried, and while of quality manufacture, some may have had less attention paid to finishing details like amount of grease on bearings, or proper moisture & temperature control before/while/immediately after laser cutting serial #s (explaining the rust).  Thus, I'd expect those systems that were rushed to have a higher probability of benefitting from some extra lovin' like ensuring proper grease amounts in the bearings.

That said, any change to the bearings alters the resistance to cycling as well as charging, so it better not make charging too easy.  

I have a spring loaded bow weight scale I could put on the charging handle and see how much weight it needs to pull loose. I still have to lock the stock into the shoulder and give the charging handle a sharp tug to get it to cycle.

@Kiranslee how did the DDC Hard Charger work with your RDB setup?

I'm really considering one of the RDBs for a PSA AR-V project, but I am hesitant to deal with the difficult charging aspect.

Get large handles on your charging handle. I have a PRI gasbuster with large ambi handles and it is not hard to charge. I can get more fingers on the handles. Leave the stock in the shoulder and give it a sharp tug.

Ok, so I want to make a spacer that will limit the length of travel since my 9mm build doesn't need all that extra space. Anyone have any ideas of a rubber bumper that would fit over the the rear of the buffer system to keep it from moving about a half inch less?

Quarters.

Really bad idea. That will make the system non-functional.

JP makes spacers for their SCS that would likely work or could be modified pretty easily.  The tall bumper in the SCS maintenance kit would probably be about perfect.

I saw that and I think that is for using the carbine system in a rifle length buffer tube. I saw a reddit post where a guy made a spacer out of delrin and added some orings on the ends for bumpers. May be an option if I can get access to a lathe. With the extra buffer travel, I have had some brass get stuck behind the bolt lock when they failed to eject.

The bumper in the maintenance kit should work as is.  The spacers are designed to use it in a longer tube, but I'm sure they could be made to work with minimal work.

https://www.jprifles.com/buy.php?item=JPSCS2-MAINT-K15

They did have an aluminum spacer for the short stroke kit. I wonder if puting the aluminum spacer at the end of the rod, then mounting the bumper next to the spacer would be enough. They add 2 aluminum spacers next to the weights on their captured buffer short stroke kits. I sent an email to Maxim to see if this is an idea they have thought of. Will send an email to JP asking about the specific parts and their take on it.

Don't make this difficult. Fashion a 1/2" spacer(thick washer, has to have a hole in it to clear the spring) out of aluminum, Delrin, or whatever, and place it under the original bumper. Done!

That's what I was I thinking. Now to source some aluminum tubing.

See below.

I looked closely at my RDS and now I'm not sure you'll be able to modify one the way you want. It appears as though the bottom steel piece has an extension that goes into the nylon bumper. If that is the case, you won't be able to put a spacer under the bumper and be able to keep the bumper centered on the rod. The only way I can envision something working would be to make an entirely new bumper the length you want to replace the old one entirely. That wouldn't be hard if you had a lathe and some material, but, otherwise...

Hopefully JP and Maxim have some ideas and I hear back from them.

So you are trying to do what I did on page 6 of this thread but longer not shorter right?
https://www.ar15.com/forums/AR-15/Maxim-Defense-RDB-review/15-780933/?page=6

Yeah needs to be about 1/2" longer bumper to reduce the length of travel of the buffer. I have a PSA AR-V using Evo style mags and the bolt travels way past the bolt lock allowing brass that fails to eject to get stuck behind the bolt locked back nub.

No need to wait. Go to post #21 and look at the pic Amphibian shows of the bumper. You can see how the steel piece below the nylon bumper is inserted into the bumper. This allowed them to have a steel seat for the spring. So, all you would have to do is make a new bumper the length you want by cutting one from a piece of Delrin rod and them machining/drilling a through hole the same size as the one in the original bumper. Push the steel collar into the new bumper and you're done. I can't see why you'd even have to taper the new piece to match the old bumper. That taper seems to be purely cosmetic.

I just use these from Home Depot

Nylon Spacer

Got a foot of delrin tube coming so I will be able to make several short stroke spacers if anyone is interested. Should be here tomorrow and I can start figuring out the length needed this weekend.

Got my delrin short stroke bumper done. Man I wish I had a mill and a lathe. Modding parts on a poor man's lathe, aka electic hand drill with part held on a bolt, sucks. Got it turned down under an inch on the outside and inside is just under half an inch. I went with 3/4" long as you have a little over an inch of space between the bolt face and the bolt hold open nub. The bolt release is much better now that the small nub on the bolt stop is gone. I can seat a mag and drop the bolt much easier with the bolt release button. Now to run a few rounds through it tomorrow.

Got to test it out. I think I will grab a JP SCS spring kit and try to tune the Maxim just a little firmer. Throws brass pretty far and still has a decent port pop so hoping a stiffer spring will hold the bolt closed just a little bit longer. Short stroke worked great and only problem I had was a couple of fail to feeds where the round looked like it missed the ramp and went straight up. Happened on a PSA mag and a Magpul Mag, both the very last round.

Just got my Maxim RDB installed in my Aero EPC upper and lower. At the same time I got my replacement barrel from Aero's upgrade program and a new CMC PCC trigger. In addition to that I was using the EPC bolt which is not compatible with the Maxim, so I got a BKF bcg to fix that. While I was going to have it all apart I decided to swap stock and grip too. So I kind of rebuilt the EPC yesterday! The good part of that is that everything is new and nice, the bad part is that if it doesn't run it could be anything.

While I had it apart I cleaned every little part and boy do blowback guns get dirty! I even got a ton of crap out of the buffer tube! I sure hope the RDB improves that. But speaking of the buffer tube, my EPC is built with the buffer tube that Aero included with the EPC buffer kit and it is PERFECT for the Maxim RDB! There is no preload on the buffer at all, but just enough space to slide a hair through. Literally couldn't be better! This was a concern as I had read others have a gap on one turn of the buffer and preload on the next, with no sweet spot.

So a lot has been made of the force required to charge the firearm with the RDB, and with good reason. After the first failed attempt I had to reset to get it right. I am going to try my Raptor charging handle to see if it helps as some have said.  I had my son try and he thought it was a trick until I showed him it could be done. Once he knew it was possible he did it first try. Next I had my 5'3" daughter, who my son calls "baby arms", give it a try. She pronounced it impossible until I told her I would get her favorite dessert if she could do it. Properly motivated she got a good grasp and charged the rifle!

It is hard enough that I would consider this a no go for a gun I might bet my life on. For competition or fun I think it is up to the shooter. For me the EPC is something to train with that has AR controls but shoots 9mm (affordable, available, low barrel wear, safe on steel at shorter ranges), and something to shoot informal competition with my friends. For that purpose the charging force is not a deal breaker so long as it lives up to the hype in the shooting experience.

On that note, I will make a separate report after I shoot it. My non-shooting report is long enough by itself!

I'd really like to hear from someone who has this & a side charger, see if that makes it any easier.

No direct experience myself, but I have read a few comments from people who have said a side charger made little to no difference. I have heard several people say that Raptor charging handles make it a little easier but nothing else they tried did, including the Aero Breach charger.

Some people intentionally preloaded the buffer and said that helps a lot, but I think that is a horrible idea defeating the whole purpose of the RDB.

All in all I think it is just something you have to deal with on this system. Again, it is not a deal breaker for me as long as the shooting experience makes up for it.

I have two with side chargers and one with a rear. The side chargers are easier, but still a bear to get moving the first fraction of an inch. The extra effort is well worth the trouble.

Yeah, it's annoying to have to rack it hard every time, but I'm getting more used to it. The added delay makes a difference and the recoil doesn't feel abrupt since there's less mass moving around. I had been using the extra heavy 11 oz KAK buffer for a long time before this and even though the added mass makes the recoil slower, it's still a lot of inertia to stop at the end of the stroke coming back and when chambering a round.

With the Maxim buffer, the recoil stroke feels light and quick and the muzzle doesn't dip when chambering a round. The added charging effort is worth it for me.

So I posted my post-install review a few days back, now it is time for the post-shooting review.

I went to my buddy's private indoor range (I am actually not kidding) today with the RDB enhanced EPC, my Scorpion SBR, and Century AP5. The scorpion was there to represent what the EPC used to feel like, the AP5 what it wants to be. I burned a considerable amount of ammo to see how they stack up, so how did the Maxim/EPC Franken-AR9 fare?

I'll get straight to what you want to know: how does it compare in recoil? I shot bill drills, Mozambique drills, accuracy tests and more. At one point I setup where I had one gun against the left shoulder and another against the right alternating shots so I think I have the recoil difference pretty nailed down. If the AP5 has a recoil force of 1, and the Scorpion has a force of 10, the RDB setup has a force of 5. I was really hoping for a 2 or 3, but it sits right between the CZ and AP5, but just a little closer to the AP5 than the CZ. It definitely has a smoother feel. My buddy has an PSA AR-V and noted how much smoother the RDB felt than his. The EPC was easily the fastest in all drills but I don't want to oversell the RDB too much. The CMC trigger had at least as much to do with that as the RDB. The Scorpion and an AP5 triggers are hot garbage compared to the CMC.  

For dot bounce I saw a big improvement.  The EPC was the worst of my 9mm PCCs for throwing the dot around before the RDB conversion. After the conversion it may be the best, the dot was very flat.

Reliability was perfect and I shot a wide variety of ammo. Low power 115 FMJ to 147 JHP and some in-between, flat points too. This is saying that the RDB did not cause any issues but it also speaks to the new EPC barrel provided by Aero in their voluntary upgrade program. FP and JHP used to be this thing's Achilles heel, but no longer. Huge respect to Aero for doing this.

With that barrel and trigger the accuracy contest was over before it started. The range is 20 yards and at that distance the RDB EPC was just putting everything through one fat hole.

I shot some suppressed to see what effect that had. With good subsonic ammo there was almost no port pop or gas to the face. With supersonic there was a puff of gas but definitely much better than in the old blowback config. It is not as quiet as the AP5 but close, way better than the Scorp, which has some pretty significant port pop.

The last observation is the effort to charge the rifle. After today's trip, and 200-300 rounds, the force required is down significantly. I would guess 20%. At this stage I would call the charging effort a non-issue. I will continue to monitor as I think it would be a bad thing if it gets too easy.

My conclusion is this: I would not build an expensive new PCC around this system. I would probably save more for a CMMG Dissent or Sig MPX. But if you are in the position I was, having an AR9 that I already registered as an SBR, and therefore don't want to sell, but wished it was better, this is a pretty good option. All in all I am pleased!

Wonder what lightening the 9mm bolt would do for reducing felt recoil with an RDB setup….

Wonder what lightening the 9mm bolt would do for reducing felt recoil with an RDB setup….

Hello guys, maybe some of you know where to order/buy maxim buffer in Europe?

Anyone have an answer for this? Wondering the same thing. I want to put this buffer with a LWRC UICW stock on my MP5 SD we have at home.
Attached File

Response from Maxim was:

"The CQB RDB is designed for our CQB series braces/stocks. We have not tested on other buffer tubes."

I'm gonna have to just buy one and try it I guess.

I ordered one for my regular ol ar9 with side gibbz charging upper.
Gas to face is brutal even with a heavy arstoner buffer and side charging upper.  Supressor is YHM R9.
My brass is dirty as shit compared to the same from a pistol.
Were talking VV N340 here.  

So maybe this thingy will help cause I havent been shooting it suppressed lately and its a fun gun.
Oh and Im lefty.  Well ambi but better lefty.

If you look back to page 6 of this thread, I posted my suppressed results and found it was loud with my CT9KS and @CJoFL was with me to concur.  
However, I think some others have posted later in this thread it worked for them.  Maybe the R9 has higher backpressure than my coaxial design CT9KS and it may be fine?
I also posted that using the Maxim RDB in conjunction with my belt fed FM9, the same CT9KS is now quiet most likely because the belt fed FM9 uses a good bit of energy to pull the belt to cycle so kind of a delay already.  That is the only use case I have for my Maxim RDB as I find the hybrid CMMG Dissent to work the best for my needs.  

I have long contemplated installing a cmmg RDB bolt/barrel and may end up there eventually.  I grabbed a AP5 a couple months ago to have a comparison.
Worth a try I figured.

Please update if you do. Not knowing the answer has kept me from buying one

First time at the range today with the carbine Maxim RDB installed.
I shot decent powered 124g JHP reloads.  5.2g of VV N340.
And 147g Summit City 147g Hitek coated RN.  Forget charge.  I think its 3.4g of WSF.
Didnt shoot tons. Maybe 75 of each.
I had zero malfunctions other than the 147g would not lock back the bolt without my YHM R9 suppressor attached.

Initially I noticed a decent reduction in felt recoil.  But that feeling went away as I shot more.  Not surprising. I wasn't comparing side by side.

I didnt buy the Maxim for its lighter recoil.  I wanted to reduce gas to face when suppressed.  
It seems to have lessened it but not as much as I would want.
Gas to face just takes the fun out of shooting suppressed for me.
I may see if n320/147g will work better for a less gassy subsonic loading.

Is there a way to slow it down more?  

Also, I had no issues charging.  It definitely has a strong initial force to overcome but my side charger was fine.
I would press the stock against my chest and pull with a strong initial motion.  Had no issues there.

I am going to strip mine down and see if I can get a heavier spring for it. I was using 147 RMRs with 3.5 gr of Titegroup. They run right at 1050 fps. I notice that the shells get thrown pretty far away on ejection so it seems the spring could be heavier. Was looking at the JP Silent Capture spring kit but I don't think their heavy spring is that much heavier.

Out of what length barrel and was it suppressed?
I've been using 3.7Gr of Titegroup w/ 147Gr projectiles for 15+ years.  Yes, I know max charge is listed at 3.6gr.
We had a guy at our SMG matches running 4.0gr of Titegroup with 147gr out of his full size UZI as well.

So I chrony'ed it a long time ago and just pulled out my data:
3 lugged full size UZI barrel which is about 9" long 5 shot average was 946 fps unsuppressed w/ 3.7.

I tested going up to 4.0 and got an average of 1053 fps.  I didn't see any signs of over pressure but sometimes my rounds would go supersonic with that velocity so I backed down to 3.7 knowing that if my powder drop wandered  +/- .1 gr I wouldn't care/notice.

So wondering how your getting 1050 with 3.5.  I used a PACT Chrony to obtain my results.  Which chrony are you using?

It is an 8" PSA barrel suppressed with a Banish 45 in full length. I load to OAL of 1.1" chrono is a magneeto sporter. Brass is probably federal and  remington. Loaded on a dillon 550.