ACR Forum banner
1 - 19 of 19 Posts

·
Premium Member
Joined
·
4,707 Posts
Discussion Starter · #1 · (Edited)
Here's a description of all the gas systems lengths for the Acr. They all have their pro's and con's. The only 4 so far is the Midlength which is standard, Carbine, PDW and Pistol length.

The Midlength system is the standard system for the Acr at this point. On a DI rifle it has slightly less hop than a shorter length system, but I don't notice it near as much on the Acr. This system works best with the basic handguard (ascetically) and can go down to around 12.5 inches suppressed. You can go slightly shorter if you don't suppress it since the Suppressor will hit the regulator.

To install a midlength system on an AR15 barrel requires a profile reduction for the piston guide, cuts for the gas block taper pins, and generally a gas port reduction.


The Carbine length is slightly shorter than the Midlength. It is, in my opinion the best length for the Acr. It allows for every length that the Midlength can use plus go down to 10.5"-10.3" suppressed. The weight is slightly moved back and the regulator is placed under the receiver. This is also the preferred length for the Remington Acr.

To install a Carbine system on an AR15 barrel requires a profile reduction for the piston guide, cuts for the gas block taper pins, and generally a gas port reduction plus piston reprofiling and trimming.

The PDW length is from the PDW Acr variant. It has a slightly shorter gas system and custom gas port placement. According to Remington the barrel is 8.25" with an OAL of 19.5"

The Pistol length system is extremely troublesome for the Acr. This requires the least work to fit the barrel but the most work to "Every" component close to the system. You need to trim down the gas block and charging handle tray extensively to allow for clearance, you also need to shorten the piston significantly and reprofile almost the entire rod. You also do NOT have any access to the adjustable regulator so there's no S or U setting for being Suppressed or Unsuppressed.



The pistol length at this point is highly discouraged by me, unless you know alot about the Acr system. It's not a quick drop in system and could deadline your rifle if a cut is made incorrectly and/or blow it up. Unfortunately it's also required for the shorter 300blk barrels.

Some pics of the systems
Carbine on the left, Pistol on the right. This shows the position of the charging handle carrier for both systems.


Pistol, Carbine, and midlength systems.







A slightly better pic of the pistons

 

·
Registered
Joined
·
132 Posts
Thanks bigdog, very interesting and really helpful. Do you know what the various gas port sizes you used for the different lengths were? Do all the lengths (other than pistol) run pretty much trouble free with different ammo?
 

·
Premium Member
Joined
·
4,707 Posts
Discussion Starter · #3 ·
The gas ports aren't really common info overall. The 10.5" is somewhere around .058 (.223) to .052 (5.56) IIRC. I don't know any of the other sizes. The 14.5 and 16 .223/5.56 should use the standard gas port.

I've had no issues with ammo selection on midlength or carbine. I've shot everything from tula or brown bear to custom 75gr match handloads with no problems.

Personally I try to avoid steel casing's like the plague.
 

·
Registered
Joined
·
80 Posts
I'm thinking about chopping my factory barrel to 12.5", and it's going to be ran suppressed 75% of the time using 5.56 pressure ammo. So should I leave the gas port as is or should I open it up a little?
 

·
Premium Member
Joined
·
4,707 Posts
Discussion Starter · #5 ·
That would be pushing the limits on the factory port. I'm not sure if Adco opened the port at all when they did the 12.5" cuts. IIRC it was opened slightly.

I'd try it and if it short strokes with lower pressure ammo, I'd open it some. If you are running 5.56 it might have enough pressure to work ok.
 

·
Registered
Joined
·
132 Posts
I find that gas porting is the biggest single issue with reliability in semi-auto rifles. You get it wrong and you just may have made your sweet running Cadillac into a big, heavy stick. I know I have in the past.
 

·
Registered
Joined
·
444 Posts
Thanks a ton for posting this info Bigdog. Quick question...when you said this:

The Midlength system is the standard system for the Acr at this point. On a DI rifle it has slightly less hop than a shorter length system, but I don't notice it near as much on the Acr. This system works best with the basic handguard (ascetically) and can go down to around 12.5 inches suppressed. You can go slightly shorter if you don't suppress it since the Suppressor will hit the regulator.

How much shorter can you go? I'd like to cut mine just aft of the M4 profile bulge, which is right at 12". Not planning to suppress (yet), and will be 5.56 99% of the time. Outside of opening the port (if needed), will it work?
 

·
Registered
Joined
·
278 Posts
That would be pushing the limits on the factory port. I'm not sure if Adco opened the port at all when they did the 12.5" cuts. IIRC it was opened slightly.

I'd try it and if it short strokes with lower pressure ammo, I'd open it some. If you are running 5.56 it might have enough pressure to work ok.
I had mine cut down to 12.5" (measured bolt closed, and using a rod to measure it to the muzzle with no brake/flash hider; it seems some ACRs are slightly longer) and had Adco open the port to 0.060. It would not properly lock back about 50% of the time using AE223. So I opened it up to .0625" (1/16" drill bit) and it works 100% with Tula, XM193 and some mild reloads with H4895, 55 grain Vmax.

Here is a short video of the action, you can see it locks back, ejection is about 2 o'clock, and the bolt does bottom out, but softly. I can certainly hold the gun better here...it was cold and I was tired *cough*hungover*cough*


-Sterling
 

·
Registered
Joined
·
278 Posts
Thanks a ton for posting this info Bigdog. Quick question...when you said this:

The Midlength system is the standard system for the Acr at this point. On a DI rifle it has slightly less hop than a shorter length system, but I don't notice it near as much on the Acr. This system works best with the basic handguard (ascetically) and can go down to around 12.5 inches suppressed. You can go slightly shorter if you don't suppress it since the Suppressor will hit the regulator.

How much shorter can you go? I'd like to cut mine just aft of the M4 profile bulge, which is right at 12". Not planning to suppress (yet), and will be 5.56 99% of the time. Outside of opening the port (if needed), will it work?
I had mine cut down to 12.5" from Adco. I verified it was 12.5 since it seemed shorter, but with the bolt closed, measured to the muzzle (no device) with a rod, it was exactly 12.5". As you can see in the pic, the shoulder of the threaded muzzle goes right up to the M4 Cut. Go any shorter and you'll loose that larger shoulder. Gas Regulator button will hit the AAC brake, so you have to depress it slightly to remove the gas regulator. If you go with a larger muzzle brake, you may not be able to easily remove the gas regulator.

1455981_366298266861789_3204127277539440048_n.jpg
 

·
Premium Member
Joined
·
2,889 Posts
It's good to have a certain amount of shoulder on the barrel to contact, if it has too little, the flash hider has a chance of digging into it and not indexing correctly, or slipping over it, which would happen on something like the scar 17.
 

·
Registered
Joined
·
278 Posts
What's the harm in losing the larger shoulder? The OD of the flash hider threads is smaller than the OR of the barrel aft of the M4 cut. Am I missing something?
I guess you could always have it threaded longer if there isn't enough shoulder, and use a nut kinda like how the Tavor works. For me I wanted the larger shoulder as I will be mounting a silencer on it (currently in NFA jail...) and didn't want to fiddle with it too much
 

·
Registered
Joined
·
1,478 Posts
I guess you could always have it threaded longer if there isn't enough shoulder, and use a nut kinda like how the Tavor works. For me I wanted the larger shoulder as I will be mounting a silencer on it (currently in NFA jail...) and didn't want to fiddle with it too much
When I had my IWI Tavor cut & pinned to 14.5in, the gunsmith did just that.

Since it was a thin profile barrel, there is not much for a muzzle device to back up too. To make sure it would stay rigid, he threaded all the way down to where the threads bottomed out on the 51T Brakeout.

He was worried over time it could wear, then be slightly off, & then a baffle strike.
 

·
Premium Member
Joined
·
4,707 Posts
Discussion Starter · #15 ·
You need to mill the gas block and charging handle carrier so the block will fit inside of the carrier. You also have to cut the front of the charging handle carrier back some.
 

·
Registered
Joined
·
43 Posts
Just bought the ACR pistol which will be delivered tomorrow. I want a 300 Blackout conversion and have a a couple spare full size conversions I could use for parts. What is the shortest 300 Blk I could safely do?
 

·
Premium Member
Joined
·
4,707 Posts
Discussion Starter · #18 ·
Just bought the ACR pistol which will be delivered tomorrow. I want a 300 Blackout conversion and have a a couple spare full size conversions I could use for parts. What is the shortest 300 Blk I could safely do?
You can’t do a 300 conversion without an older generation charging handle assembly. The new generation charging handle trays have a vertical spring that will come out If you machine the charging handle trays to fit a gas block under them.

I am working on a new gas block for 300’s specifically that I’m hoping to have very soon. Read before shot show if all goes well.
 

·
Registered
Joined
·
3 Posts
How much do you have to cut down the op rod to be at carbine length; what dodes the overall lenght need to be? Im going with an 11.5 barrel.
 
1 - 19 of 19 Posts
Top