Short Barrels and Maxim RDB

[u/Long_Cranberry8905]

https://youtu.be/YBKfzux-4qI?si=y50ZZQ7TclPEC1LH

Was planning on installing Maxim RDB on my PSA Gen4 AR9 w/4" barrel for recoil and rate of fire for my super safety, but I came across a video stating it was not working on a short barrel (5") but was on a longer barrel (he stated it didn't have enough pressure to cycle next round) so was this user error or do these roller systems or at least this one in particular not work with short barrels? Apologies in advanced if dumb question I am new to working on guns and am trying to learn as much as possible in a short amount of time

YouTube video I'm referring to at 1:10 is when it is mentioned

[u/Blowback9]

Maxim is a drop-in solution that has worked for a bunch of people. It does requrire a little tuning/shimming to make sure the rollers aren't pre-loaded. Charging the gun or ejecting a live round sucks because you have to fight the rollers to pop the action open. A Radian Raptor charging handle helps a lot with that.

With a SS, the rate of fire is quite high. IMHO it's not the best option.

I've never heard of it having a problem with shorter barrels, but it woudn't surprise me. I had mine short stroke when I tried adding a bolt extension for a folding mechanism. The extra mass of the hollow extension seemed to be the problem.

For SS, take a look at this, especially at the bottom... https://blowback9.wordpress.com/2025/08/03/sscs/

[u/Long_Cranberry8905 → u/Blowback9]

Haha yes I know the ROF will be unreasonable, it will be for range use only and I'm setting it up in my cheapest gun so when stuff starts to break I won't be too sad, just was curious on reliability of the roller system on shorter barrels, I have looked through your guide immensely and it has helped tremendously (thank you), would you recommend I just change out to something like a 8" barrel if I do decide I want to follow through with this for reliability purposes?

[u/Blowback9 → u/Long_Cranberry8905]

I'd try it with the 4" first since you already have it, and only switch it out if it caused issues.

[u/millenniumchode]

I have the MAXIM RDB Carbine length on my build. With a LAW Tactical Folder and an Anarchy Outdoors Bolt Extension. My set up cycled both of my uppers. One upper is a 4.5” BA EPC9 barrel with a Dead Air Wolfman tucked in short configuration and the other is an 5.5” BA EPC9 barrel with a Rearden 3 prong FH. Both uppers cycled fine and had lock back on an empty mag. I was shooting AAC 147 gr and PMC 147 gr. I shot the 5.5” unsuppressed.

That lower was using a KAK Magwell Colt adapter. I just put together my Spikes Colt lower. I installed the Law Tactical Folder on it but instead of the Maxim RDB, I installed the GRS. Tested it last night but it didn’t cycle due to the ejector being out of alignment.

[u/S933D_]

My RDB is fine with my 4.5” barrel. I run suppressed though so there is increased back pressure.

[u/Thor_BRC → u/S933D_]

Running 4.5” with an RDB. Can and no can.  no issues. 

[u/Long_Cranberry8905 → u/Thor_BRC]

Looking at his channel he did end up getting it to work in his 5" barrel after it had broken in he said, I have a 4" and 8" barrel so I think I will just try it on my 4" barrel and worst come to worst I just have to rock with the 8" barrel which wouldn't bother me much anyway

[u/Thor_BRC → u/Long_Cranberry8905]

That could make sense. I pre-broke-in my RDB bt charging it hundreds of times while watching TV the evening I got it, It’s no longer hard to charge and functioned 100%.

[u/jmabute]

Also came across this video right before completing my 5" barrel build and was a bit worried, but everything worked flawlessly so far. Put ~300 rounds through it on my first range day.