Deep Throat? (No, this isn't porn)

[Quicksilver]

How deep is the rifling supposed to be on a AR style weapon?

I have called Armalite repeatedly to get answers, but finally just got "flagged" and they will only say that the information is "Proprietary". Well shit.

On my rifle, the standard .308 is of course 2.800 OAL, however this places my bullet .163 from the bullet lands!!!!!!!! That's one HELL of a jump! Despite that, I still manage 1/2 moa accuracy
on a good day with handloads, but wondered just WHY in the hell its so far out? Is that just the way it is with the AR platform or what?

I think receiving a answer of "proprietary" is bullshit, but all I want to know is IF I payed the money to have another barrel made by someone else, what would be the consequence of moving the
lands to say, 2.830 or 2.850? Is that deep throat style an integral part of how the AR works??

I actually spent the time to WD-40 the throat and entrance and lands and make a "port" mold using the same stuff I use on cylinder heads to get a port mold, so I can see this magical chamber
that they won't give me information on (even though I HAVE all the information!!!!!!!!!!) I just want to know if this is "how it has to work" or if this is some safety precaution taken to prevent
dumbassery for crap ammo?

I have taken fired brass, loaded a bullet .200 longer than normal (so, 3.000 rather than 2.800) and painted the bullet with machinists blue and chambered the bullet, checked, made another bullet .002 shorter, painted, checked, light light marks ect until I came up with a definite .163 till the bullet engages rifling if using the standard 2.800. What gives?

 

[Quicksilver]

Also for those who are curious- you can make a port mold by sticking a ear plug about 1" in the barrel from the throat, spray a LIGHT coat of WD-40, carefully pour in the rubber port mold mix, wait... wait... then extract in about 24 hours. Great way to see whats going on in there. As for measuring anything accurately, obviously its a rubber style compound and likely has some expansion/shrinkage that will not allow accurate measurements, but you will be able to see how the chamber was machined and rifled in general. You can also use BONDO, but at your own caution.

 

[scudzuki]

The throat is a good bit shallower than the distance the bullet protrudes from the case... the ogive of the bullet is usually a good ways into the rifling of the barrel. The bullet only begins to touch the lands where the diameter of the bullet is equal to the diameter of the lands, in the case of a .308, that's .300 diameter.

COL is relevant only to what maximum length a box magazine will fit or the action will allow. Ammo that is precisely assembled is measured at the location where the bullet diameter equals the diameter across the lands and cartridges assembled that way will vary in COL due to the variation in ogive shape within a lot of bullets. I load for my Remington AAC-SD to 2.88 COL since it's the longest my magazines will reliably feed, but the bullet is nowhere near the lands (typical Remington factory chamber). Still, the lands are quite a bit closer than 2.800 from the bolt face, probably much closer to 2.5" from the bolt face.

Joe

 

[ksthomas]

Quicksilver,

The "proprietary" answer is one you'll get a lot of in this industry, sometime for good reason, sometimes, not so much. In most cases, it revolves around something you can (and should) measure for yourself anyway. In the case of throating, there's several variable involved, including those that scudzuki alluded to. The ogive of the bullet and how it "finds" the leade, that sort of thing, all go into determining what the correct OAL is for a given round. And as was mentioned, in an AR format, the primary factor that dictates this is the magazine length, assuming that you want to fire it semi-auto? Not being facetious here, as there are several combinations (the various 80, 82 and 90 grain .224" bullets) that were designed to be single loaded and never fired from a magazine. Without knowing what you're planning on using this rifle for, I don't know if that's an issue for you or not. If not, then you might want to use something like the Hornady/Stoney Point gage to determine how far out (OAL) you need to seat a particular bullet for it to be some specific distance off the lands. From there, the groups should tell you what the rifle has a preference for.

 

[Quicksilver]

Here's a picture of the mold. The first 2 black lines is the shoulder, the 3rd is the end of the cartridge length (which is surprisingly deep, you could be running a case trimmed to 2.035 and still have a smidge of space at the end) and the red is where the rifling begins.

 

[Quicksilver]

I have been using the rifle for short range middle of nowhere competitions and occasionally go out with my buddy to the 1,000 yard gun range in Austin. I wouldn't take a shot at anything other than paper with the rifle over 500 yards, and usually its just for coyote/varmint/hogs under 500 yards. That's pretty much it. I would like to be able to shoot mini clay's at 300 yards with the rifle, but it may not be capable. My magazine max length I can load is 2.860 before I start having feed issues, and to get the bullet "out there" to the lands, I have to feed in one at a time from the side and the length of that bullet (.005 from the lands) is 2.958. Seems really long as compared to my wife's bolt action .308 and its a savage arms... not the best, but still not bad. Her lands are at 2.850, which is a considerable amount shorter than mine. That's why I asked if the longer engagement is just a semi auto rifle thing or ...?