For owning just a handle of AR15s, buying 2 complete BCGs as backup is likely overkill. But if the new BCGs will specifically replace defective parts, that's fine. Adding AR15 parts here and there is common way for folks to say "Hey, I have a bunch of components on hand. I bought 2 more items and lookie here -- a new AR15!" Then the process of accumulating "spare" parts starts all over again.

IIRC, I have a spare complete BCG tucked away somewhere. I also have a spare JP bolt, which I took from the prize table at a rifle match. It's debatable if I will ever use either one.

Combining personal experience of AR15 use with that of instructors who've seen rifles with tens of thousands of rounds down the pipe, my take on useful parts life.
- The bolt carrier is pretty much bullet proof, as long as the carrier key is properly installed. It should be replaced when a bolt with new gas rings is pretty loose. One instructor said he's seen that once on an AR15 -- a rifle with an extremely high round count. Well over 20k rounds, maybe even more than 30k rounds. So maybe the bolt carrier replacement is in the 20-30 thousand round ballpark. Very, very few gun owners will ever reach this.

- It appears that the bolt's biggest issue is lug wear or shearing lugs. I've seen a couple of AR15s with a sheared lug -- one wouldn't cycle, the other cycled OK. Some gunsmiths & trainers suggest replacing the bolt when a barrel is shot out. I've done this with both barrels that I retired. #1 was a Wilson Combat parkerized bolt -- it has noticeable wear on one lug, a touch of wear on another lug, and the extractor spring feels week. I doubt I'll ever use this bolt again, but it could still be in my basement somewhere. #2 was a Wilson Combat NiB bolt -- hardly any wear on any lugs, the extractor spring feels good. My 'smith said I could easily use this bolt again, but he was fine with my putting a new WC bolt into the new barrel. Especially since the replacement barrel is tight-chambered Bartlein. Based on my limited experience, I will never own a parkerized bolt again.

My AR15s are built for match-quality results, so I retire their stainless barrels sooner than someone with a chrome-lined barrel who can live with less stringent accuracy requirements. But it still makes sense to retire a bolt when a chrome-lined barrel goes south, as the round count might be much higher than with my stainless barrels.

- The cam pin should be inspected here and there. They definitely show wear on high-volume ARs. Maybe 10k rounds? Maybe more? At least it's a cheap part. I may have an extra somewhere.

- The firing pin will eventually fail. Could occur at tens of thousands of rounds, I don't know. I have a couple of spares from JP, also from rifle matches.

- Bolt gas rings might last a long time, or they might give up the ghost quickly. IMO the internet "test" of supporting the weight of the complete BCG means little. My most accurate AR15 fails this "test", but it cycles perfectly. Many ARs function just fine with only 2 of the 3 rings installed. I switch out rings here and there, mainly to go to a 1-piece ring set. I have yet to replace a 1-piece ring set. Rings are cheap, so change them as you see fit.

- The firing pin retaining pin is likely the first part that an AR15 owner changes out. Maybe the only part changed out due to function. Not because it fails, but because it bends over time and becomes difficult to re-insert. The pins are really cheap, so change them as you see fit.

Thank you all, and particularly fritz, for the comprehensive replies.

I will likely never shoot out the BCG in my primary rifle, which is a Sig M400 Enhanced. I buy some of this stuff just to solve the political problem of having spares if any Democrat gets the sharp idea of banning or licensing this stuff.

FYI, I just bought a 10.5 inch upper to build into a pistol. I don't particularly want an AR pistol. I've shot a 10.5 full auto M16 lower. The blast is impressive and the noise is epic. Muzzle rise is a thing, and the burst setting rather than the firehose is your friend.

But I'm afraid the bastards are going to try to ban stuff like this just like they tried with the "pistol brace", and I don't want to be caught short.

quote:

Originally posted by fritz:

- Bolt gas rings might last a long time, or they might give up the ghost quickly. IMO the internet "test" of supporting the weight of the complete BCG means little. My most accurate AR15 fails this "test", but it cycles perfectly. Many ARs function just fine with only 2 of the 3 rings installed. I switch out rings here and there, mainly to go to a 1-piece ring set. I have yet to replace a 1-piece ring set. Rings are cheap, so change them as you see fit.

Gas rings are a weird thing. I’ve seen them last 10k. Hell, I’ve had guns I never maintained and never had an issue.

My current rifle fails the test about every 3k. Gas rings are about $2.50. I replace them and move on.

quote:

What is a reasonable number of rounds lifetime should I expect out each original BCG?

Complex question. Really depends on the type of life your rifle has. BUT it is a wear item and thus most think its reasonable to have spares in the context of the world we live in. My personal experience is life is between 5K and 10K rounds, but those have not all been premium parts. For sure you will need at a minimum rebuild parts if you shoot a reasonable amount. Given how cheap they are as a complete unit its easier to have your spares as ready to go than piece parts IMO. That discussion is for 5.56 guns. The 9mm universe is completely different as there are no locking lugs, gas rings and cam pin (assuming normal Colt design) but you will break a whole bunch of different parts (like bolt catches) on a regular basis.