Mr. N

Exactly. You know that and I know that, but in my experience many manufacturers and sellers either don't know that or don't seem to care. For example, as per the relevant information (or lack of relevant information) on their web sites, both Ballistic Advantage and Faxon apparently intend that customers use the same "rifle length" gas tube regardless of what kind of rifle they're building. Yep, and this is precisely why it's so important that manufacturers and sellers provide useful specifications on their ".308" parts (compared to the case of an AR-15), which many or most of them studiously refuse to do. Getting information out of some of them is like pulling teeth; many have refused to answer any of my reasonable questions about their parts. Furthermore, in my unfortunate experience, some manufacturers and sellers even seem to use the well known lack-of-standardization as a fig leaf to sell defective, not-as-advertised or out-of-spec. parts (i.e. their own internal specifications). With the exception of lower receiver internal parts, I've literally had to send every other part of this .308 build back to the seller or manufacturer at least once, including the Aero M5 lower receiver, which had a ridiculously narrow mag well. If I had known in the beginning what I know now I would have never started this build in the first place.

Thanks for the info; I didn't know anyone was selling custom gas tubes. Unfortunately though, the primary obstacle which prevents me from using a BA barrel is the fact that the distance between the front face of the barrel extension flange and the shoulder of the gas journal is 11.9," which is about 0.3" too short to use an A2 or Magpul MOE handguard. (BTW speaking about BA .308 barrels, one of the things I don't quite understand is how/why they sell the same "rifle length" (in this case 15.125" long) gas tube for both "AR15 & AR10 rifles" and the same gas blocks. I wonder about this since in an "AR10" the bore of the gas tube entry port on the upper receiver is 0.062" farther away from the bore of the barrel than is the case for an AR15. In light of this, it seems to me that using a standard AR15 gas tube and gas block on an "AR10" would result in the gas tube making contact with the front of the AR10 barrel nut, thus being canted upward slightly as it entered the upper receiver, potentially causing misalignment with bolt carrier gas key).

Actually I didn't notice the relief cut in the Faxon barrel at first. It's somewhat hard to see unless you're looking for it because the width of the cut is only a few thousandths larger than the thickness of the handgurd cap. It wasn't until I assembled the upper and felt the handguard move that I realized there was an issue. I don't know why so many of these barrel manufacturers can't or won't turn the gas journal of a .308 barrel to a square shoulder. I bought a Faxon 20" "SOCOM" 5.56 mm barrel a few years ago for an AR-15 build and I incorrectly assumed their .308 barrels would be similar. What irritates me is that Faxon and many other manufacturers don't mention these cuts in the descriptions of their barrels nor do they usually provide any high resolution pictures or pictures showing a close-up view of the gas journal area. Considering that the "AR 308" platform isn't standardized, you would think that most manufacturers would provide some basic specs. on their barrels, e.g. gas system length, gas journal diameter and length and gas port diameter, but no, you have to try to call and/or email them and hope they provide the information. I've built four AR-15s including machining and anodizing the lower receivers and I've had nothing but a few minor problems, but this "AR 308" build has been a nightmare from the beginning.

The thing is, because of the relief cut, the handguard cap doesn't just sit there like it's supposed to; instead it moves around (radially) relative to the barrel and that's the problem. Unlike AR-15 barrels, most .308 barrels I've seen seem to have have some kind a relief cut in front of the gas journal shoulder. (Attached is a picture of a barrel for sale on Gunbroker. It's not a Faxon barrel but you can clearly see the cut right in front of the shoulder). A notable exception are barrels made by Ballistic Advantage, which don't have the cut, but Ballistic Advantage barrels can't be used with A2 of Magpul handguards because the gas system length is about 0.3" shorter than that of Faxon barrels. (Apparently they are intended to be used with standard 12.125" AR-15 gas tubes).

Hello all, I'm building a ".308" and I'd like to use A2 or Magpul MOE handguards rather than an aluminum free-float handguard. I bought a Faxon 20" "Big Gunner" barrel because I was told that the geometry was right, i.e. the distance between the front face of the barrel extension flange and the shoulder of the gas journal was approximately 12.2" (which it was) but unfortunately I can't use the barrel because for some reason there's a relief cut in front of the gas journal shoulder right where the handguard cap would be. I'm wondering: Does anyone know of a 20" barrel with the correct gas system length which does not have this relief cut? Regards, Mr. N