AR15 Forearm Selection: "The Math"

[Chuck R.]

Guys,

Still working on my AR15 300 Blackout pistol project and am in the barrel VS FF forearm selection stage. The numbers I've found to use when comparing a manufactures barrel length to get to the "protruding length" from the receiver front are (and they're approximate):

.5" of barrel inside of the receiver (not barrel extension, just barrel)
.625" (or .600") of the barrel is threaded for a muzzle device.

So, a 9" barrel should protrude 9.8" from the receiver face when coupled with a 1.95" muzzle device (that covers threads).

Does this sound about right???

I'm looking at pairing the above barrel with an 8.5" FF handguard which should leave me with 1.3" of muzzle device protruding from the handguard and a sight radius of about 15.5".

Understand that a suppressor cannot be mounted with the above combination.

 

[MistWolf]

Barrel length is measured from bolt face to the muzzle (or the end of a permanently affixed muzzle device). There is a bit of distance between the back of the chamber end of the barrel and the bolt face. Therefore, a part of the length of the barrel extension is included in the barrel length.

 

[Chuck R.]

Barrel length is measured from bolt face to the muzzle (or the end of a permanently affixed muzzle device). There is a bit of distance between the back of the chamber end of the barrel and the bolt face. Therefore, a part of the length of the barrel extension is included in the barrel length.

Agreed, But do you know "how much". I'm thinking I read approx .5" of barrel is in the receiver/barrel extension and will not count towards OAL. Not "legal" measurement from bolt face, but from edge of receiver to muzzle crown.

 

[Varminterror]

A pistol does not have to be less than 26” overall length, nor does it have to have a barrel under 16”. All which inclusively defines a rifle does not define the contrary specifications for a handgun, and do not independently define a rifle if all stipulations are not met.

A 10.5” barreled SBR with a UBR Gen 2 will have minimum fixed length over 26”.

 

[Chuck R.]

Got that....I'm trying to sort out how long of a FF forearm to order to maximize rail space, but not exceed the muzzle.

 

[Varminterror]

Not every handguard sets against the receiver, and not all manufacturers produce handguards to EXACTLY their specified nominal length - so there are no guarantees. Not all barrel manufacturers cut their barrels to their exact nominal length either.

The barrel extension should be a half inch forward of the end of the barrel, and the threaded receiver mortise is 0.44”, such the barrel is within the boundary of the receiver wall. So you’ll see within 1/10th of an inch of of the barrel length compared to the distance from the receiver flat face to the muzzle - a short tenth on the short side.

So if you have a 9” barrel and a 9” handguard which seats against the face of the receiver, your muzzle would be about 50thou shorter than the handguard.

 

[MistWolf]

Agreed, But do you know "how much". I'm thinking I read approx .5" of barrel is in the receiver/barrel extension and will not count towards OAL. Not "legal" measurement from bolt face, but from edge of receiver to muzzle crown.

My point is that the barrel length includes a portion of the barrel extension but your estimates do not include that length.

 

[Chuck R.]

Guys,

THANKS! Probably going to "take a knee" and order the barrel & muzzle device 1st, then take some measurements of what I really have. I'm getting another SLR handguard and they seem to be spot on with their measurements, but to be on the safe side I'll exercise some patience.

 

[Skylerbone]

You’ll also need to keep in mind that the threaded portion will be used (internal to) the device you choose, so that if a barrel were in fact 7” and the muzzle device 2”, you will not have 9” OAL but 9” - the threaded portion, + any crush washer/jam nut thickness.

If the device is a linear comp or flash can and its diameter is small enough to fit inside your chosen rail then you could potentially cover the entire device without trouble. Therefore 7.5” barrel + 3” LC - 1/2” threads = ~ 10” or a 9.5” rail with 1/2” sticking out.

 

[greyling22]

so if I have a 10.5" ar barrel and I use a 10" handguard that is flush the with receiver I should have enough space to screw on a suppressor that won't fit inside the handguard? or will is be slightly too long? because there are lots of 10" handguards, 9" is harder to find. esp. on a budget....

 

[Varminterror]

@greyling22 - I would not bet the farm on it.

Recall - a 223/5.56 AR barrel will have 0.625" long threaded muzzle tenon, so you'd have the barrel shoulder at 9.875, relative to the muzzle at 10.5", but you'll have about 50 thou of that barrel sunk into the receiver at the other end, leaving you with about 9.835" from the receiver face to the barrel shoulder... Not 10".

You might be able to sneak it together with a QD mount which stands off of the shoulder a bit, like the SiCo ASR brake for the Omega, as an example, to buy some length back - that SHOULD work with a 10.5" barrel and 10" handguard, assuming the rear "nut" of the QD brake mount moves the rear of the can at least .2" forward of the barrel shoulder.

 

[greyling22]

well poop. I'll be direct threading it.
And so begins the hunt for a 9" handguard that isn't $25 ebay garbage (black paint, sharp corners) and isn't too nice either. I had been hoping to get one of these https://www.ebay.com/itm/223-10-Inc...rentrq:33217fc91660ac3d5432faeffff3533b|iid:1

they are actually anodized a nice matte black that natches an ar receiver, the sharp corners are rounded off..., it's not a bad rail. but it only comes in 7" and 10" (and well, 12 and 15)

 

[Skylerbone]

There are a number of 9” rails available.


SLR makes a 9” and a 9.7” but pricey. $120-$190
YHM a 9.29” $120ish
Odin Works 9.5” $150ish
Aero 9” $120
Ghost 9” $85
Troy 9” $120
Foxtrot Mike 8.5” $62
Seekins 9” $150
Troy 9.25” $120ish
Strike 9” $100
Lots of others in the $180s and up, but all of the above are free floating with some slims here and there.

 

[Varminterror]

Midwest makes a 9.25” which is $150 normal, but has been on sale off and on a couple years now for less at Midway.

 

[Chuck R.]

You’ll also need to keep in mind that the threaded portion will be used (internal to) the device you choose, so that if a barrel were in fact 7” and the muzzle device 2”, you will not have 9” OAL but 9” - the threaded portion, + any crush washer/jam nut thickness.

If the device is a linear comp or flash can and its diameter is small enough to fit inside your chosen rail then you could potentially cover the entire device without trouble. Therefore 7.5” barrel + 3” LC - 1/2” threads = ~ 10” or a 9.5” rail with 1/2” sticking out.

That's what I was trying to get at, the threaded portion for the "MD" is listed on prints as .625", but can vary some. Then there's approx .5" of barrel in the receiver. To make my 9" barrel work, with the MD partially covered, I've got to go with an 8.5" forearm. Which is why I'm looking at another SLR.

 

[Skylerbone]

Cheapest 9” I could find with a quick google: https://www.joeboboutfitters.com/Matrix-Arms-AR-15-Foxtrot-M-LOK-Handguard-p/ma-foxtrot-mlok.htm

$53.95 for a Matrix Foxtrot MLOK