I am new to ARs. I have a Seekins SP10 in 6.5 CM. The rifle seems very well made. I got the heavy barrel (0.85” OD at muzzle). My experience is with bolt actions. I’m looking at the cam pin on the BCG and watching the Utube showing the animation of the action. The question I have is related to what makes the bolt lock up tight? Or does it not? On my rem 700, it looks like one of the bolt lugs have the rear edge (rotating direction) machined with an inclined plane, making it cam forward on the action lugs, to get on the far edge surface of the action lugs, when you cycle the bolt clockwise, it cams forward. Does the AR10 do that ? There are a couple unique looking barrel extension lugs on the barrel at the 6 o’clock position. Do they make the bolt cam forward? I’m basically asking if there is not a camming action, it seems like you are relying on very precise machining and the plunger spring forward force to ensure the bolt rotates. There are no 90 degree edges colliding preventing the rotation of the bolt. Can anyone explain how that works? It works great. I’m just trying to understand how. Thanks
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Perhaps this video will help. The AR10 functions the same was as an AR15.
Pressure from the buffer spring and the mass of the buffer is what holds the bolt carrier group forward and locked in place.
Pressure from the buffer spring and the mass of the buffer is what holds the bolt carrier group forward and locked in place.
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That is a great video. I see how the bolt rotates, once fully forward , with the cam pin. I’m just wondering how the back edge of lugs from the bolt don’t get caught on the front edge of the lugs on the barrel extension? im thinking the edges just aren’t sharp, 90 degree cuts. Maybe more pronounced on the front edge of the barrel extension lugs? Only one of the surfaces would need to have a slight miter and as action operates, those surfaces would wear in. You would have to very precisely cut the length of the lugs , so that with the buffer spring pushing forward those lugs just mate perfectly. It would require machining to within thousandths. I just expected the miter cuts to be more pronounced? I guess with CNC machining this is possible. Seekins claims their CNC holds to 20 microns, which is 0.00078”. So 8/10,000. That’s 80% of 1/1000. Since reloaders measure to 0.001”, that’s easy to believe. Whatever they do it works. If a human was doing this on a milling machine, the cuts would likely be more pronounced. I’m assuming as you get a few hundred rounds thru the rifle, these surfaces wear in nicely. But to function correctly as a new rifle, that’s impressive. With a manual bolt action, you can feel (in your hand) and see the cam forward action of the bolt as the lugs engage. This must be happening with an AR, using the buffer spring and cam pin on the BCG, combined with the precise lug machining. Very impressive design.
The flats will hold against each other (extension and backside of the bolt) with the cam pin rotated left. The buffer and spring only gets it to lock, but doesn't hold it there (in battery). When the gas (DI version) travels down the tube towards the gas key it forces the carrier rearward and in turn places the cam pin into the chargehandle channel letting it travel into the buffer tube (part of the carrier). The buffer and spring only delays the unlocking/decocking of the bolt.
If you watch anything on YouTube at all, there is (in my opinion) a really decent channel to subscribe to or follow called School of the American rifle. And you will find it is a great resource for things just like this. Lots of great info there. Also, this isn't meant to be one of those posts ridiculing the op for not doing any research. Just wanted to share in the wealth of knowledge that people might not know about.
I just pulled up that channel. That guy seems like a real expert. I watched him do some feed ramp polishing on a rifle that wouldn’t feed due to a burr on the right side of the feed ramp. It fixed the issue. He also doesn’t use a bore snake more than 100 pulls because he says when they break they are very difficult to remove, even by a gunsmith. He also doesn’t use wire brushes for cleaning, only nylon. And the scraping tools for cleaning the inside parts of BCG can cause gas leaks that lead to operational issues. The interior clearance dimensions of the BCG need to stay tight in order to not cause gas leakby. Thanks for the channel reference. Appreciate it very much.
Glad I could help. There's so much crap out there that it's really nice when you stumble across a channel like that. Somehow I came across a deal last night that was filmed in 1990 and it was mhikal Kalashnikov and Eugene stoner meeting and sitting down together and talking about their design and function of each of their historic rifles with each other. Maybe I'm the only one who hasn't seen it before, but it was really cool. Sorry if this was a derail.
