Metal Injection Molding (MIM) BCG Gas Keys

Discussion in 'AR-15 Discussion' started by eastby, May 24, 2013.

  1. eastby

    eastby New Member

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    Is there anything wrong with a MIM gas key on a BCG? I've read some complaints of cracked MIM gas keys where they were staked -- particularly on LMT and PSA full auto/M16 BCGs. I guess the circular MIM mark is between the two rivets/fasteners, to distinguish it from a forged gas key (with no mark). I have a PSA M16 BCG that has a MIM gas key that looks fine. Should I be worried?
     
  2. JonM

    JonM Moderator

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    Gas keys can and do break on any type of build. They are routine maintenance parts and if found in a state that shows looseness should be replaced. Everytime you clean your rifle you should inspect the gas system and the entire gun regardless of the rifle design.

    The only way to tell if a part is MiM or not is if it breaks and was poorly made you can see metal looks like powder, mold marks are left on the part (indication of lack of technique or care), the manufacturer says its a MiM part. Some yahoo on the interweb telling you something is MiM is NOT a good source of information.

    When metal injection molding is done right there is no way to tell the difference between that part and one made from billet steel without an advanced degree in metallurgy and manufacturing processess. In other words you could take 500 MiM parts and 500 forged or billet parts and no interweb "expert" could successfully sort them out.

    Butbutbut mim is bad right forged and machined from billet is better??

    Well its no worse than any other way of making parts. Garbage in garbage out. All steel starts as iron ore and has to be melted and injected or poured into molds. Use crap or a bad proccess and you end up with crap. The way batch steel is made is a metallurgist takes a guess mixes ores in batches weighing hundreds of tons, melt it to liquid form, take a small scoop dip some out run tests then determine what needs to be added to get the steel they want. The fraction thats actually tested is like baking a cake then tasting one crumb to determine how well its baked.

    So once you understand that metallurgy in pouring enourmous batches of billet steel is more about withcraft and guesswork, albiet highly experienced and educated guessing, you start to understand that quality control on the actual manufacturer of your parts is more important than how the metal got in the shape its in.

    The reason milspec calls for shot peening and mpi inspecting is because even carpenter steel can have voids or bubbles or cracks. Casting steel into blocks of billet material is no different really than taking powdered steel and using pressure to heat the metal to liquid inside the part mold. Its just faster.

    The whole MiM is bad thing comes about because its a easy cheap way to make lots of identical parts and really bad manufacturers use it with cheap metal powder to make cheap parts.

    So if a company like jennings or lorcen is using mim i would say yeah that parts gonna break... but if a company like bcm colt rra noveska daniel defense is using a mim part im not gonna worry about. One type of outfitis going to do it half *** the other type is only going to have the best.

    So, in my estimation its more about who does the work and the quality control that goes into the part and materials. How the part goes from ore in the ground to a part in my firearm is irrelevent to me
     
    Last edited: May 24, 2013

  3. Quentin

    Quentin New Member

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    Great post, JonM! Very informative...

    PSA says it does not sell a carrier with a MIM gas key but their numbered keys are softer than normal, leading to ugly staking. It's been a hot topic over at AR15.com.

    I also a PSA BCG with the numbered key and YFS screws. Right now it's only a spare but I'm watching this topic for more info. A BCM key and screws may be a wise purchase.
     
    Last edited: May 24, 2013
  4. JonM

    JonM Moderator

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    im extrapolating from your reply quentin and this isnt directed at you :)

    there is a reason that psa comes in a scootch cheaper than bcm and its evident that they use lesser materials in some areas. they couldnt deliver a cheaper gun than bcm unless corners were cut somewhere. doesnt mean its bad or wrong to do it as it doesnt make any difference if someparts arent built of indestructinium. it is however bad to say psa sucks because they didnt use aircraft grade screws that boeing uses on fighter jets to secure the buttplates to the stocks of their guns but brand X DOES USE THEM!!!!...

    im assuming thats where this question arises since i dont read ar15.com on any real or regular basis.

    replacement parts i keep on hand are spare lower parts kit, bolts, extractors, springs, detents, and..... CARRIER KEYS along with their attendant screws!! those are parts any ar15 owner should have on hand to get the gun running quickly.

    while i was doing the armory thing in the army the only issues i ever saw were ones involving extractors and carrier keys. takes like 5 minutes to put a carrier key on and they arent that pricey. pop the old key off soon as it starts to loosen DO NOT RETORQUE OR RESTAKE IT!!! and replace it with a new one. the reason you replace it is because in all likely hood the weapon was fired with it loose and this can easily introduce micro-cracking which leads later to a catastrophic deconstruction of your rifle. seen it happen.

    now regardless of rifle type whether its an AK47 or AR15 or grandpa's Browning BAR deer getter, if there is an issue with the gas system REPLACE ALL PARTS in question. do not "fix" or "refit" the old ones. cant stress that enough. there are a lot of great guns that are thrown away because someone cheaped out on buying a 15$ gas plug/screw/piston/key reused the failed part and blew their gun up.
     
  5. pawjr74

    pawjr74 New Member

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    Last edited: May 24, 2013
  6. JonM

    JonM Moderator

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    same is true of parts whittled out of steel. use crappy steel or a grade not up to snuff and milled parts fail at an astonishing rate when put under stresses the steel isnt rated for.

    its not the fault of the MiM process its the fault of those running the machines designing the parts and inspecting the materials.
     
  7. pawjr74

    pawjr74 New Member

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    Good point.

    I guess the moral of the story is to order from a quality supplier. If an issue does come up a good company will stand behind their product.
     
  8. JonM

    JonM Moderator

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    That is spot on. Its something we all lose track of. Its lessons companies have to learn over and over because they continously hire college degree types that have yet to learn lessons in providing a quality product first.
     
  9. eastby

    eastby New Member

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    Thanks very much for the superb info, gentlemen, and thanks for correcting my confusion concerning MIM vs forged parts. Still a little puzzled why the gas key on a PSA M16 BCG is [apparently] made of a softer steel than the carrier. Must all come down to dollars and cents -- as Jon mentioned -- and what constitutes "acceptable" quality of the key while still technically meeting the MILSPEC requirements for the carrier and bolt.
     
  10. Quentin

    Quentin New Member

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    I'm assuming the numbered carrier key is softer than the unnumbered key PSA also uses. The reason for this is the ugly, broken staking where it appears the MOACKs staker cuts the softer metal. I don't know for a fact that it's softer but it makes sense.

    Still, my PSA carrier key is on tight and when removing the bolt to test for alignment with the gas tube, the key slides over it perfectly, not hitting on left, right, top or bottom - it's flat dead center. So I expect it to function properly.
     
  11. JonM

    JonM Moderator

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    the keys dont need to be made of super hard material since they arent hitting anything. the psa issues some people are experiencing may be isolated o a bad run of a select few parts. it does happen to all companies.

    whats really important is how is psa handling any carrier key failure?? if they are fixing the issue in a timely responsive fashion i wouldnt be concerned and still consider psa top notch