Am I calculating this correctly?

[Black_Sheep]

I’m in the process of building a mid-length 14.5 rifle, everything has been ordered but the barrel (Criterion CORE) and BCG are 2-3 months out. I would like to use the same WC Accu-Tac FH as I used on a recent 16” build, assuming it makes the required 16” minimum on a 14.5 barrel. Since I don’t have the barrel and BCG to mock up I measured the 16” barrel with the FH using a cleaning rod inserted in the barrel. From the bolt face to the tip of the FH is 17 15/16” (17.9375”), if I subtract the 1.5” difference in barrel length it comes out to 16.4375” which exceeds the 16” legal minimum when pinned and welded.

Is this the right way to make my calculation? I would like to order the FH so it’s already in hand when the barrel and BCG show up.

Thanks in advance

[AndyLate]

There is no guarantee that your 16" barrel is 16 inches. It won't be shorter but is almost certainly longer.

Your method is correct for measuring the barrel length with muzzle device included.

How long is your flash hider? An A2X FH is 2 inches long and designed expressly to create a 16" barrel from a 14.5.

Andy

[Black_Sheep]

There is no guarantee that your 16" barrel is 16 inches. It won't be shorter but is almost certainly longer.

Your method is correct for measuring the barrel length with muzzle device included.

How long is your flash hider? An A2X FH is 2 inches long and designed expressly to create a 16" barrel from a 14.5.

Andy

The manufacturer states the FH is 1.990 not including the crush washer. I could not find the OAL dimension of the A2X on the BCM site for comparison.

[lysander]

Generally, the distance from the bolt face to the back of the barrel is 5/32 inch.

Generally speaking, the muzzle threads are 5/8 inch long.

For calculation I would assume 1/10 inch for the crush washer.

Generally speaking 2 to 2.25 inches of flash suppressor are required to comfortably get a 14.5 beyond 16 inches per ATF rules. (That includes the washer.)

EDIT:

By my calculation, a 1.990" long flash hider permanently installed would make a 16.106" barrel. That's pretty close, so I would assemble and check with all the proper parts and not rely on calculations.

[Black_Sheep]

I did try calculating based on the 5/8 muzzle threads, FH OAL and crush washer as well, the numbers don’t jive so it’s safe to assume that my 16” barrel is actually a bit longer than 16”. I appreciate the responses, it looks like I have to consider other options.

[Renegade04]

When using a 14.5" barrel and pinning and welding a muzzle device to make it non-NFA, you have to subtract 5/8" because of the threaded area of the barrel. If using a crush washer, you pick up about 1/8", so you only end up losing about 1/2". This means that you will need a FH with a minimum length of 2" to meet the 16" non-NFA length.

[GH41]

If the new barrel is really 14.5, the MD is really 1.990 long and the thread length is .600 (Typical) the math is easy... 1.990-.600+14.5=15.890. The crush washer compressed would have to be .11 or thicker to make it work. I would order the barrel and MD and if I came up short use a combination of accu-washers to make it work. I don't like crush washers anyway.

[AndyLate]

I think you should wait until your barrel comes in, mock it up, and then measure it. It seems odd they would build a flash hider that is almost, but not quite, long enough to turn a 14.5" barrel a 16" p/w.

https://www.m4carbine.net/showthread...overall-length

Andy

[lysander]

When using a 14.5" barrel and pinning and welding a muzzle device to make it non-NFA, you have to subtract 5/8" because of the threaded area of the barrel. If using a crush washer, you pick up about 1/8", so you only end up losing about 1/2". This means that you will need a FH with a minimum length of 2" to meet the 16" non-NFA length.

If the new barrel is really 14.5, the MD is really 1.990 long and the thread length is .600 (Typical) the math is easy... 1.990-.600+14.5=15.890. The crush washer compressed would have to be .11 or thicker to make it work. I would order the barrel and MD and if I came up short use a combination of accu-washers to make it work. I don't like crush washers anyway.

You are forgetting the ATF measures to the bolt face.

The 14.5 in an M4 14.5 inch barrel is the actual length of the naked barrel. You have to account for the additional distance from the back of the barrel to the face of the bolt.

[ViniVidivici]

I’m in the process of building a mid-length 14.5 rifle, everything has been ordered but the barrel (Criterion CORE) and BCG are 2-3 months out. I would like to use the same WC Accu-Tac FH as I used on a recent 16” build, assuming it makes the required 16” minimum on a 14.5 barrel. Since I don’t have the barrel and BCG to mock up I measured the 16” barrel with the FH using a cleaning rod inserted in the barrel. From the bolt face to the tip of the FH is 17 15/16” (17.9375”), if I subtract the 1.5” difference in barrel length it comes out to 16.4375” which exceeds the 16” legal minimum when pinned and welded.

Is this the right way to make my calculation? I would like to order the FH so it’s already in hand when the barrel and BCG show up.

Thanks in advance

I find your post coupled with your sigline EXTREMELY ironic.......