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-   -   backup bolt carrier group? (http://www.firearmstalk.com/forums/f20/backup-bolt-carrier-group-88460/)

StainlessSteel215 04-09-2013 02:33 AM

backup bolt carrier group?
 
Was considering a nickel boron coated BCG from a DPMS Oracle A15....for a backup to have on hand for any future repairs. How many folks have an extra BCG on hand for long term storage in case of breakages?

Gatoragn 04-09-2013 02:44 AM

I have a complete extra BCG for backup.

StainlessSteel215 04-09-2013 02:47 AM

How will you know when the bolt or the group assembly parts begin to wear out? Visual inspection? What exactly seems to break on a bolt carrier group....the bolt itself?

Sgt_Skrb_25 04-09-2013 02:48 AM

I actually have a few. Not specifically for a back up. I use them for builds I do for friends. Although not a bad idea, I actually have taken a spare to go shooting. But if it's not a critical situation, maybe you get away with carrying and extra complete bolt, gas key and bolts, firing pin?
Don't think much can go wrong with the carrier. But it would be easier to swap out the assembly.

Sgt_Skrb_25 04-09-2013 02:49 AM

Quote:

Originally Posted by StainlessSteel215
How will you know when the bolt or the group assembly parts begin to wear out? Visual inspection? What exactly seems to break on a bolt carrier group....the bolt itself?

I would say the extractor on the bolt, and firing pin is all I have seen worn or damaged.

StainlessSteel215 04-09-2013 02:51 AM

Thanks guys.
And yeah SGT: Exactly what I was just thinking....a dulled firing pin or damaged extractor (specially if you fire steel cased ammo which I DONT) are likely to burn out first. Maybe I'll just keep those items on hand and save myself the $250 for emergencies

purehavoc 04-09-2013 03:05 AM

Quote:

Originally Posted by StainlessSteel215 (Post 1207862)
Thanks guys.
And yeah SGT: Exactly what I was just thinking....a dulled firing pin or damaged extractor (specially if you fire steel cased ammo which I DONT) are likely to burn out first. Maybe I'll just keep those items on hand and save myself the $250 for emergencies

Oh geez , not the steel cased ammo wears stuff out quicker again :rolleyes: buy your self a extra bolt and put the rest in cheap steel cased ammo

kbd512 04-09-2013 03:39 AM

StainlessSteel215,

With NiB coated bolt carrier groups it's not easy to see hairline cracks because the coating is imbedded into the steel, but it is possible to see excessive or uneven wear. Mind you, NiB is very hard and smooth so wear resulting from friction should be improved over mil-spec bolts.

Here's what you'll see with a phosphate coated (Parkerized) bolt carrier:

Hairline cracks around the locking lugs near the extractor, cracks around the cam pin hole, and unevenly worn locking lugs (assuming there's a problem with the machining or heat treating of the bolt, how the bolt interfaces with the barrel extension, or chipping/galling of the bolt which is another sign of a heat treatment problem. Eventually the locking lugs, typically the ones nearest to the extractor, will shear. Firing pins can develop cracks or shear, but having a spare firing pin fixes that problem.

Now, you'll only see stuff like this if you fire thousands and thousands of rounds through your AR so keep that in mind. A properly machined and heat treated Carpenter 158 bolt's design lifespan is between 7K-8K rounds or so if memory serves. Mind, you duty cycle has an impact on how long a component will last (not time, but round count). If you fire lots of full auto over the course of that 7K rounds, expect to see failures sooner rather than later. Most bolts will last 7K rounds or more in the hands of the military and because civilians typically won't fire any full auto or do lots of rapid fire the bolt may last considerably longer (or it may not).

Some misinformed or misleading manufacturers like JP Enterprises may imply that 8620 is mil-spec AR bolt steel and that it probably won't make it to 6K. They're either not knowledgeable about what mil-spec steel is (Carpenter 158 steel alloy) or they're intentionally misleading you to entice you to purchase their SAE 9310 bolts. The geometry of the bolt has been improved over mil-spec with better locking lugs, but I'm not sure SAE 9310 is qualitatively any better than Carpenter 158. Again, heat treatment is critical. Apparently that part is magic or trade secret because commercial manufacturers like JP don't give you much inkling about how closely controlled the heat treating process is. If 8620 bolts were significantly cheaper than Carpenter 158 bolts, I'd just buy cheaper bolts and replace them as necessary.

At a minimum, it's a good idea to have a complete spare bolt, cam pin, firing pin, and buffer spring. A better idea is to just have a spare bolt, bolt carrier, and buffer spring as a drop-in replacement. The NiB coating is supposed to increase the design life of the coated parts by reducing wear from friction, but I'm not sure how much better that is in relation to performing regular maintenance (lubrication and replacement of worn or damaged components). After you put 7K-10K through a carbine, a spare operating parts set seems pretty cheap by way of comparison. I wouldn't run an AR without lube no matter what the manufacturer of the NiB coated components says.

The type of lubrication is important but only with the understanding that light teflon infused oils like CLP burn off, due to the direct gas system blowing hot gas back into the operating parts, leaving your carbine without that critical lubrication it needs to continue firing. I prefer light coatings of high-temperature greases to light oils if I'm gonna put a lot of rounds through my AR. Synthetics are better than petroleum products, but any lube is better than no lube. If all you have is Mobile 1 synthetic, you have what you need. There's no special requirement for lube except that it not be burnt off, gum up, or run off the coated part.

A simpler solution to the heat problem is to carry a small bottle of lube with you. When you get a chance, lube your carbine. The AR doesn't need to be nearly as clean as most would have you believe, but you can take it to the bank that it absolutely requires lubrication.

A spare bolt carrier group and a small bottle of lube (for light firing like target practice I keep a small CLP bottle with me) are really good things to have on hand.

Hope that helps. I prefer having 2 or more bolts from the same manufacturer. I have a Colt carbine, so I have spare Colt bolt carrier groups as replacements.

kbd512 04-09-2013 04:08 AM

As an aside, I'd be willing to bet that if the geometry of the locking lug design, especially near the extractor, was improved like the LWRC and JP Enterprises bolts have and a strictly controlled double heat treatment process was applied like SEI does to M14 bolts, bolt failures would come at a much higher round count than with the current mil-spec design. Of course, all of that comes at an increased cost per bolt. Perhaps a NiB coating on the bolt and barrel extension could be applied to reduce friction wear and make cleaning easier, but I wouldn't bother with a NiB treated bolt carrier and hammer. The bolt carrier still requires lubrication, even if the requirement is greatly reduced, so what's the point?

The current design isn't flawed, but it could be improved to the nth degree using what we know now. I guess the real question becomes, what are civilians willing to pay for a qualitatively better bolt design? The military just runs stuff until it breaks and replaces it. The military would rather keep the manufacturers busy so that they can continue to purchase parts rather than have them produce a perfectly machined component that nearly lasts a lifetime but ultimately bankrupts the company that produced it and the military because of high initial cost and the fact that the company won't be receiving any more large orders for their product after the initial production runs.

Quentin 04-09-2013 06:20 AM

I don't have a spare BCG, they always morph into rifles. That's not all bad though, I do have two spare ARs! :D

I plan pick up a milspec BCG from PSA this year but since I have a spare complete lower ... I know where this is going again!


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