I don’t bother timing the gas port.True, it was too rude and I was caught up in the moment. I should have said in order to time the groove you would need to do the following:
1. Precisely measure the twist to the supplied barrel since some manufacturers twist do slightly vary ever so slightly from advertised.
2. Very carefully calculate where on that barrel blank that you want that hole to end up. Now that you have that determined…
3. You will need to carefully measure from the hole to the tenon shoulder and turn your shoulder to match that distance.
4. All that was probably the easy part. Now the barrel would need to be threaded for the extension. Mind you, it needs to be threaded and timed so the extension, when torqued tight, is in alignment with the hole. Any honest machinist knows the variables and odds of that happening perfectly.
5. You’ll need it timed perfectly at top dead center to align with that tiny hole that’s currently inside the headstock of the lathe and not even visible. Any misalignment(1/2 a degree) and you’ve missed lining up with that tiny narrow groove, which could be different depending on barrel manufacturer; 4,5,6, grooves etc. Depending on the gas port hole size, it might be bigger than the groove itself. I’ve had a couple barrels from a major premium brand AR barrel manufacturer measure .128” with pin gages!!. When shinning a light down the barrel, you could clearly see the rifling on the other side when looking thru the hole!
5. You might be lucky, or it might be off a bit. You’ve got some options to try salvage your expensive blank: You can time the extension by removing a little shoulder (still need to tighten it up too,remember). But Arrgh… changing the shoulder changes your tenon-to-gas port length, so not a good option unless you only need several thousands.
Or you could take a fuzz off the front of extension(Arrgh…Hope you have another lathe), or maybe you keep a bunch on hand and try several hoping one is close enough. I’ve tried that before when trying to salvage a good barrel that had a bad extension. Since I usually have a bunch on hand and they do line up differently. It’s a pain.
But what if it lines up 7/8 of a turn off? And it will because you don’t want it to. You would need to turn the shoulder back 0.0546” to line it up. In this experiment .0005” error will mean you missed the alignment.(Arrgh…gotta calculate how far it will turn when torquing, remember). If you take that much off the shoulder, now that affects the gas tube depth into the bolt carrier, Arrgh. Since gas blocks a drilled at the same spec. And gas tubes are pinned at the same standard lengths, you typically don’t have much options there. There is some forgiveness in the port hole line up on gas blocks but .054” off is a bunch. So that’s not a good option either.
Maybe the hole wasn’t drilled yet and you carefully measured and marked out multiple options depending on how many grooves your barrel has. That would give you more chances to line up with. Hopefully those locations were indexed on the barrel where they can be seen and while barrel is in the lathe. Or maybe the barrel will be just removed and dialed back in a half dozen times while verifying line up, Arrghhh.
So with that option, now you will need to drill your hole based on that barrel extension’s torqued location. Any length variation or slightest error in rotation, and you’ll miss the groove. Remember, the average groove width, depending on groove count, is literally the same width as the hole you’re drilling, .070-.098 (We’re talking 16-20-24” barrels, not short carbines). The faster the twist, the less error you can have too. So any, ever ever ever so slightest error, and you’ve missed it and overlapped the hole! All that work, and it’s what? Scrap? Throw it in the “seconds” bin? No, off it goes to the customer without a concern, because you did everything else right, and that baby will shoot good regardless of the hole timing.
All that rant just to say, “manure”.
I’m not saying you can’t time the hole with the rifling. God bless those with the time and resources to do so. I’m saying it’s not economically realistic. What are you willing to pay for an AR barrel? There are a lot of variables working against you trying to time a gas port hole for no real benefit. The bullet has already been swagged to match the rifling before it gets to the hole, as long as the hole is burr free, that bullet is going to sail on by without damage.
Didn’t mean to be rude to the original poster of that comment, no judgement was meant to you. There’s just so much misinformation out there in the rifle world that some people are just lead to believe some of the craziest things without any real manufacturing knowledge or machining experience to know what’s truth or just some old guy telling you a story.
Sorry if I offended anyone. Believe what you want.
My opinion is it’s not practical to time gas ports and I haven’t seen a difference in the many I’ve done.
The gas port being your gospel that dictates the tenon is a hassle I’d like to avoid.
