HELP! I've run into an issue with a carbine-length AR build that I want to run by you. My 16" 300AAC was not cycling very well with ammo (only supersonic so far). So I've yanked it apart to measure the barrel gas port size (0.109"). That looked perfectly acceptable for supersonic rounds. Then I lined up the dimple on the barrel with the rear hole in the gas block. I found a potential problem there. I had a hand guard end cap between the gas block and ledge where the barrel increases in size. I didn't realize it when I built the rifle, but the gas block's dimple is off center when the end cap is installed. It lines up perfectly without it. I have a pic-rail gas block and I don't want an A2 front sight. Do they make gas blocks specifically for use with and without end caps? I really don't want to spend the money to switch to a free floating system if I don't have to. I don't think one would work with this gas block anyway since the pic rails are the same height as that of the upper receiver. Thanks in advance!
If I understand you correctly, how about milling/sanding/etc some of the material off the endcap side of the gas block? May have to relieve the gas tube also.
Or get a different gas block...
RR
Interesting, I never considered that that thickness is factored into the design. Actually it is probably OK if the gas port or the dimple doesn't exactly align with the holes in the gas block. The port on the gas block is usually oversized. The dimple is mainly to clock the gas block into position, and the set screw will still do that even if not aligned fore-aft. Shaving some thickness of the gas block is also a valid option. I don't see how it would affect the gas tube.
You may want to consider an adjustable gas block for 300AAC so you can switch between supersonic/subsonic or suppressed/unsuppressed.
I just received this PM from jturbo:
"The gas ports are in 2 general locations on the barrel. the G.I. location is .295" on center from the gas block grind shoulder on the barrel. The alternate location is .265" which allows for elimination of the front handguard plate and is specifically done for the DCM Service Rifle Free Float tube.
I have not been able to get a straight answer from any of the gas block manufacturers regarding the gas port location specification in the gas blocks. Some of the gas port diameters in the gas blocks are up to .130" in diameter which allows for use on either barrel gas port location.
The way I see it is you have a few choices.
1. If you like the gas port you are using, grind about .030" off of the back of the gas block and lock tite it in place.
2. Try another gas block with a larger gas port hole. (you may have to really dig to get a diameter from the mfr.)
3. Open up the gas port on your gas block. This would have to be done with a tapered reamer so your transition would be smooth."
This is excellent info on something I've never seen on a forum anywhere! Thanks, jturbo!