Swapping bolt carrier groups

[Cheflife15 455]

I was reading somewhere that it's normal to hand a spare bolt carrier group in your kit incase it fails in your rifle.  I was looking at picking up a wilson combat BOLT CARRIER ASSEMBLY, AR-15/M16, 5.56 NATO, AUTO, LOW MASS, POLISHED NICKEL BORON. 

After googling my understanding is these are all interchangeable between rifles.  I would be putting this in a 5.56/.223 daniel defense ddm4v7.  

Also,  what is headspace? I'm reading changing out bolt carrier groups could weaken the bolt and cause the gun to fail completely.  

Tldr: I'm lost in the sauce with all the conflicting information on the internet.  

[Displaced Texan 11,731]

K.I.S.S answer:

Just so we are clear on terminology…

First pic is your BCG assembly. 
Second pic is your BOLT. 

Think of it this way….Your BOLT (not the BCG assembly) will wear with the barrel extension that is pinned inside your barrel assy as the weapon is fired. 
 

When you change the BOLT to a new one, because of parts breakage,  you now have a new bolt going into an old barrel extension. They MAY have a tolerance issue. 
Headspace gauges are used to see if the bolt/barrel extension is within tolerance. Normally, a GO/NO GO gauge is used. 
 

In MOST CASES…headspace on an AR isn’t a problem, because of the inherent design, but it’s not a bad idea to check it, if you have to swap a bolt. That being said, I personally wouldn’t get my panties in a twist over checking headspace, unless I was replacing a bolt on a very high round count rifle. That’s just me. 
 

You probably just need a mil-spec bolt, complete with gas rings, extractor, and such for spare parts. 
Most people don’t replace the entire BCG assembly (in most cases). 
Whatever you do, buy a mil spec bolt that have been MPI proofed and tested. No fancy coatings are really needed. Your carrier will probably not fail…(unless the gas key falls off, but that’s another subject). 
 

Clear as mud? 

[High Exposure 5,661]

53 minutes ago, Cheflife15 said:

I was reading somewhere that it's normal to hand a spare bolt carrier group in your kit incase it fails in your rifle.  I was looking at picking up a wilson combat BOLT CARRIER ASSEMBLY, AR-15/M16, 5.56 NATO, AUTO, LOW MASS, POLISHED NICKEL BORON. 

After googling my understanding is these are all interchangeable between rifles.  I would be putting this in a 5.56/.223 daniel defense ddm4v7.  

Also,  what is headspace? I'm reading changing out bolt carrier groups could weaken the bolt and cause the gun to fail completely.  

Tldr: I'm lost in the sauce with all the conflicting information on the internet.  

I keep a spare bolt and some other small parts (gas rings, extractor springs, cam pin, firing pin, trigger and hammer pins, etc…) in an Altoids tin in my rifle bag. I have a second tin with a complete LPK in it in my teaching bag. A little insurance for a frustrating day.

Don't go crazy with a premium, fancy coated Wilson BCG just to keep as a spare. Just get a standard Mil-Spec set up from a reputable shop.

No need for boutique kit in this role (whether non-spec coatings on AR parts are a good idea is a whole other conversation).

Spend the $$ you save in a GO/NO GO gauge and an armorers class.

[Cheflife15 455]

11 minutes ago, High Exposure said:

 I have a second tin with a complete LPK in it in my teaching bag. A little insurance for a frustrating day.

What's an lpk ? 

[Displaced Texan 11,731]

Just now, Cheflife15 said:

What's an lpk ? 

Lower parts kit. 

[Cheflife15 455]

33 minutes ago, Displaced Texan said:

K.I.S.S answer:

Just so we are clear on terminology…

First pic is your BCG assembly. 
Second pic is your BOLT. 

Think of it this way….Your BOLT (not the BCG assembly) will wear with the barrel extension that is pinned inside your barrel assy as the weapon is fired. 
 

When you change the BOLT to a new one, because of parts breakage,  you now have a new bolt going into an old barrel extension. They MAY have a tolerance issue. 
Headspace gauges are used to see if the bolt/barrel extension is within tolerance. Normally, a GO/NO GO gauge is used. 
 

In MOST CASES…headspace on an AR isn’t a problem, because of the inherent design, but it’s not a bad idea to check it, if you have to swap a bolt. That being said, I personally wouldn’t get my panties in a twist over checking headspace, unless I was replacing a bolt on a very high round count rifle. That’s just me. 
 

You probably just need a mil-spec bolt, complete with gas rings, extractor, and such for spare parts. 
Most people don’t replace the entire BCG assembly (in most cases). 
Whatever you do, buy a mil spec bolt that have been MPI proofed and tested. No fancy coatings are really needed. Your carrier will probably not fail…(unless the gas key falls off, but that’s another subject). 
 

Is there any real benefit to changing a bolt carrier group? Anything noticeable? Are coatings useful?

I actually see heritage Guild does classes on building ar15s. I may sign up for that.  

[Displaced Texan 11,731]

Not really, other than it’s about 2 seconds quicker. 
 

Honestly, just buy a complete bolt with gas rings, and keep it in your kit. 
 

Coatings…..sigh, that’s a whole source of internet argument. 
I personally don’t see a benefit to anything other than the standard, mil spec phosphate coating. It’s easy enough to clean (and I rarely clean my ARs). 
 

I think the fancy, NiBoron and other such boutique coatings are snake oil, but some people will argue  that to the ends of the earth. 
 

I would highly recommend an armors course, it’s more in-depth than a ‘build and AR’ class. 
But the build class certainly wouldn’t hurt. 

[High Exposure 5,661]

5 hours ago, Cheflife15 said:

Is there any real benefit to changing a bolt carrier group? Anything noticeable? Are coatings useful?

I actually see heritage Guild does classes on building ar15s. I may sign up for that.  

No. Some say they are easier to clean, or need less lube. It’s a gimmick.

[My1stGlock 144]

If you are looking to grab one for a spare this weekend would be your best bet. Deals all over the place. 

 

[B Gem 23]

True that I think I just saw a Brownells email with a ton of Black Friday specials in it. That reminds me I probably should check and make an order out.

[silverado427 10,675]

I have NIB,  phosphate , and nitrided bcg's  they all preform the same.  Nothing wrong with having a spare bcg or bolt. Spare gas rings should be your only worry.  In my opinion the nitrided bcg  are the slickest when lubed up. 

[ESB 244]

NiBo/Nitride coatings are nice but not needed.  They require less lube and make it significantly quicker to clean than Mil Spec Parkerized/Phospate.  Having had both (MilSpec and NiBo), this is definitely true, not just a gimmick.  For an extra $20, its worth it to me.  

As others said, you don't need a spare BCG as a backup, but just a bolt.  On the condition you know how to change out the bolt in a BCG.  Which you should as that is part of cleaning it.  If you have MilSpec (ie Parkerized/Phosphate coated) BCG, and wanted NiBo, you should get an entire NiBo BCG, not just a NiBo bolt to put in your MilSpec BCG.  

 

This is a complete BCG (Bolt Carrier Group)

 

 

This is the Bolt (this is the part the fails or wears and needs to be replaced)

 

 

 

This is a BCG without the Bolt

 

 

This is a Parts Kit which includes the parts that fail in a BCG.  The rings, extractor, extractor spring, firing pin, etc.  

 

FYI, you can get a complete Toolcraft NiBo BCG for only $79 from PSA right now.  Run this, and keep your current MilSpec as a spare.  AR-15 specs and tolerances are such that you should not have to worry about swapping BCG's of same caliber.  Like at all.  If there is an issue, either you have a crappy AR that is out of spec (or a crazy high end one that was hand fitted) or its pretty worn and probably should be replaced anyway.  

https://palmettostatearmory.com/pa-15/parts/toolcraft-non-logo-premium-5-56-nickel-boron-bcg-with-carpenter-158-bolt.html

 

 

 

[ESB 244]

Also you would want a regular mass, not a low mass BCG, unless you plan on also changing to a lighter buffer and an adjustable gas block.  

[raz-0 1,259]

On 11/26/2022 at 3:53 PM, Cheflife15 said:

I was reading somewhere that it's normal to hand a spare bolt carrier group in your kit incase it fails in your rifle.  I was looking at picking up a wilson combat BOLT CARRIER ASSEMBLY, AR-15/M16, 5.56 NATO, AUTO, LOW MASS, POLISHED NICKEL BORON. 

After googling my understanding is these are all interchangeable between rifles.  I would be putting this in a 5.56/.223 daniel defense ddm4v7.  

Also,  what is headspace? I'm reading changing out bolt carrier groups could weaken the bolt and cause the gun to fail completely.  

Tldr: I'm lost in the sauce with all the conflicting information on the internet.  

Spare bolts? Yah. WHole spare BCG... not so much. Although if you like building ARs, you eventually wind up with them. 

No, Carriers and bolts are not universally interchangeable across all ar-15 pattern guns. 

5.56 and 300 blackout guns use the same bolts.

7.62x39 and 6.5 grendel use the same bolts. (bolt face is .445" diameter)

6.8SPC uses something real close to the bolts (bolt face is .442" diameter)

Then you have pistol caliber BCGs which don't have a bolt, and are very different from a standard mil spec 5.56 BCG. 

Then you have annoying bullshit like wonderbolts and other things that start messing with the lugs and have to be headspaced different or need special extensions. Avoid them. 

As for the BCG, all those bolts will fit in a standard carrier. But if your gun is a piston/op-=rod gun and not a normal direct impingement gun, the BC is non-standard. Additionally if your gun is set up with a roller camp in (this is a POF gun thing), you can't use a normal carrier with a normal cam pin. This stupidity looks like this. I mean it works, but it fixes a problem that doesn't exist and creates proprietary uppers that don't work with standard BCGs right. 

 

 

 

Headspace is the measurement telling you how the bolt fits to the chamber/barrel extension. Too much and too little means the gun will have issues, potentially catastrophic if it is too far out of spec. Hence go/nogo gauges. In spec bolts will generally mesh with in spec barrel extensions and everything will be fine. If you want to be certain it is fine, you get go/no-go gauges. You can also do a ghetto version of this by using an in spec round and a couple pieces of masking tape. As mr. google about that if you need to know.

Typically what fails is the following in the BCG area. 

1) Bolt cracks at the camp pin hole. This is the #1 failure mode over time... by a lot.

2)  Bolt sheers a lug. This is what everyone worries about, and is much less common. It happens, but typically the gun will keep running, possibly less reliably, but keep running. It is an inevitable failure mode over time, but the reality is you usually never get here because of #1 unless there is a defect in your bolt. 

3) Gas key comes loose

This isn't a common long term failure mode. It is however something that can happen because the BCG wasn't built right or something was defective. This will usually happen fairly early on if it is going to happen. 

4) Gas key cracks or is eroded. This is a longer term failure mode. It is the result of the gas key and the gas tube not being concentric, and is typically due to an assembly error on the upper, or a seriously out of spec key on a carrier. But the tube and the key rub when the BCG goes into battery, and if that wear is not symmetrical, you can chew one edge of the key or tube away and get a gas leak and the gun doesn't run reliably any more, or at all. Worst case is asymmetrical rubbing turns into impact rather than just rubbing, and this can damage the gas tube, or crack the gas key, or cause the gas key to loosen. Spares typically don't help here because the root cause is typically a misaligned gas tube, and the damage occurs to both the gas tube and key so a spare BCG is insufficient. 

 

On 11/26/2022 at 4:57 PM, High Exposure said:

I keep a spare bolt and some other small parts (gas rings, extractor springs, cam pin, firing pin, trigger and hammer pins, etc…) in an Altoids tin in my rifle bag. I have a second tin with a complete LPK in it in my teaching bag. A little insurance for a frustrating day.

Don't go crazy with a premium, fancy coated Wilson BCG just to keep as a spare. Just get a standard Mil-Spec set up from a reputable shop.

No need for boutique kit in this role (whether non-spec coatings on AR parts are a good idea is a whole other conversation).

Spend the $$ you save in a GO/NO GO gauge and an armorers class.

This is a solid list of spares. I'll just add that you can buy bolt rebuild or bolt spare parts kits that are basically all the BCG bits of this in a container. I will also add that a lot of fancy BCGs are doing something where the tradeoff is less reliability. 

On 11/26/2022 at 5:18 PM, Displaced Texan said:

Not really, other than it’s about 2 seconds quicker. 
 

Honestly, just buy a complete bolt with gas rings, and keep it in your kit. 
 

Coatings…..sigh, that’s a whole source of internet argument. 
I personally don’t see a benefit to anything other than the standard, mil spec phosphate coating. It’s easy enough to clean (and I rarely clean my ARs). 
 

I think the fancy, NiBoron and other such boutique coatings are snake oil, but some people will argue  that to the ends of the earth. 
 

I would highly recommend an armors course, it’s more in-depth than a ‘build and AR’ class. 
But the build class certainly wouldn’t hurt. 

I agree with you and the keep a spare, fully populated bolt in your kit idea. I do that as well as keep a box of replacement parts. 

Aww.. you should give your opinions on coatings. That's the kind of stuff we are here to argue about. 

I will. 

COATINGS ARE BAD. 

SURFACE TREATMENTS ARE GOOD. 

Coatings sit on top of the metal. Surface treatments basically become one with the metal due to chemicals transformation. 

Also "nitrided" is an ambiguous term, and it has started to be abused in the retail sector. 

Mil-spec phosphate is fine. It is a surface treatment. 

IMO the only improvement on this is going to a polished, melonited carrier. This may also be called a nitro carburized, QPQ, or nitrided. Why is this potentially better? There are only really three ways to mess up a mil-spec carrier. 1) machine it out of spec, and this isn't really something you can tell from advertising, and is an issue of QC for a given brand. 2) Attachment of the gas key.  Which is once again a QC issue, but if the marketing pics don't look like they did a good job, they probably don't do a good job.  3) Do a poor job at applying the chrome coating in the inside of the carrier where bolt goes. It's part of the mil-spec requirements, but just by it's nature there's minimal room for error, and chrome is chrome and can fail in addition to the channel being machined out of spec, or the polishing step being done wrong and driving it out of spec dimensionally. For a melonited carrier, the coating can't fail. If machining and polishing were good, melonite won't alter the dimension or finish. And polished melonited carriers are easier to clean than phosphated. Also polished and melonited can't hide any sins in machining or surface prep. The matte phosphare can to a small extent. 

Things like TiNi, Ion bond, NiBoron, etc are coatings. They can flake off, and potentially interfere in the smooth operation of the gun. This is primarily a concern of chrome and nickel boron as they are thicker coatings, and one the coating is compromised, the damage will likely spread. TiNi, ionbond, and many others are PVD coatings, much thinner, and damage is less likely to spread. They are probably ok, but why pay for them when the better choices are cheaper? 

Unfortunately some PVD coatings are black and contain nitrogen and will be referred to as nitride. So do some digging. 

Personally I've been very happy with BCM mil spec BCGs, JP, PRI, rubber city armory, and spikes melonited BCGs. 

As for bolts, I just buy JP bolts and let my wallet complain. They are the only company that puts extra meat wound the cam pin hole without doing something else stupid. Their QC is excellent and they insist on pretty tight tolerances. Many of the good, more boutique barrel makers head space to their bolts because of this. And their warranty and CS was good 20 years ago and is still good today. That's from personal experience. I know a lot of people who use their stuff, and I've never run into anyone with anything bad to say about them. They are pricey, so I don't indulge in everything they make, but their bolts are worth it and I've just stopped dicking around with anything else.  

[Displaced Texan 11,731]

1 hour ago, raz-0 said:

Aww.. you should give your opinions on coatings. That's the kind of stuff we are here to argue about. 

Snake oil. 
 

Mil-spec phosphate is all you need. 
Cleaning a mil-spec bolt ain’t that hard!! 

[High Exposure 5,661]

Guns turn ammo into dirt (carbon) and heat.

All that heat and dirt has to go somewhere. The engineers have built considerations for how to handle the heat and dirt into their plans (including coatings as well as manufacturing and assembly processes).

“There is no free lunch”. So, when you change things to make the gun run cooler or cleaner, that heat and dirt end up someplace else.

That may be a place that was not designed to handle said heat and dirt. This can be problematic.

I like to prevent problems in my life-saving equipment at all costs. If I can’t prevent them, I will do everything I can to avoid them.

For me, step one is to stay away from all proprietary parts (like that roller cam pin or piston driven uppers). Step two is to avoid fancy coatings and the like.

So far, it’s worked for me.

[Displaced Texan 11,731]

3 hours ago, raz-0 said:

As for bolts, I just buy JP bolts and let my wallet complain. They are the only company that puts extra meat wound the cam pin hole without doing something else stupid. Their QC is excellent and they insist on pretty tight tolerances. Many of the good, more boutique barrel makers head space to their bolts because of this. And their warranty and CS was good 20 years ago and is still good today. That's from personal experience. I know a lot of people who use their stuff, and I've never run into anyone with anything bad to say about them. They are pricey, so I don't indulge in everything they make, but their bolts are worth it and I've just stopped dicking around with anything else.  

Plain ole Toolcraft for me. Standard mil-spec phosphate. Carpenter 158 steel. 
HPT/MPI. 
 

I have a couple of spares laying around. 

Reminds me, I should get one for my AUG. 

[Scorpio64 5,147]

2 hours ago, High Exposure said:

Guns turn ammo into dirt

Guns turn money into noise.

[raz-0 1,259]

9 hours ago, Scorpio64 said:

Guns turn money into noise.

Sometimes they just turn money into guns though. 

[Displaced Texan 11,731]

That spare castle nut in your parts kit seems to be able to magically turn into a complete rifle…

[silverado427 10,675]

I had to stop extra parts seeding, crop rotation was destroying my finances.  

[raz-0 1,259]

On 12/1/2022 at 8:47 AM, Displaced Texan said:

That spare castle nut in your parts kit seems to be able to magically turn into a complete rifle…

My spare parts pile is dangerously close to critical mass. It has some good stuff in it. 

[High Exposure 5,661]

39 minutes ago, raz-0 said:

My spare parts pile is dangerously close to critical mass. It has some good stuff in it. 

Lol. Same. I probably have 4 more AR pattern rifles than I really need because of this.

I have grabbed a single part and turned it into a complete rifle in less than a day on more than one occasion….

I was trying to sell off a bunch of spare parts, just to keep that from happening again…..  I have too many other firearm projects I want to finish. 

[RUTGERS95 890]

On 11/26/2022 at 4:57 PM, High Exposure said:

I keep a spare bolt and some other small parts (gas rings, extractor springs, cam pin, firing pin, trigger and hammer pins, etc…) in an Altoids tin in my rifle bag. I have a second tin with a complete LPK in it in my teaching bag. A little insurance for a frustrating day.

Don't go crazy with a premium, fancy coated Wilson BCG just to keep as a spare. Just get a standard Mil-Spec set up from a reputable shop.

No need for boutique kit in this role (whether non-spec coatings on AR parts are a good idea is a whole other conversation).

Spend the $$ you save in a GO/NO GO gauge and an armorers class.

I keep spare bolt, pin and gas rings in my grip.  'Never go home without it'   lol

[My1stGlock 144]

3 hours ago, RUTGERS95 said:

I keep spare bolt, pin and gas rings in my grip.  'Never go home without it'   lol

More like never leave home without it!

[RUTGERS95 890]

On 12/19/2022 at 6:39 PM, My1stGlock said:

More like never leave home without it!

yes lol....I"m an idiot!  mess that quote up:)

[High Exposure 5,661]

If anyone bought a Wilson BCG via their recent Blem sale - there are internet reports of functional defects, not just cosmetic blems.

I don’t have one, so have no firsthand experience. Just passing along what I am seeing. Remember - this is the internet, so grain of salt and all that.

[RUTGERS95 890]

1 hour ago, High Exposure said:

If anyone bought a Wilson BCG via their recent Blem sale - there are internet reports of functional defects, not just cosmetic blems.

I don’t have one, so have no firsthand experience. Just passing along what I am seeing. Remember - this is the internet, so grain of salt and all that.

good to know as I bought one.  

The good thing is that as part of the bullet proof line, it's a simple rma with them and they'll fix or replace it (with a new one at that)  I did a visual inspection of mine but now will break it down and review today and report back on mine.

to add to that, these are generally considered the top of line of chrome bcgs

[RUTGERS95 890]

On 11/26/2022 at 5:18 PM, Displaced Texan said:

Not really, other than it’s about 2 seconds quicker. 
 

Honestly, just buy a complete bolt with gas rings, and keep it in your kit. 
 

Coatings…..sigh, that’s a whole source of internet argument. 
I personally don’t see a benefit to anything other than the standard, mil spec phosphate coating. It’s easy enough to clean (and I rarely clean my ARs). 
 

I think the fancy, NiBoron and other such boutique coatings are snake oil, but some people will argue  that to the ends of the earth. 
 

I would highly recommend an armors course, it’s more in-depth than a ‘build and AR’ class. 
But the build class certainly wouldn’t hurt. 

I actually hate niboron and wont' use them and the nitride, while good, doesn't hold the oil as well as phosphate imho.  My BCGs are always Colt, AO Precision, LMT or FN, I don't stray.  I do like Chrome as you can see from the post above this one and have several.  

 

OP, if close you are always welcome to my place to do some learning!  lol

[Cheflife15 455]

46 minutes ago, RUTGERS95 said:

I actually hate niboron and wont' use them and the nitride, while good, doesn't hold the oil as well as phosphate imho.  My BCGs are always Colt, AO Precision, LMT or FN, I don't stray.  I do like Chrome as you can see from the post above this one and have several.  

 

OP, if close you are always welcome to my place to do some learning!  lol

There may come a day where I take you up on this! I actually took my bcg apart the other day and really cleaned everything.. I took the advice here and bought some replacement parts incase I need them. Very thankful for the education I get from you guys.