Bolt carrier groups

[Robocop1051]

Looks great!  <thumbsup>

[Guill732]

This is awesome. Thanks! Anyone have the prints for the bolt carrier or receivers (upper/lower)? Also, what steel is usually used in these?

[tucson308]

I will do the bolt carrier eventually...4140, 9310 and Carpenter 157. S-7 is also a great choice, but would be difficult for the average guy to obtain without overpaying too much. 4140 is the weakest of the bunch for this application, but is used more than all the others combined in the firearm industry. It's low cost to strength makes it very attractive to manufacturers.If you are making just a few parts the savings is not that big per part, but if you look at the big picture it will equate to thousands of dollars in the long run. Yes, manufacturers look down to the nearest penny when comparing cost if they are making high volume runs.

I commonly am looking at ways of saving .1 of a cent at the company I work for. We deal with large contracts to as much as 100,000,000  PPY and tenths of a sent equal big $$$ in the end. China is one of the biggest scrutinize-rs of cost down to the point of choosing an inferior material to make a product. We all make fun of "Chinese junk", but in a lot of cases if they used a better material to manufacture there part it would be equal to the competing American part. This should also be a red flag to you when comparing something that cost's so much less than the U.S. made unit. I would not use 4140, or 4150 unless it was an last resort for an AR15 7.62 X 39  or 6.5 Grendel bolt, but would be just fine for the .223/5.56 and .308 bolt.

The .308 bolt is especially strong, inherently due to the size of the .308 bolt to a regular AR15 bolt. I would be confident with a .308 bolt made of 4140, or 4150. The other three will be stronger and have better wear properties, but 4140 is adequate for it's call of duty in this situation.

The case head diameter of the 7.62 and the Grendel make the lug area weaker than all others and thus creates the failure that you hear about with the AR15 bolt's. I built my first Grendel with a 7.62 standard AR bolt  and it broke in under 400 rounds. I since then replaced it with a 9310 unit and have had 2000 rounds put through her with no issues.

The .223/5.56 case head is much smaller and does not weaken the lug area as much as the 7.62 or Grendel does. Many manufacturers still use the 4140 or 4150 (has a little more chrome in it) for there .223/5.56 specific bolts.

Carpenter will be very hard to obtain as it is proprietary to Carpenter and is only made by them and will cost the home guy some big $$$ to obtain even if you could find a 1" dia piece.

This leaves you with 9310 and my favorite. It is super tough, strong and everybody, well most everybody has it. It is a bit more pricey than 4140, or 4150 but not too bad if you shop a little. Avoid Mcmaster, or MSC for buying raw material. they charge a premium for all there raw materials, but they will have them in stock.

I purchase mine from the same outfit that supplies our tooling steel(Arizona Tool Steel), but a local tool steel supplier will, or can have it for you in little time.

[Guill732]

Thank you very much for the info. I have a round bar of S7 I've had since high school that i think is the right size to make into the BCG, so I was curious if this was appropriate material.

[tucson308]

It will work great. We use it in many forming, coining  and some cut operations where, CPM, Carbide, or other steels fail due to the inability to handle the shock.. I'm talking about parts that are smaller and have a lot less support than an .308 AR bolt and run for  weeks, or even years at 50,000 cycles per day with a minimum of 60,000 PSI without failure. You sacrifice some in the way of wear properties, but if the part fails before you get to that point, Who cares.

You will want a RC of 55-57 when finished. You will need the ability to heat the material to 1250 degrees then soak  for 45 minutes at that temp and then raise to its final temp of  1725-1750 degrees for 1 hour, followed by a temper cycle, or two at 500 degrees is best in order to heat treat the S-7. Do NOT heat, treat it with a torch, it will get hard and may hold up, but you will not know what you are dealing with. Unless you can follow the above process with your wife's turkey thermometer and the Harley mechanics torch from down the street, you are best to take it to a heat treater, or look for a tool and die shop that is local and ask them to heat treat it.

[constructor - Banned]

For the record, 416 stainless isn't a better material (for a barrel) over 410 stainless.  That's why you see alot of 416 barrels, and not very many 410 barrels - 410 is supposed to be a bitch to machine.  Bravo Company makes a few 410 barrels, for specific applications, but there aren't many other manufacturers out there that do it.  It's pricey stuff. 

Now, is a 416 stainless carrier better than a 8620 steel carrier?  I have no clue, whatsoever.  I doubt that the 416 is better than 8620 tool steel, when a 410 ss barrel is better than 416 ss barrels, for strength...  Why not make that carrier from 410 ss, then? 

Bottom line, I'm not trusting ANY stainless steel carrier, until I see numbers.  Comparisons.  Show me the differences between the two.  Not 416 to 410 - that's already there.  Show me both 416 and 410 to 8620.

Bolts should be from 158 Carpenter steel, as well as tool steel extractors and ejectors... 

Bottom line, a machine shop can't pull this off.  This is poop that they're not used to, and materials a typical machine shop can't even get ahold of.  They won't have any idea where to source this stuff from.  When they find it, they'll pay out the ass for it, and so will their customer, after machining.  Unless they commit to this 110%, buy bulk, and go big, then they won't get discounts on the proper materials.

Now...  all this is my ass-umption that they're just a machine shop.  All this might backfire on me, when I find out that they're truly that badass, and already have cheap sources for true gun-steel.

Here's what they need to pull off, in addition to the carrier:

Bolt carrier group (auto version) for your AR15, M16, or M4.  Parkerized exterior and chrome lined inside carrier. Machined to USGI specifications.  This M16/M4 carrier includes the proper Mil-Spec gas key.  Gas key is chrome lined and heat treated per GI specifications.  Gas key is secured to carrier via USGI domestic mfg Grade 8 fasteners and properly staked per Mil-Specs.  These properly hardened parts insure a strong staking and proper gas system functioning.

The bolt assembly is machined from the correct Mil-Spec Carpenter No. 158® steel, shot peened for increased strength, includes tool steel machined extractor and ejector.

•Milspec Carpenter No. 158® steel

•HPT Bolt (High Pressure Tested/ Proof)

•MPI Bolt (Magnetic Particle Inspected)

•Shot Peened Bolt

•Chrome Lined Carrier (AUTO)

•Chrome Lined Gas Key

•Gas Key Hardened to USGI Specifications

•Grade 8 Hardened Fasteners

•Key Staked Per Mil-Spec

•Tool Steel Extractor

BCM doesn't make 410 barrels, they are made for BCM by Kreiger's Criterion division. They are apx 38-42 Rockwell.

When SS is heat treated it loses it's corrosion resistance, it is not a good material when it needs to be 58 Rockwell. 8620 is standard material for  AR10 carriers AND bolts. AR15 bolts are Carp 158.

[98Z5V]

BCM doesn't make 410 barrels, they are made for BCM by Kreiger's Criterion division.

Doesn't matter who makes them for them, really - BCM only sells 410 stainless barrels, and doesn't sell and stainless barrels that are 416.  They have clearly stated in the past why they do this. 

[tucson308]

Sounds like your looking for nobtanium. A "small machine shop" is just as capable as the people that work there.. 158 is not the only steel to use. 9310 is just as capable and maybe better based on its chrome percentage you could also use S-7, or many other choices. I deal with material structures, have been a mechanical engineer for 25 years and manage the tooling operations for 4 plants 3 in the U.S. and one in MX. That make over the top prescision tooling for the electrical, medical and dental industries,Just to state my qualifications. .308 bolts are not known for breaking, but could wear out. 9310 is both a great shock and wear steel. "poop there not use to"? come on, is this stuff going to take you into orbit? these are very basic parts to machine and could be done by a weekend warrior  with the correct equipment. chroming specific areas was diffacult in 1936, but even the run of the mill chroming operation could chrome these parts, or coat them with whatever you would like. As far as getting 158, 9310 or tool steel. See my post above. Call a local tool steel supplier. every moderate size city will have one, or more.

I will show you some blowups of some bolts from reputable companies that look like they were machined with a chainsaw, but to the naked eye they look great. They do not have these things made by some special shop who gets nobtanium, and have discovered the holy grail. It is not that complicated. on another post here in 308 there is a pic of a bolt that I had made in one of out tooling shops after I reversed engineered it. You will see the ultimate in locking lug machining. Run of the mill shops, won't be able to do that type of machining because it was done on a $195K Agie Progress EDM machine. EDM is over kill for this application, but if you want A++++ better that perfect and have one at your disposal, why not use it?

Hope this helps!!!

[98Z5V]

Hey, hope that wasn't for me, brother - All I intended to do was provide "the minimums" with what I posted.  If you find something better, by all means, go for it.  Just don't use something that's not on par with the materials listed above...  <thumbsup>

I saw the pics of the 308 bolt you already made - the thing is a work of art.

[Matt.Cross]

Sounds like your looking for nobtanium. A "small machine shop" is just as capable as the people that work there.. 158 is not the only steel to use. 9310 is just as capable and maybe better based on its chrome percentage you could also use S-7, or many other choices. I deal with material structures, have been a mechanical engineer for 25 years and manage the tooling operations for 4 plants 3 in the U.S. and one in MX. That make over the top prescision tooling for the electrical, medical and dental industries,Just to state my qualifications. .308 bolts are not known for breaking, but could wear out. 9310 is both a great shock and wear steel. "shit there not use to"? come on, is this stuff going to take you into orbit? these are very basic parts to machine and could be done by a weekend warrior  with the correct equipment. chroming specific areas was diffacult in 1936, but even the run of the mill chroming operation could chrome these parts, or coat them with whatever you would like. As far as getting 158, 9310 or tool steel. See my post above. Call a local tool steel supplier. every moderate size city will have one, or more.

I will show you some blowups of some bolts from reputable companies that look like they were machined with a chainsaw, but to the naked eye they look great. They do not have these things made by some special shop who gets nobtanium, and have discovered the holy grail. It is not that complicated. on another post here in 308 there is a pic of a bolt that I had made in one of out tooling shops after I reversed engineered it. You will see the ultimate in locking lug machining. Run of the mill shops, won't be able to do that type of machining because it was done on a $195K Agie Progress EDM machine. EDM is over kill for this application, but if you want A++++ better that perfect and have one at your disposal, why not use it?

Hope this helps!!!

That's very encouraging to hear brother, appreciate that input.

[tucson308]

No, Not for you. Just trying to set the record straight. Hope I did not offend anybody.

I wrote the post while making dinner and did not preview, but I was not trying to be a "Douche nozzle".

[98Z5V]

<lmao> <thumbsup>

[dengoose]

Just a heads up Harris Tactical has a pre-order going for Nickel Boron DPMS pattern BCG's. Not exactly sure the make but guessing WMD as they did a pre order on their 5.56 BCG's as well

http://www.harristacticalonline.com/308-762x51-Nickel-Boron-Complete-Bolt-Carrier-Group-NIB-X-FA-HT-NB-308.htm

[imschur]

thanks

[Justintoxicated]

Just a heads up Harris Tactical has a pre-order going for Nickel Boron DPMS pattern BCG's. Not exactly sure the make but guessing WMD as they did a pre order on their 5.56 BCG's as well

http://www.harristacticalonline.com/308-762x51-Nickel-Boron-Complete-Bolt-Carrier-Group-NIB-X-FA-HT-NB-308.htm

Is this a good BCG?  it sounds nice but there is no forward assist.  I was originally going to go with a JP before parts got scarce, which also does not have forward assist, but it's stainless steel and light weight.  My Upper has a forward assist, but I guess it would be ok to get a BCG without it?

[EasyEJL]

Is this a good BCG?  it sounds nice but there is no forward assist.  I was originally going to go with a JP before parts got scarce, which also does not have forward assist, but it's stainless steel and light weight.  My Upper has a forward assist, but I guess it would be ok to get a BCG without it?

I think i'd rather live without a forward assist to get a nickel boron coated BCG at that price. Besides, its not like you can find a forward assist now either :D

Thanks for posting the link dengoose, I needed one and i'm ordering one!

[Justintoxicated]

I think i'd rather live without a forward assist to get a nickel boron coated BCG at that price. Besides, its not like you can find a forward assist now either :D

Thanks for posting the link dengoose, I needed one and i'm ordering one!

Yea, waiting on BCG so I can give my bolt to my barrel maker to have them headspace it, I told myself I would be patient to get this just right, but now I have a regular buffer and recoil spring rather than a fancy JP one, and no JP BCG...  Oh well at least I'm not paying ridiculous prices for stuff...  Ordered!  Thanks for posting the link.

[edgecrusher]

Guys, keep in mind, if you have to use a forward assist on a .308, you have  problems and should stop shooting immediately! Buying a BCG without one will not affect you rifle with a forward assist on it, you just won't be able to use it.

[Justintoxicated]

Guys, keep in mind, if you have to use a forward assist on a .308, you have  problems and should stop shooting immediately! Buying a BCG without one will not affect you rifle with a forward assist on it, you just won't be able to use it.

Well I reload, so if my cases are not sized perfectly, and if I have a very tight chamber the forward assist could come in handy. But then again, another reason could be a squib, and with no forward assist you HAVE to take the BCG out and look :)

[edgecrusher]

I say that because of the pressures of the .308 should be plenty enough to cycle the action. I see your point, but I want to caution the use of one to solve any "problems" of the bolt not going into battery… or any other reason someone may try to use it where it will become a safety issue

[98Z5V]

Forward assist works on 308 ARs.  It doesn't ratchet the BCG down the upper receiver like the AR-15 does, but it catches the shoulder on the BCG and locks it into battery.  It would be the equivalent of the "last click" of a forward assist on the AR-15.

It works on all DPMS and Armalite patterned 308s, notches, serrations, smooth doesn't matter.  I can't vouch for the RRA guns.

[unforgiven]

Same principal on RRA haven't used it on either 5.56 or .308 doesn't bother me.Was gonna take it off but no valid reason to.It's not like the weight is an issue. <lmao>

[tucson308]

The first bolt came out of heat treat this morning. I'm not going to NiB coat the bolt until after I test the current design. I altered some of the lug areas for more strength and a better lockup, but need to do some extensive testing starting this weekend. If all goes well I will have the bolts available for sale in the $325 ish range in about two weeks.

[tucson308]

oops, That should say the $225 ish range .

[98Z5V]

Is that $225 for a bolt, or $225 for a complete BCG?